MarkD

The flywheel sensor can cause erratic behavior
For the relatively little time and money, I would eliminate it as a possibility

Testing it requires an o-scope
Doesn't always throw an error code or show up on durametric.
It can wreak havoc with ignition...

Good luck and be sure to post the fix

bcameron59

Grasping at straws here, but see this link where head temp sensor was the issue,

https://rennlist.com/forums/993-foru...re-sensor.html

Daxmanian

Time for an update!

Finally found some time to do some work on the 993.
Tried the suggestion to mount an extra ground wire, mounted an extra from the point above cylinder 3 to the chassis point above the left strut. Did a test drive, same results

Attachment 847007

Found the missing ground point in the left wheel well, cleaned the wire and ground point, no results

Head temp sensor was read with Scantool and gives normal results, no spiking in temps.

Now going to try find someone who can test the flywheel sensor.
Not giving up, the search continues........

AllMine

Thanks for the update. Does the dura metric display knock counts or total timing? Would be interesting to see if it happens at the same time (seconds since key start)? Also if the air charge temp and cylinder temp match on stone cold key on engine off.
Curt

Daxmanian

The Durametric was not mine, I borrowed it from a friend of mine but if I'm not mistaking it does not display knock counts.
I currently use the Scantool from Rennlist with the OBD cable from ToreB, no knock counts on this.
I'll check if the IAT and CHT match on stone cold start.

Daxmanian

It has been a long time since I updated this so here I go.

Problem is still there.
I was able to buy the engine wiring harness new very cheap so I decided to swap the wiring harness, no joy, problem was still there.

After talking to a mechanic he presumed it has something to do with the mixture not being correct. He thinks maybe fuel pump acting up or the engine is drawing in air somewhere it isn't suppose too.
He suggested checking the rubber seals under the connection where the injectors are plugged in.
Changed all six of them but again, no joy.
I don't think its the fuel pump because then the problem would be more random and not only when warming up.

I noticed that when the engine is acting up the exhaust makes a puffing noise in both tailpipes and you can also feel it pulsating in both pipes so i think the problem is not just one cilinder but both banks, all six cilinders.
I'm going to lend my friends Durametric one more time so I can trend a few sensors and values and maybe learn more.
I'll keep you updated if I ever find the problem.

myflat6

Wish I had suggestions for you that you already haven't tried however I would like to ask how difficult it was to replace the wiring harness and any tips you would recommend.

Ed Hughes

Im not sure I'd rule out anything, including fuel pump. Maybe it acts goofy at a certain point in its day?

Daxmanian

@myflat6 : replacing the wiring harness was not very difficult but I have a '94 so without a varioram making the engine a lot more accessible. I took off the inlet manifold for easy acces and routing the harness. The most difficult part were the wires leading to the starter.
If you have acces to a lift then it would be a bit easier.

@Ed : maybe I should monitor the fuel pressure when the engine is acting up. I'll try to borrow a fuel pressure gauge and see what that gives.

Ed Hughes

Fuel pressure check is a must. And would effect both banks.

AllMine

So is there no way to check long term and short term fuel trim on these cars? Sounds to me like it is going into closed loop after the internal timer has expired. Then something is being altered to make the program happy. Will these systems stay in open loop without a switching O2 signal?
Curt

dutchcrunch

if you can duplicate this problem without driving the car I would run the car on just one ignition circuit to see if one in particular is causing the problem. best/safest way is to disconnect the ignition module plug to the corresponding coil. also whats the health of your caps and rotors and are they factory bosch?

Gt3stig

I'd take a compression check right about now.

Daxmanian

@Allmine : since this is an obd1 car I don't think I can see the fuel trims
@dutchcrunch : with the service of last year the rotors and caps were replaced with oem porsche parts.
@Gt3Stig : compression was tested about a year ago, all within tolerances.

If all goes well I should have acces to a fuel pressure gauge this weekend so I can test the fuel pressure.

Will keep you updated.

techman1

Read thru the thread, twice. Did not hear a mention of checking the main ground, from the battery to the body. I was getting a few electrical gremlins on my '95. It was recommended I check/replace that ground strap. "That ground strap will build resistance, and the point where it meets the body loses contact." Replacing it does two things. Replaces the cable, and when you do that, you make a good body connection again. Could you get the same result by removing the cable, clean the area & cable, and reinstall? Maybe. I replaced it and gremlins left.