Rough running

[Quarterflash]

My car is an '89 turbo with nearly 160,000 on the clock. When it runs ok the performance is sparkling and faultless.
This is a most frustrating problem. It has similarities to other problems in that it goes away the minute it enters the Porsche garage for fixing. I have had the problem on and off for about 3 years.

The problem is one of rough running. So I can usually start the car and drive anywhere without any problems but if I stop for some reason and then subsequently start it again the engine goes very rough - as though the choke is out, if there were one. Most recently when this occurred the car got me about 4 miles and then wouldn't go above 2000 revs. The usual way to correct this phenomenon is to let the engine cool for 5, 10, 15 or 30 minutes and then it will be fine and continue as though nothing had happened.

The car has just had a full service with replacement distributor cap and rotor, a length of return fuel line replaced and all the other usual stuff. Previously we had tried replacing the throttle body/valve and MAF meter unit to no avail. It appears to come and go for no apparent reason.

It will be going back to the garage (Pro-9) for diagnostics and further investigation.

All comments would be appreciated.

 

[DivineE]

Sounds like a good mystery you have there. Could it be a faulty temperature sensor (the ones at the front of the block under the intake manifold)? I don't know why anything would have such a drastic effect but something used for the cold start that always thinks the engine is cold when its started may have an effect.

Other than that maybe something clogging the fuel filter? Has that been checked? Debris in the fuel filter can do some funny things around starting/stopping. Once had something a little like this on a golf.

 

[AndrewS]

Check the AFM (Air Flow Meter) connection. A poor AFM connection will cause those syptoms.

Regards,
Andrew

 

[alwigley]

Could well be the DME temp sensor - try disconnecting it when you get the rough running and see if it stabilises. Had some very odd problems with mine and turned out to be nothing more than a £15 temp sensor..... took ages to figure out.

 

[barks944]

http://www.clarks-garage.com/shop-manual/elect-19.htm

 

[Quarterflash]

Hi
Thanks everyone for your posts. At the moment I am leaning towards the cold start injector - maybe not directly but indirectly picking up an incorrect input. When the car is cold it would work perfectly and see the engine through warm up but if it were still thinking the engine was cold when it was hot then it would give the symptoms that I am experiencing. Of course when the engine cools it would perform its job correctly again.
A replacement fuel filter was fitted in the service.
ThanksQ

 

[vince944red]

I had weird problems of misfiring and rev limiting until I decided to clean the injectors. Once I did that I noticed the injcetor electrical connections were a bit iffy and lost their clips and barely holding on - making poor contact. Once I changed the connectors all was 100%, cleaning the injectors defintly helped but the poor connection was random explaining the coming and going of the problems as the engine warmed or vibration shook the connector.

 

[peanut]

I'd second Devine's advice about the engine temperature sensor . You're right about the effect of having the cold start injector working when the car is hot although you should be able to smell overfueling .

One other possibility is is an intermittant sticking of your FPR valve which could cause erratic fuel pressure in the fuel system. You could fit a temporary fuel pressure gauge and observe when the problem next occurs.

I second the suggestion to clean and test all 5x injectors. There is a mail order specialist in Devon called Tytherleigh

I thoroughly agree with Vince's suggestion too. My injector connectors are very brittle from age and over-heating. There are some new injector connector repair kits on ebay currently and they are cheap.