944 Turbo Frequently Asked Questions

1.0 Introduction
2.0 Differences Between the 944 and 944 Turbo
3.0 944 Turbo Model Year Changes
4.0 General Description of Turbocharger and Boost Control System Operation
5.0 Common 944 Turbo Questions
6.0 944 Turbo Problems

1.0  Introduction

     The 944 Turbo or 951 (internal factory designator) made its international debut at the Geneva Auto Show in March 1985. Around 176 cars were released for the European market in 1985 with the first cars for the US being released in the 1986 model year. Even before the first 944 Turbo debuted at Geneva, a pre-production 944 Turbo competed at and won the 24-Hour endurance race at Nelson Ledges in June 1984.

2.0  Differences Between the 944 and 944 Turbo

2.1  Foreword

     In Section 2, we will compare the differences between the 944 and the 944 Turbo. Bear in mind that these comparisons only apply to the 1985 and 1986 944 Turbos as compared to 944s built up to that time. Later models of the normally aspirated cars were equipped with much of the same equipment as their turbocharged counterparts. In a later section, we'll look at how the 944 Turbos changed with each model year.

2.2  Engine

     The 944 Turbo engine retained the same 2.5L displacement of the normally aspirated 944s. However, the addition of a KKK K-26 #6 turbocharger, along with other supporting modifications, yielded some very impressive performance numbers.

Performance Comparisons

1985 911 Carrera

1985 928S

1985 944

1985 944 Turbo


3164 cc

4957 cc

2479 cc

2479 cc

Horsepower (SAE)





Torque (SAE ft-lb)

185 @ 4800

302 @ 2700

137 @ 3000

243 @ 3500

0- 60 mph





Top Speed (mph)





Fuel Consumption












* Estimated numbers at the time of release.

Note: The numbers in this table were originally published in the March 1985 issue of Panorama. However, the 0-60 mph number for the 944 Turbo was actually a 0-100 kph number of 6.3 seconds. Since the 0-60 mph time was available from another source, it is included for a better point of reference.

     Unlike the 924 Turbo, the turbocharger on the 944 Turbo was moved to the opposite side of the engine from the exhaust. This helped reduce the turbocharger inlet temperatures by approximately 160 °F. Additionally, the turbocharger has a water-cooled bearing housing. During normal operation, the cooling water is supplied from the engine cooling system via a tap on the low temperature side of the radiator. The cooling water returns to the water pump suction via a small thermostat or to the expansion tank if the thermostat is closed. During shutdown, coolant is supplied from the radiator and expansion tank and circulated through the bearing housing and back to the expansion tank. The water in the expansion tank is cooled by being siphoned through the makeup line to the radiator. Coolant circulation through the turbocharger bearing housing is supported by an electric pump. The combination of lower inlet temperatures and water-cooling to the bearing housing dramatically increased the turbocharger life expectancy over the 924 Turbo and 911 Turbo.

     The 951 cylinder head received several modifications as well. The cylinder head exhaust ports have ceramic liners cast into them. This results in higher exhaust temperatures and better turbo response. This is somewhat of a contradiction to moving the turbocharger away from the exhaust side of the engine. However, what it does is provide insulation that allows the energy to be retained in the exhaust gases and converted into work in the turbine rather that being lost as rejected heat into the cylinder head. An additional benefit of this is a reduction in the heat load on the cooling system. The 944 Turbos also use sodium filled valves in the cylinder head for better heat dissipation.

     An intercooler is used with the 944 Turbo system to cool the charge air before it enters the engine. The 944 Turbo intercooler reduces the charge air temperature by approximately 135 °F to provide a denser charge of air and a noticeable increase in horsepower. Because of the increase in engine performance, the engine also received thicker cylinder walls, forged pistons, a larger oil pump, and heavier head gasket. 944 Turbos were also equipped with an external oil cooler as opposed to the normally aspirated cars whose cooler is integral to the block.

2.3  Transaxle

     The 944 Turbo received a transaxle with a different gear ratio from the normally aspirated cars. The 944 Turbo transaxle employed an external oil cooler which was not used on the normally aspirated cars. Also, a transaxle with limited slip differential and external oil cooler was available as optional equipment. Below is a table which compares the 951 gear ratios to the 944s. The different transaxles used in 944 Turbos will be discussed in a later section.


