Last year I bought a beautiful black 1992 C4, with just 96.000km. The car had one previous owner and was maintained by one OPC for all ten years.

After a month one problem developed:
whenever I start the engine after it has been off between 5 en 15 minutes, it starts to run very rough and won't accept throttle, as soon as I try to push the throttle, it dies. Then after some 30 seconds or so, the revs suddenly go up to 1200/1300 and then after a few seconds to the normal 900. Its is then possible to drive away like normal. My independent Porsche specialist has checked DMe realy and engine temp sensor. Bosch Hammer does not report any problems with Lambda sensor.
Does anyone recognise this problem?

 

Dear Bert,
This is not an uncommon issue especially if the outside air temp is hot and humid. The engine heat soaks (gets hotter) in this period of time. The result is that the fuel in the rail tends to vapourise. You need to practice (only in these situations) holding the accelerator pedal in the best position to help the start. Do not pump the pedal. I have seen this so many times on 964s including my own that I worked a way around it because you cannot fix these types of things. All I can advise is not to start up in this 5 to 15 minute period and wait until the weather cools and see if you still have the problem. A DME reset can also help because you adapt the idle to the current air density (roughly).
Ciao,
Adrian
964C4

PS: On my C4 this happens very rarely but if it is going to happen it is always in the mentioned time frame.

 

Hi Adrian, thanks for your quick reply. The problem I described is also there in wintertime, regardless of the weather conditions. Starting the engine is not a problem, you just have to wait for approx 30 sec or so before the engine will accept opening the throttle. It seems like the engine is running on a very lean mixture all this time, and then suddenly, as if a relay closes, the revs to up to 1200/1300 for a few seconds and then back to the normal 900. Engine then runs smoothly and no problems to drive away. Could it have to do with the fuelpump or maybe one or two leaking injectors?

 

Dear Bert,
I would recommend a DME reset first and foremost. After that I would be looking at the idle speed control valve and the idle microswitch. Hunting idle can mean fluctuating airflow or fuel. The fact that you say it only happens when the engine is hot could also suggest a problem with airflow sensor potentiometer/vane assembly.
Leaking injectors would cause you problems all the time and you would also smell the unburnt fuel. I doubt that the problem is the fuel pump. You may have a bad fuel pressure regulator I suppose but lets start cheap and work towards exspensive. The fact that you say it has been diagnosed using a Hammer and nothing shows up tends to suggest it is a mechanical problem rather than an electronic/control problem.
Ciao,
Adrian
964C4

 

Dear Adrian, tonight I did as you said. Took the car out for a half hour drive to warm up, then disconnected the battery, meanwhile I fixed a faulty HVAC servo (using the guidelines on www.porsche 964.co.uk/technical/servos.htm)
Then reconnected the battery, started the engine and let it idle for 3 minutes, took it out for an half hour drive again.
Then switched off the engine and to my disappointment, when I started after 5 minutes or so, it appeared that nothing has changed. So, what do you recommend to be the next step?

 

Dear Bert,
My next step would be to check the idle microswitch on the throttle quadrant for correct operation and adjustment, check the cable tensions when the engine is hot (this means check if the throttle quadrant is sloppy) and then I would advise that you check the idle speed control valve. This valve sits right in the middle of the engine. You cannot miss it. You have a 1992 model so the ISV will have a muffler installed. Black round sealed at one end tube sticking out. Check the muffler is not damaged. Remove the valve and clean it. The valve may actually be failed but we have found recently that cleaning sometimes works. The next step after this is to check your reasonance valve. Make sure it is not stuck open but we will get to that later. When you drive do you often exceed 5000 rpm?
Ciao,
Adrian
964C4

 

Adrian, I have checked all the things that you mentioned. My independent let me play with the Bosch hammer last week and I was able to check all sensors and actuators, and they are ok, incl idle switch etc.
Yesterday I discovered that the problem also occurs wwit a cold engine. Not the first cold start, but after drving the car to the fuelstation 200yards down the road, the same thing happened after the car had been standing there for 4 or 5 minutes with engine off.
Could it be a problem with fuelpressure?

