Last Lemming

Sorry if this has been ruled out but is your coil good?

Texbuckeye

coil was replaced

Texbuckeye

Update: I drove over to Wal-Mart and it drove fine on the drive over. It's about 5 minutes away. When I started it 10 minutes later it was doing it again. The car never got hot. So either it is getting worse or it is not temp related.

Texbuckeye

New alternator and temp II sensor did not fix it. Anybody have AFM lying around?

Tom M'Guinn

Put a volt meter on Pin 7 of the dme while it is running and check if the voltage goes whacky above 2k rpms. Should be somewhere in the .5-.9v range at idle, and you can watch the voltage go up a tad as you slowly rev it. If the voltage jumps around or goes haywire right when it starts to misfire, then AFM is probably the culprit. If voltage seems linear/smooth as revs climb, then I would pop open the DME and KLR and inspect closely for solder joint cracks.

Did the alternator help the voltage problem, or is it still low on volts. If still low, check the draw in case something in the engine management (or elsewhere) is shorting out or over-drawing.

EDIT: Oh, and humor us all by cleaning the bell-housing ground wires. Those things can get nasty and cause endless variations of problems. See pictures in the link below.

Texbuckeye

Tom,

The new alternator did fix the voltage problem. It is putting out 13.5 now.

http://www.the944.com/afm.htm

Does this link apply to my 86 951 AFM?

Sean loosened the bell housing ground wires pictured on your site, he then re tightened them. I was there when he did it and they didn't look corroded to me. I didn't see any blue copper corrosion like I see in your picture. Those buggers are a pain in the back side to get to.

We also noted that the either the DME or the KLR got hot when the car was driven indicating resistance from somewhere. Sean will know which one it was.

Nice garage you have there.

txhokie4life

Could also be faulty or overheating voltage regulators or fuel injector drivers.

If you were in Austin, I'd let you swap components to test.

m

Texbuckeye

Update

We are making progress. Sean swapped the AFM out with one borrowed from GARG ie. Gary. It is behaving differently but still doing it. It started hesitating and stumbling after heating up from driving in residential neighborhood. It started doing it again but then when it cooled down it stopped. When the car heated up again from driving around in residential neighborhood again it did it again and the second time it took longer to cool down and then it stopped doing it again. We turned it off for a couple of minutes and restarted it was still doing it but when we drove at 50MPH at around 2000RPm for 15 minutes it cooled off again and stopped doing it.

I think Gary is going to let me borrow the DME and KLR from his 88 Turbo S. Hopefully swapping out that will tell us if it is one of those.

Texbuckeye

Update:

Swapped out the DME still did it. Swapped out the KLR and it still did it. With the swapped out parts it doesn't start misfiring and hesitating until it gets to 3000 RPM's. They came off of a 88TS.

What else can be the problem?

Tom M'Guinn

I'd be inclined to monitor the key DME inputs on a scope (AFM, IDC, Coil, TPS, Speed/ref sensors) when the problem is occurring, and check the fuel pressure when the problem is occurring. Fuel flow could be compromised, or the engine management harness might have a short (or frayed wires under rubber covers) or one of the components you thought you tested might only show signs of failure when the motor is missing. Look over post 15 also and make sure you've looked at all the things listed there -- including every last piece in the ignition system. I've seen lots of folks post up challenging problems like this over the years, and as often as not the problem turns out to be something previously "ruled out" for the wrong reasons. Also check for blink codes in the diagnostic port -- doesn't sound like one of those issues, but too easy to check not to try it.

shortyboy

I had a similar problem on my 2.8L. I was running a stock fuel rail with VR maf, smt6 and wasted spark kit. Everything was running fine until I installed the CEP bullet rail. I was running the rail with a bosch low profile fuel damper that came with the sale. I've tried running countless days but it would miss fire or hesitate about 2k sat full WOT. After While I swapped out the bosch unit for the CEP unit and no more hesitation and missing. Try and checking your fuel system including the injectors, damper, regulator and pump.

Texbuckeye

Fixed!
SeanR put a fuel pressure guage on and it confirmed low fuel pressure when hot and misfiring and stumbling. The Fuel pump and fuel filter were replaced and it is fixed! Car pulls stronger and feels more peppy. The fuel filter was last replaced in 1996. So if you have the same problem check out the fuel filter and see if it is 17 years old.

Thank you guys for all your help. Thank you to Gary for letting me borrow the AFM, DME and KLR. Now I just have to talk my wife into letting me keep it.

Tom M'Guinn

Great news -- congrats!

You might now confirm that you have healthy voltage at the pump, in case those 25 year old wires were part of the issue (they are not Porsche's most "over-engineered" part of the car...). The 17 y/o filter was probably putting extra load on the motor, and when combined with low voltage from the electrical issue you were having, caused the pump to overheat, burn windings, etc. Testing the old and new pumps side by side with a multimeter would probably confirm that.

Texbuckeye

Bad news it is not fixed. Seems to go longer before it happens but it started after a 45 minute drive twice now.
What else could it be.

jmj951

Is it still giving you low fuel pressure when the problem starts? If so, did you get a chance to check the fuel pump wiring at the pump when it starts to stumble?