1986 944 (ROW) 016J

1986 944 (US/Japan) 016K

1986 944 Turbo 016R

1st Gear




2nd Gear




3rd Gear




4th Gear




5th Gear




Reverse Gear




Final Drive Ratio




2.4  Body

     The 944 Turbo introduced some body changes that carried over to later water-cooled models. Most noticeable is the aerodynamic front end. The new front end integrated both the fog lights and the front bumper into a single panel. The front panel has several ducts that provide cooling air flow to the engine. The 944 Turbo has a larger frontal silhouette than the normally aspirated 944 (1.89 m2 vs. 1.82 m2). However, the smooth lines of the new front end resulted in a lower drag coefficient for the turbo (0.33 vs. 0.35). In addition to the spoiler mounted to the rear hatch, the 944 Turbo also carries a spoiler mounted underneath the rear bumper. Air flow between the bottom of the car and the spoiler provides negative lift which improves the car's stability, particularly at high speeds. The 951s also have side skirts mounted at the bottom of the rocker panels.

2.5  Suspension, Brakes, and Wheels

     The 944 Turbo front brakes have a smaller surface area than the normally aspirated cars (86 cm2 vs. 92 cm2 per wheel). However, the turbo uses the same brake pad in the rear while the normally aspirated cars use a smaller pad (63 cm2 per wheel). This results in a total brake area of 344 cm2 for the turbo and 310 cm2 for the 944. Additionally, the 944 Turbo uses 4-piston fixed caliper brakes while the 944 uses single-piston floating caliper brakes. Both models use vented rotors.

     The 951s came equipped with 16" cast aluminum wheels (7J x 16 front, 8J x 16 rear). The tires were 205/55 VR 16s front and 225/50 VR 16s rear. Forged aluminum wheels were available as a factory option. Standard equipment on the normally aspirated cars was 7J x 15 wheels with 205/60 VR 15 tires. As optional equipment the 944s could be equipped with the 7J x 16 forged alloy wheels used on the 911s with 205/55 VR 16 tires.

     The 944s came equipped with 20 mm front stabilizer bars as standard equipment. Rear stabilizers were only available as an option (14mm). They were also available with 21.5 mm solid or 23 x 3.5 mm tubular front stabilizers. On the 944s rear torsion bars were 23.5mm. The 951s came equipped with 22.5 mm stabilizer bars in front and 18mm stabilizers in the rear and 23.5 mm torsion bars. Tubular front stabilizers were also available for the turbos (24 x 3.7 mm). Until 1985.5, 944s came equipped with steel control arms. Later models were equipped with light alloy control arms. 951s were equipped with the light alloy control arms. Due to the wide variations in shocks, struts, and springs used on the 944s and 944 Turbos, we won't try to discuss them all now to avoid confusion. However, in a later section we will discuss the differences between 944 Turbo model years.

2.6  Interior

     There is essentially no difference in the interior of the 944 and 944 Turbo. The only noticeable difference is the addition of a Boost gauge underneath the tachometer.

3.0  Model Year Differences

3.1  Foreword

     Now that we've compared the first 944 Turbos to their normally aspirated counterparts, we'll try to take a look at how the turbos changed with each model year. I'll try to cover as many of the changes as possible. If I miss some of the changes or corrections need to be made, please let me know.

3.2  1987 Model Year

     There were minor modifications to the 951 engine for the 1987 model year. Most notable is the change from the eccentric roller cam belt tensioner to a spring tensioner arrangement. This change also occurred on the normally aspirated cars. Also, an oil level sending unit was added to the oil pan which illuminates a low oil level light on the dash. The 1987 model cars were provided with a one piece oil pressure relief valve as opposed to three piece unit on the 1986 models. As a result of the addition of the cam belt spring tensioner and changes to the water pump pulley arrangement, a new set of timing covers were produced for the 1987 models.

     Beginning in the 1987 model year, the 944 Turbo transaxle came without and external oil cooler as standard equipment. However, the transaxle with cooler as well as the transaxle with limited slip differential and cooler were still available as optional equipment.

     In 1987, ABS was offered as optional equipment on both normally aspirated cars as well as turbos. This required the wheel offset to be increased for the 1987 model year from 23.3 mm to 52.3 mm. All cars received the increased offset wheels even if they were not equipped with ABS. Tubular front stabilizers were used on the 1987 model cars (25.5mm x 4 mm).

     The M030 suspension option was made available starting in the 1987 model year. With this option the front tubular stabilizer size increased to 26.5 mm x 4mm. Through the end of the 1988 model year, FS black struts were standard equipment on 944 Turbos with Koni yellow struts as optional equipment. With the M030 option, height adjustable Koni yellow struts were added. Also with the M030 option, the strut bearings have a harder rubber compound than earlier models which can be identified by a green stripe on the underside of the bearing. The spring rate for all 944 Turbos not equipped with the M030 option is 21.8 N/mm with an unsprung length of 251 mm (through the end of the '88 MY). The M030 option springs have a rate of 28 N/mm with an unsprung length of 220 mm.

     For the rear suspension, all 944 Turbos through the end of the 1988 model year were equipped with FS grey shock absorbers as standard equipment. Koni yellows were available as optional equipment. With the M030 option, the Koni yellow shock absorbers will have two white dots offset by 180°. Also with the M030 option, the torsion bar size was increased to 25.5 mm.