 

Dear Bert,
Then obviously the problem is something that the DME does not monitor. My first guess would be an inspection for leaks both air and vaccum. I would inspect all around the air intake area. I have heard of underside cracks. Do you have the headlight adjuster system? If you do check that it still works. This runs off engine vacuum and if it leaks will cause problems.
A harness problem would throw up faults on the hammer so I will discount that at the moment.
Fuel pressure or a failed fuel pressure regulator are not monitored by the DME nor is the fuel pump. The only way to check these systems is to hook up pressure guages to the test ports.
If I was going to guess in the fuel system I would start with the fuel pump not generating enough fuel pressure.
What about the fuel filter, when was this last changed? I would expect constant problems with a fuel filter but it is still worth mentioning.
I do not know where you would prefer to start but I would recommend a leak check of the manifold and fuel pressure test.
Ciao,
Adrian
964C4

 

Bert,

You should check the residual fuel pressure also. Make sure that fuel pressure is retained when the engine is shut off. A bad fuel pressure regulator or a bad fuel pump check valve will cause excessive residual pressure loss which will cause hot start problems. Your mechanic should know how to test this.

-doug

 

Just returned from a three week holiday in the France Alps! Grtea time with the 964!!
The problem has not gone away (as expected) so today I went to my independent. He suggested to try yo disconnect the engine temp sensor before checking fuel pressure. The effect is that the DME detects a broken sensor link and from then on assumes a temperature value of 120 oC (approx 248F). After disconneting the sensor the cold start was bad, as exepected as the DME now does not entich the mixture for the cold engine. After warming up the engine ran as normal and when switched off and restarted after appreox 5 minutes the problem was not there anymore!!!
Apparently the sensor is faulty but still gives values to the DME that lt within the range it expects. Hence no error is logged by the DME!!
Tomorrow I am going to make an appointment for having the sensor replaced.

 

Bert,

Porsche uses Motronic on the 964, Motronic is used in a lot of cars, so I would check the 'Bosch Dienst' .

When the sensors are o.k., fuel pressure is o.k. [you can give it a higher pressure by make the 'return fuel line' flat with a clamp or so. The return fuel will not go back to the tank, so the pressure will rise, may be this will help you check out if the pressure fails.

Are you sure nobody messed around with the spring from the air flow meter?

Is you valve adjusment correctly? Are your sparks o.k.? Is your distributor [ignition] o.k.? Do you have enough spark power coming from the coils?
You have to check all those Items first.

May be a short call to Lammertink Porsche will give you a answer, call 0547-384233 and ask for Edwin Lammertink, Torens of Ad. Al super technicians who work all day long on 911's. I am sure they will help where they can.

 

Last night I have replaced the faulty engine temperature sensor, and the problem has gone!!!!
Lesson learned are these:
DME can't detect a faulty sensor unless the wire(s) are broken
Checking a temperature sensor at one temperature (in this case 20oC) means nothing. My sensor went wrong at very high temperatures only.
Anyway, thanks for all the help and good advice, especially Adrian!!

 

Bert:

How sure are you about the statement "DME can't detect a faulty sensor unless the wire(s) are broken"? I'm not trying to be a smart-***, but I was under the impression that that the DME was capable of detecting and reporting sensor faults that are out of spec. Is it possible your sensor was operating on one of the threshold limits of the sensors, or does the DME identify different failure conditions for different sensors (for example, out of spec might apply to an O2 sensor, but broken wires might apply to the temp sensor)?

I'm curious about this because, as you stated in your previous post, you indicated that "My sensor went wrong at very high temperatures only"..

Anyone know the answer to these questions?

 

Maybe the sensor gave wrong values with hot engine but still within the "normal" range. Suppose the sensor indicates 0oC at a real engine temperature of 150oC, the mixture would be far too rich and as a result the engine would not run very well.
As soon as I find the time to do it, I will heat up the old sensor in a bath of oil with an Ohm-meter connected and see what is going wrong.

 

Dear Bill,
The out of spec fault monitoring was removed from the 964 series DMEs. Only the earlier 911 series DMEs had this feature. The reason it was removed was because it fired up so many false fault codes. Here is a case where it would have given a correct answer.
Ciao,
Adrian
964C4