PRBob

Having trouble with my 87 924S after going through just about everything that I can think of. It is idling very roughly and high. Frequently missing and occasionally backfiring. It will sputter and die if I don't keep my foot on the gas until it warms up. Here is a video: Sometimes, it will die when trying to idle. Then occasionally, white "mist" (not really smoke as far as I can tell) comes out of the intake very slowly and dissipates very slowly. Looks like coolant to me?

It was running fine in 2013, then one day would not start. I cleaned starter and ground contacts, and found that fuel pump was dead. Replaced it, fuel filter, and the fuel lines with push-lock hoses and AN fittings and then got it to fire up but it immediately ran approximately the way that it does now. Got a fuel pressure gauge and fuel pressure seems to be at spec. Replaced spark plugs, wires, cap and rotor, speed and reference sensors, throttle position switch, adjusted MAF to spec, checked for all vacuum leaks and replaced all hoses. Replaced the ECU with a rebuilt one.

Brought the car to a Porsche specialist and they supposedly tested everything but could not figure it out without opening up the engine. To them, it sounded like a broken valve spring. So I began to tear it apart myself. There was no broken valve spring. Had the head professionally rebuilt for good measure. Replaced gaskets, timing belt, balance belt, water pump and all rollers/bearings. Verified timing is correct per marks. (have not checked with a timing light but it should be ok?). Cleaned all grounds under hood and chassis. Tested ignition switch and new fuel pump relay. Compression was consistent (average 185? iirc) within 5 psi of each other on all cylinders. Tried BG-44K fuel additive and 93 octane fuel. I am sure that I have tested other things that I am not thinking of right now.

Any idea what else I should try?

mel_t_vin

Sounds like a mixture issue.

All large/small vacuum hoses were replaced? J-boot, as well? If not, look at the underside for cracks between the ribs.

Appears you've replaced most of the sensors under the hood, but did not see the fuel pressure regulator or DME coolant temp sensor mentioned. If you pull the vacuum line off the FPR, can you see or smell gasoline?

968workaholic

+1 for the coolant temp sensor

PRBob

Thanks, I just retested the DME temp sensor per Clark's garage and everything is ok. There are no leaks from J-boot and I replaced all the hoses other than the J-boot, large and small. Pulled the line from FPR and didn't see/smell anything. Rechecked vacuum hose connections. Everything is snug. Retested speed/reference sensor outputs at DME plug and they are at spec. Disconnected O2 sensor plug and it runs rougher (new O2 sensor a year back). Any ideas?

PRBob

The tach just stopped working too

Auto_Werks 3.6

I think your tach wasn't working correctly in the video. It seemed to idle a bit high, but not 2500 rpm high. Also the oil pressure warning light is on????? Triple check your electrical things. My NA runs about like yours does for several seconds at startup; no power, poor response to throttle input, then it clears up. On my car it's a direct correlation to installing some funky junk yard fuel injectors. I didn't notice you having the injectors serviced anywhere, so that's a possibility

Auto_Werks 3.6

I think witch hunter can clean and test for fairly cheap

PRBob

I had them serviced at witch hunter and I have since rechecked myself by removing the fuel rail. They spray evenly.

The RPMs were high in the video because I had my foot on the pedal. When idling, it ranges from about 700-1400 RPMs going up and down frequently, sometimes more frequently than others but it seems random.

I agree that it may be an electrical gremlin. Not sure what else to check at this point, though. I visually inspected wiring and the harness and don't see anything funky. I will keep on looking until I do. Any ideas of what else to check?

curtisr

I'm probably way off the mark but I once accidentally disconnected number 7 in the picture enough to have a problem similar to yours. That is, the car wouldn't hold an idle unless I kept my foot on the gas.

PRBob

Thanks curtisr, I have all of those vacuum pieces in place and clamped gUdentiten

mel_t_vin

Have you swapped out your AFM for a known working unit, or opened yours up to ensure the wiper arm has a clean track?

PRBob

I opened up my AFM and it looked clean and intact. I also tested it electronically per Clark's Garage and everything was perfect.

PRBob

Does anyone think it might be worth it to get a new coil even though the old one's resistances are ok? I have heard that sometimes they are still bad regardless of resistance values.