     On the interior, the 1987 944 Turbo became the first production car in the world to be equipped with driver and passenger side air bags as standard equipment. On the normally aspirated 944s for that year, the driver's side air bag was standard equipment, but the passenger side air bag was optional. As already mentioned, a low oil level light was added to the dash. Also, a 180 mph speedometer was added as opposed to the 170 mph speedometer on the 1986 model Turbos.

3.3  1988 Model Year

     There were no significant changes to the 944 Turbo until the middle of the 1988 model year. One minor change involved changing the chip arrangement in the DME unit. Earlier 944 Turbos (86/87) have a DME with a 24-pin chip. Beginning with the 1988 model year, a DME with a 28-pin chip was installed. It's not exactly clear why this change was made, but it has been rumored that it was in anticipation that more control features were going to be added to later models. At any rate, the 1988 DMEs received the same map as the early cars and I've been told that the extra 4 pins on later DMEs were never used.

     In 1988½ Porsche introduced a new 944 Turbo which was designated as a 944 Turbo S. The first example of the 944 Turbo S were a limited production run. Exact numbers on the limited production 951 S cars varies, but seem to be between 750 and 1000. All of the limited production vehicles came with a Silver Rose Metallic exterior and burgundy plaid cloth interior.

     The Turbo S received the M 44.52 engine as opposed to the M 44.51 engine of the earlier models. The engine itself remained essentially unchanged from the early 1988 models. However, the turbocharger was changed to a K-26#8. The Turbo S was electronically limited to the same maximum boost (1.75 bar) as the early 944 Turbos. Also both models achieve maximum boost at approximately 3000 rpm. However, the K-26#8 turbocharger was capable of maintaining maximum boost until 5800 rpm while the boost on the early turbos would decrease from 1.75 bar at 3000 rpm to 1.52 bar at 5800 rpm. This resulted in a 30 hp increase in peak horsepower and a 15 ft-lb. Increase in peak torque.


Performance Comparisons

944 Turbo (M 44.51)

944 Turbo S (M 44.52)


217 HP @ 5800 rpm

247 HP @ 6000 rpm


243 ft-lbs. @ 3500 rpm

258 ft-lbs. @ 4000 rpm

0-60 mph

6.1 sec.

5.5 sec.

Top Speed

152 mph

162 mph

    The Turbo S transaxle came standard with a limited slip differential. However, there were improvements to the differential over previous models. The Turbo S differentials have inner plates which are 2.5 mm thick as opposed to 2.0 mm on earlier models. This required the two differential thrust rings to be made 0.5 mm thinner for the same overall thickness. Also, the inner plate and differential shafts are molybdenum coated for additional hardness. The clutch had a two-stage torsional spring damper with the friction material being bonded and riveted to the clutch disc instead of just rivets on earlier models.

     The wheels on the Turbos were milled forged 7J x 16 in front and 9J x 16 in the rear (52.3 mm offset). The tires were 225/50 VR 16s in front and 245/45 VR 16s in the rear. The M 030 suspension package was standard equipment on the Turbo S models as well as ABS.

3.4  1989 / 1990 Model Year

     For the 1989 and 1990 model years very few changes were made to the 944 Turbo. 1990 was the last year the 944 Turbo was imported to the United States with less that 150 cars imported to the US that year.

     The only real differences between the 1989 and 1990 model year cars and the 944 Turbo S was the size of the wheels used. While the 1989 models still had 7J x 16 and 9J x 16 wheels with the same size cutout as the Turbo S wheels, the offset was changed to 65 mm in front and 60 mm rear. Also available were wheels with smaller cutouts at 7J x 16 front with a 65 mm offset and 8J x 16 rear with a 52.3 mm offset. For the 1990 model year, the wheels were 7.5J x 16 with a 65 mm offset in front and 9J x 16 with a 52.3mm offset in the rear.

3.5  1991 Model Year

     As the saying goes, "all good things must come to an end". The end came for the 944 Turbo in 1991. However, Porsche sent the 944 Turbo out in style by introducing a 944 Turbo Cabriolet in the first half of the 1991 model year. Exact figures on the number of cars produced for the 1991 model year vary greatly. However, there were somewhere in the neighborhood of 875 cars total produced in 1991*. Of those there were approximately 525 cabriolets produced with 255 exported outside Germany.

     Due to lack of a hard top, additional chassis stiffening was required on the Cabriolet. This along with the electric motor used to raise the top, the Cabriolet is actually 110 pounds heavier than the Coupe. However, the added weight apparently did not significantly affect the car's performance. Also, the windshield was lowered by 2.4 inches and raked back 0.6 inches to the rear to reduce the wind noise with the top down.

