mstrfishy

Hey there guys, I've been combing google for weeks trying to find a solution for my 87 928 S4. The car tries to fire when you turn the key after it turns over a few times, but wont run unless you hold the gas down very gently. After it runs for 5 or 10 mins it idles fine (if a bit lopey) but only with the car in neutral. It will stall in gear at a stoplight.

I have an old Volvo that uses lh2.4 so I have a basic familiarity with the setup. If the IAC was broken one would think the idle would not be hunting but simply not there. We have a new fuel pump. I've heard people say the crank pos sensor goes bad and can cause this issue. I've also heard people replace the relays for efi and ignition to some effect.

Questions #2, the rad fans come on super frequently and run for 5 to 10 seconds then turn off. The lights flicker when the fans turn on making it look like im flicking my high beams on and off to the people in front of me. I hear a relay click when the fans turn on but ALL the fusebox and relay diagrams for 87 say I don't have a relay for the rad blower fans. There is no fan relay AT ALL, just the one for the automatic radiator vent louvers. What is up with this?

I appreciate your help guys! We've had it to 5 Porsche specialists and they have come up dry. One of them broke the fuel tank and charged us for it...

John Speake

If the idle speed s hunting then it suggests the LH ECU and ISV are trying to stabilise the idle at 675rpm when hot.

I would start by measuring intake vacuum at the vac port on the fuel damper at the front of the engine, you looking for around 21"Hg at idle speed. Let us know how that test goes.

Did any of your Porsche specialists put a diagnostic tester on the car ?

mstrfishy

They claim they did but I wouldn't be surprised if they didn't. I'm going to check that today thanks for the idea I'll report back.

fraggle

Low voltage can bugger up CPU operation. I would clean all chassis grounds as a first step.

mstrfishy

On that note, it was having a grounding problem when I first got it. Could the computer still be getting lower than normal voltage if the gauge reads nominal all the time??

stuartbrs

I had very similar problems with my 87 S4 manual. In my case replacing the ECU with a JDS unit cured the problem immediately. I then tried one of the upgraded ECUs and the car ran even better!

mstrfishy

Hey there lovely Porsche people! I got some time to do a little troubleshooting yesterday. Turns out it was the ground strap for the battery the whole time, Google was right. Thing fired right up after replacing even though the old cable LOOKED good. It was actually rusted from the inside out.

Thank you guys for the snappy intelligent responses!