     The top is raised by electric motors, but must be latched by hand using two handles located in the cassette storage bin. The car was shod with 17" wheels sporting 225/50 and 245/45 tires. The M030 suspension package, ABS, and limited slip differential were all standard equipment.

Note: Most of the information on the 944 Turbo Cabriolet comes from Jerry Sloniger's article in the October 1991 issue of Excellence. Please read Jerry's article if you'd like to learn more about the Cabriolet.

*  From the 924/944/968 FAQ.

4.0  General Description of the Turbocharger and Boost Control System Operation

4.1  Foreword

     The information in this section is taken from a number of different sources including articles in Panorama, Excellence, the 944 Turbo Workshop Manual, some of my own experience, and different versions of the 944 Turbo Problem FAQ that are being maintained by devoted Porschephiles.

4.2  Turbocharger

     As previously mentioned, the stock 944 Turbo uses a KKK K-26#6 turbocharger for the '86-88 model cars (engine M 44.51) and a K-26#8 turbocharger for the 88½-91 model cars (engine M 44.52). For KKK turbochargers, the size of the compressor housing is designated by the first number (K-26) and the size of the turbine or "hot house" is indicated by the second number (#6 or #8 in this case). As we mentioned before, the larger turbine of the K-26#8 allows it to maintain maximum boost longer than the earlier cars which results in a higher peak torque and horsepower.

     The turbocharger is supplied from the car's exhaust via an insulated two-to-one crossover pipe. The exhaust headers are also insulated to prevent the exhaust's energy from being lost before it reaches the turbine. On the compressor side the air inlet comes from a plastic pipe connected to a pocket in the left fender well, through the air filter housing, through a "barn door" type air flow sensor, and a rubber inlet plenum boot to the turbocharger compressor. The compressor discharge goes to an air-to-air intercooler which is mounted underneath the nose panel between the headlights and back to the throttle body at the intake manifold.

4.2  Wastegate

Wastegate      The wastegate on a turbocharged car performs two functions. First, it limits the speed of the turbocharger to keep it from destroying itself. Second, it limits boost pressure to prevent damage to the engine. How does it do this? When the wastegate opens, it bypass part of the exhaust flow around the turbocharger turbine, limiting the speed of the turbine and the amount of boost it can produce. On some cars, the wastegate is built into the turbine casing on the turbocharger. However, the 944 Turbo uses an external wastegate. The wastegate is attached to a collector pipe which taps off the crossover pipe from the exhaust headers to the turbo. The wastegate discharge line taps into the exhaust just downstream of the catalytic converter. The wastegate discharge line also contains a small catalytic converter.

     The valve portion of the 951 wastegate is almost identical to an intake or exhaust valve on your cylinder head. The valve stem is threaded into a spring-loaded diaphragm that sits on top of the wastegate body. The spring pressure of the diaphragm pulls upward on the valve stem to hold it against its closed seat. Exhaust pressure enters the side of the wastegate body (on the stem side of the valve) and acts to try and force the valve off its closed seat. The top of the diaphragm assembly also contains a chamber which has a pressure line attached to a solenoid boost control valve which is referred to as the cycling valve.

4.3  Cycling Valve

Cycling Valve 1     The cycling valve on the 951 is located underneath the intake manifold. It has three ports. One port is connected to the inlet side of the turbocharger between the air flow meter and the turbocharger, one to the discharge of the turbocharger via a banjo bolt on the discharge pipe to the intercooler, and the aforementioned line going to the wastegate diaphragm. Normally, the cycling valve is "open" which allows the pressure from the turbocharger discharge to be bled back to the turbocharger inlet. This is possible because the port on the cycling valve coming from the turbocharger discharge has an orifice in it. When the cycling valve moves toward the "closed" position, it is closing off the port to the turbocharger inlet. This allows pressure from the turbocharger outlet to pass through the cycling valve to the line going to the wastegate diaphragm. This causes pressure to build up on top of the wastegate diaphragm. When the combined pressure on top of the diaphragm and exhaust pressure against the valve seat overcome the spring pressure in the diaphragm, the wastegate will open bypassing the exhaust flow around the turbocharger thereby limiting boost pressure.

Cycling Valve 2      The cycling valve is controlled by the KLR Unit (Knock Regulator Unit). The KLR receives signals both directly and from the DME Control Unit to determine how the cycling valve should be controlled. The chips in the KLR have a boost limit which comes preset from the factory. When the actual boost reaches the computers preset limit, the KLR sends a signal to the cycling valve to close which allows exhaust pressure to open the wastegate. If the cycling valve fails, loses power, or a problem occurs in the KLR unit, the cycling valve will fail fully closed. In this mode boost is limited to approximately 1.2 bar. This is essentially the boost at which the force of the exhaust pressure will overcome the wastegate diaphragm spring pressure.

5.0  Common 944 Turbo Questions

5.1  What does the boost gauge reading on my dash mean?

The 944 Turbo boost gauge reads the absolute pressure on the car's intake manifold. That means that the gauge not only reads boost pressure, but indicates vacuum in the intake manifold when the car is "off boost". So, when the gauge reads less than 1 bar, there is a vacuum in the intake manifold and the turbocharger isn't providing any forced flow to the cylinders. Above, 1 bar, the turbo is providing positive forced air flow to the engine. By comparison, the 911 Turbo indication is gauge pressure which means that it only indicates when the turbo is "on boost".

5.2  What is the maximum boost I should see on my boost gauge?

This has long been a point of contention among 944 Turbo owners. For early 944 Turbos, I've heard ranges from 1.7 - 1.75 bar while the numbers for the Turbo S cars range from 1.75 - 185 bar. However, the factory shop manual clearly states that the maximum boost for both models is 1.75 bar. "Maximum boost pressure of 1.75 bar (absolute pressure) is reached at approximately 3,000 rpm. At higher engine speeds (engine type M 44.51) the boost pressure drops off again and reaches a boost pressure of 1.52 ±0.03 bar (absolute pressure) at 5,800 rpm. With engine type M 44.52, the boost pressure remains constant at 5,800 rpm (1.75 bar absolute pressure) and drops only after this value has been passed."

M 44.51 1986-1988 Model 944 Turbos
M 44.52 1988.5-1989 Model 944 Turbos

There does seem to be some variance between models, but I don't know that anyone's ever actually proven if the later model cars produce a higher maximum boost. The factory does indicate that there is a 10% tolerance at maximum indicated boost. So, that means that the indicated maximum boost could range anywhere from 1.58 bar to 1.93 bar. Start looking for problems if your maximum boost is consistently less than 1.6 bar.

5.3  I hear a humming noise under the hood after I turn the ignition off. Is that normal?

The noise you hear is the turbocharger cooling pump. You should hear it run every time you turn the ignition off. The pump also works off a temperature sensor mounted in the turbo cooling water outlet line. However, the pump relay has a timer which will run the pump for 30 seconds after the car has been turned off regardless of temperature.

6.0  944 Turbo Problems

6.1  Foreword

     This section of the FAQ deals with problems that 944 Turbo owners have experienced in the past. This information is taken from the 944 Turbo Problems FAQ which was originally developed by Chris Mellor. There were a number of contributors to the FAQ including Jim Dresser, Chris Hanlon, Clark Archer, Stu (?), Farzaan Kassam, Bob Tucker, and many others.

6.2  Turbo Does Not Boost Correctly

Diagnostic Suggestions: Clamp off the line from the cycling valve to the wastegate and drive the car. This will close the wastegate and direct all the exhaust to the exhaust turbine. Now drive the car but do NOT fully depress the throttle. Just go over 3,000 rpm to see if the turbo boost is working. (If it is and you depress the accelerator a lot then the turbo could spin too fast and over-boost the engine causing damage.) If the car then boosts OK it means that the cycling valve and line are where the problem lies. The wastegate is not being controlled properly. This could mean that the cycling valve is being given bad information due to a faulty DME, knock sensor or 'control units' or is faulty itself.

If the car doesn't boost properly then look for:

These checks should point you in the right direction.

Other diagnostic checks:

6.2.1  Mechanical Turbo Problems

Pull off the turbo intake hose and check for freedom of movement in the turbo. The turbo unit itself may be broken. E.g. the compressor wheel could be broken.

6.2.2  Air Intake Problems

Look for a vacuum leak. A vacuum leak will be a boost leak once the turbo spools up.

Suggestion: Disconnect the timing valve. It is located between cylinders 3-4 just under the intake manifold. It has three hoses attached to it and an electrical connector at the top. (The top is white). To check if it works, place a stethoscope (automotive) on the top of the valve with the engine running. Then very quickly throttle the car past 60% and listen for a clicking sound on the valve. It sounds like a very fast "blap/blap/blap". You have to do this very quickly because you do not want to over-rev the engine. The valve is controlled by the KLR.

If the intercooler was full of oil or somehow blocked it could limit the turbo boost.

Note. Leaks in the inlet manifold area should trigger a blink code. However, if they are very small leaks then pressure could leak out but the leak not register via the blink code.

Suggestion: Have the Porsche specialist shop force a fault to make the blink code work. E.g. remove the throttle connector and it should start blinking. IF it does that tells you that the KLR is somewhat working.

Check all inlet piping extremely carefully for a leak. Even the tiniest crack will reduce boost to nothing.

Make sure the intercooler is not split, corroded or cracked.

6.2.3  Wastegate Problems

You have to use pressure to make the wastegate open. With the car idling the by-pass pipe should be cold or just a little warm. Use a hand vacuum/pressure pump to check to see it the wastegate is working.

Hook the vacuum/Pressure pump to the wastegate and apply max .5 bar of pressure. The pressure will open the wastegate and release the hot gases into the by-pass pipe.

6.2.4  Exhaust System Leaks

Check with a hose type stethoscope. Check there are no exhaust leaks from the header. Make sure there are no cracks in the turbo's exhaust housing. If you have a catalytic converter make sure that it is not clogged.

6.2.5  Other Mechanical Problems

A kink in the throttle cable can stop the turbo from spooling properly. One car car produced 2 bar at full throttle in first gear, but 2nd through 5th would not produce anything. This was because the throttle cable was kinked. The fault also seemed to get worse as the car warms up.

A faulty throttle switch can stop the turbo spooling properly. There could be a bad connection due to corrosion on the throttle position switch (located on the throttle body, forward of the intake manifold). On one side is a cable that controls the throttle opening, and on the other side is a black plastic switch that reads the throttle position. The connection to this device can become corroded. Typically moving around the connector a bit will fix the problem temporarily. Disassembling the connector, cleaning and putting on a coat of dielectric grease on it will fix the problem semi-permanently.

The timing belt may have slipped a tooth and the engine is thus retarded.

An oil trap leak can drop the boost.

Check the cycling valve connections. If they are corroded or broken off, the car will only register 0.2bar (1.2bar absolute) and will give no error codes.

6.2.6 Engine Computer Problems

The 944 Turbo engine is controlled by a Bosch Motronic DME unit. It links to a KLR turbo control unit. If either of these units are faulty then the turbo may not behave correctly. A Bosch diagnostic unit can be connected to the DME box and interrogate it. If the DME has detected a fault it may return a fault code to the Bosch diagnostic system which describes the fault when it is decoded.

A DME unit can perform incorrectly due to broken solder joints and/or broken/bent pins in the plug which connects it to the rest of the engine and its sensors.

A new Bosch Motronic unit is c £1,100 in the UK ($1,000 in the USA?). A re-conditioned unit is c £400. There are service operations, such as Network 500 Ltd in the UK, which diagnose and recondition engine brains.

6.3  More Pronounced Turbo Lag.

Check the vacuum pipe which connects to the top of the air recirculating valve. It is also referred to as the blow-off or pop-off valve. A failure in the valve can cause this pipe to be blown off the valve. The vacuum connector then does not do its work. The valve's function is to prevent a sudden slowdown of the turbine compressor wheel when the throttle is let go. If you then re-accelerate you would have to wait for the compressor wheel to spool up to high speed again - turbo lag. When the throttle is closed (accelerator pedal released) a vacuum occurs in the inlet manifold. This vacuum is detected by the air recirculating valve via the vacuum pipe. This valve opens and causes the compressed air coming out of the turbo to flow back into the turbocharger's air inlet space thus recirculating the compressed air and maintaining turbo compressor wheel speed.

The air recirculating or dump valve is located on top of the large diameter ribbed plastic pipe which goes between the airflow meter unit and the input side of the turbocharger. The vacuum pipe goes between the valve top and a 'T' piece close to the regulator valve on front of the fuel rail.

Testing the valve requires you to remove it. Unfasten two hose clamps, pull off the vacuum pipe and move the valve out and away from the air pipes attached to it. Clean the two air pipe connections on the valve and blow hard down each air connection while feeling for air leaks from the other connections. There should be no leaks. If there are it means that a diaphragm inside the valve has failed. A new valve costs about £20.00.

6.4  Turbo Overboosts

Problem statement: By far the biggest problem (at least so it seams) with my new 951 is that it cuts the fuel VERY abruptly during heavy acceleration. It is almost like someone is hitting the brakes! It usually happens around 4000rpm but will not occur if I just accelerate "normally". Also, my boost gauge is stuck at one when the engine is on (it returns to 0 when engine is turned off). I believe I can hear the turbo spinning during the acceleration so somehow the boost gauge must be connected wrong?

Second Problem statement: The car idles and runs fine, as long as you don't get on the boost hard. If you floor it, the boost gauge goes up to about 1.8, then, a couple of seconds later, the car jerks hard, and the boost goes away (fuel shut off??). I believe that either the cycling valve or the wastegate isn't working. Is there any way to check out which of these 2 items isn't working? Does anyone know of better designed parts to replace these?

Diagnosis 1: This is an overboost situation. One of the most common causes of overboost is the pressure line leading from the wastegate. A crack here will cause overboost for sure the DME then shuts off the fuel. This hose is rubber & can dry rot over time.

Diagnosis 2: You have a broken vacuum/boost line (hose) between the intake manifold between cylinders 3 and 4 and the KLR (Turbo control computer) box in the passenger footwell, or your KLR box is broken. If this hose is somehow cut or is leaking, the KLR will be reading atmospheric pressure, which will result in no real power for you. The KLR doesn't know how much boost, so goes into overboost protection mode (which is the abrupt cutoff of boost.)

Diagnosis 3: One possibility is that the connection to your air flow meter needs to be cleaned. The next is your wastegate is stuck. The next is that you have a leak in your lines going to the wastegate so that it doesn't measure the proper boost from the turbo and keeps letting boost build. Result: your DME/KLR will shut down the fuel so you don't blow your head gasket and you fly through the windshield. Inspect your hoses to and from the wastegate and cycling valve.

6.5  Turbo Pump/Fan Failure

Problem statement: I have a 1988 turbo S and I haven't heard the turbo water pump come on for some time. Temperature on the engine stays on the cool end when running (2nd mark) and goes up to the 3rd mark after the engine is shut off (after the fans have come on). Is the turbo water pump supposed to run for 30 seconds or so, or is it temperature regulated? How should it act?

Diagnosis 1: It should come on period. If it does not you are shortening the life of your turbo. Check it directly by disconnecting the wires and jumping it with a hot it should come on. If not check the relay(G19 on an '86 944 Turbo). If these things do not help then replace the part. Your turbo needs it. I replaced mine ($100) along with all the hoses leading to it. It took the better part of a Saturday but was well worth it.

Diagnosis 2: The turbo water pump should run for ~30 seconds after shut off independent of coolant temp. It is possible that the pump is binding and that loosening the hoses and adjusting things will free it up. Check the 944 FAQ for more on that subject.

6.6  Fitting A Boost Gauge.

Statement: I want to install a VDO boost gauge in my 951. I have it already mounted in the console and now need to do the vacuum line connection.

Response 1: There is a banjo bolt on the intake manifold just above and slightly forward of the cycling valve. The cycling valve is between cylinders 3 and 4, under the intake manifold (round, cream colored about the size of a ½ dollar). This banjo bolt has a vacuum line leading from it, with a T or Y connector (3 way) in the line. I replaced that Y connector with an X connector (4 way) and simply hooked the VDO gauge plumbing to that connector, and reconnected the other vacuum lines to it. You should be able to find an X connector of the proper size at (in the USA) Western Auto or other parts house. You will have to get flexible tubing to connect the hard tubing from the gauge to the X connector.

Response 2: The boost gauge can be hooked up by using a "T" connector to tap into the vacuum line from the intake manifold to the KLR control unit. This nice thing about connecting here is that it minimizes the length of the vacuum line required for the hook up and you don't have to worry about running the vacuum line through the firewall. With the vacuum line on the 944 Turbo, you can use a 1/4" compression fitting. However, it will be a tight fit an you'll have to work at it to get the compression nut and ferrule on the tubing. Most aftermarket boost gauges can be purchased with a hook up kit. They can come with varying sizes of tubing, but most I've seen come with 1/8" hard plastic tubing and usually come with a compression fitting. Be careful when tighten the compression nut for the 1/8" tubing because the 1/8" compression ferrules crush very easily.

6.7  Turbo Pump Stays On When Engine Running

Statement: I cleaned the '87 951 engine compartment today with a pressure hose and as usual something goes awry. The turbo pump now stays on after the car is started. If the relay (G22) is removed (car is not running) the pump will stop after the relay is re-inserted. However the next time the car is started the pump will run on again. Disconnecting and reconnecting the two pin connector has no effect - other than stopping and starting the pump. BTW it is only about 15 minutes after the wash and I am hoping the problem will disappear after everything has dried. I hope the Porsche Gods are listening (compassionately - yeah!).

I have done the following with no effect:

Diagnosis: Sounds like you damaged the thermo switch for the pump. It is mounted into the turbo water pipe, look for a single wire connector with a black Bakelite covering. If you disconnect this switch and the turbo water pump does not immediately start when you turn on the ignition (without starting the engine), then that is your culprit.

6.8  Random Boost Fall Problem.

Statement: I own a 1986 944 Turbo that I bought with 71K miles. Shortly after buying the car I began experiencing problems with the turbo boosting. It will happen randomly and always at a bad time. The problem consists of the turbo only boosting to 1.2 bar rather than the usual 1.8 bar. This becomes extremely annoying while trying to pass on a country road. The problem cures itself when the engine is shut off and immediately started again. I have tried shutting it down and starting several times while coasting into toll booths and I have never had a problem after the restart. About seven months ago I took the car to a Porsche Dealer. The mechanics worked on the car for two hours, which included a few calls to other dealerships. Unfortunately, they could not find the problem. Also has a rough cold idle.

Diagnosis 1: To really nail this down, one would want to do a complete checkout of your exhaust system, consider removing the turbocharger and re-gasket it, and evaluate the waste gate. Putting it on a dyno could help, too, because you could evaluate the exhaust gases at the same time. One reason for the loss of boost can be lack of fuel - leaning - which would be picked up by the sniffer. If fuel were the problem, it could be related to failing fuel pressure regulators on the fuel rail, or a vacuum leak which would compromise the effectiveness of the fuel enrichment system. The frustrating part of this sort of thing is that it is still intermittent, and probably did not show itself to the shop you took it to.

Diagnosis 2: Your problem could be a bad throttle position sensor. The fact that the problem goes away after restart would tend to confirm it. My 87 951 had the same problem early in its life.

Solution: The problem turned out to be the KLR chip.

6.9  Excessive Heat From Intake Manifold

Statement: I have a technical problem. It's a 951 '86, and I just removed the turbo and replaced all seals in the area. Everything runs like a charm, great power and smooth idle. But, I notice rich running and EXCESSIVE heat coming from the intake manifold. At first, I said it was the turbo heating up. But the water temp is not at all high, in fact it just barely simmers at ½ way on the gauge as usual. The intake manifold is so hot it can't be touched after is stops running. The DME is running the mixture rich to cool things down. But what is causing the heat?

Diagnosis 1: Sounds like you damaged the thermo switch for the pump. It is mounted into the turbo water pipe, look for a single wire connector with a black Bakelite covering. If you disconnect this switch and the turbo water pump does not immediately start when you turn on the ignition (without starting the engine), then that is your culprit.

6.10  Turbo Oil Leak.

Statement: My '86 944T produces an oily smell from the exhaust when accelerating moderately-hard. (Although no smoke is visible). There is no smoke/smell at start-up but the first time the engine is revved there is some oil smoke which clears quickly. (This only happens when the car has stood over-night). I suspect oil is getting into the engine from the turbo as the air pipes leading from the turbo charger to intercooler to inlet manifold have considerable oil deposits in them. Puddles of oil are evident at the low points along the route, e.g. at connections in/out on intercooler and at throttle body. If I clean out the pipes the smell goes away for a day but then returns, together with the oil.

The oil is not coming from the breather pipe as I temporarily disconnected it and plugged the hole left in the intake. Leak-down test has shown that all is well with cylinders. The valve guides/seals have recently been replaced during top-end overhaul - it made no difference. The problem has slowly got worse over the past 18 months. I suspect turbo bearing/seal.


6.11  Cycling Valve Not Working Properly.

Statement: The cycling valve on my 951 is not working properly. The valve is ok (tested), but it never gets any signal from the computer. The valve should open when the air flap in the air flow sensor swings +60=B0. Since no signal is coming from the computer (wiring and pressure line seem ok), I was wondering what the origin of this problem can be. Apart from a fault in the computer, which sensor inputs can result in no signal to the cycling valve (knock sensor, etc...)?


6.12  Turbo Oil Consumption.

Statement: I have found that the more I use my turbo, the more oil I consume, by a big margin. When I first got the car, I drove very conservative and used/lost about ½ qt in 1500 miles or 1 in 3000. When pushing the car a lot, I find that my consumption has been as much as 1 qt in 600 miles. Is this common, or what may be my problem? The car is an 86 turbo with 115k on it, 150 compression across all 4 cylinders. Over boosts also.

6.13  Oil In The Intercooler.

Statement: Should there or should there not be oil in the tubes going to and from the intercooler? There are no puddles just a film of oil on the inside surfaces. Is this normal or an indication that the turbo seals are going bad. This 86 951 has only 50K original miles and I hope a turbo rebuild isn't required just yet.

Answer: It is normal for there to be a light film of oil there. Over time the oil will also coat the intercooler, reducing intercooler efficiency. You will want to want to clean the intercooler out from time to time as the build-up increases.

6.14  Boost Rises Then Falls

Statement: I have a weird problem with my '86 951. As I depress the gas pedal, I see the boost rise, but just when it should give me that power kick, I actually experience a power loss - it feels as though I had downshifted. My current theory is that the boost is somehow not available at the intake manifold, but the onboard computer thinks it is and begins to inject more fuel into the air stream - causing a power loss because the air/fuel ratio gets messed up. But I really fail to see how it is that the boost is not making it to the intake.

Diagnosis: There are several things you can try to fix this:

  1. Unplug the air meter box connector and clean the connector off.
  2. Check for unplugged vacuum hoses
  3. Check the idle speed screw, see if you have one (I don't know if they fall out or just get forgotten)

What is happening I think, and this seems to be a common problem, is that the computer thinks you have an overboost situation and is shutting the fuel down. Now, you may have a real problem. The wastegate may be stuck, the timing valve may not have power etc.., so if you can't find the fix take it to a trustworthy mechanic. It took me a year and several mechanics to actually fix my problem. In the meantime, one had me replace the computers, one told me my wastegate was stuck, "bla bla bla".

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