jotaking

Firstly...I'm no expert.

FWIW

Seems that car runs okay when running within cold/start parameters.

When car reaches temp the "running temp" parameters cause car to stall when returning to idle with zero load.

Can the "running temp" parameters be tweaked to replicate or "closer" to "start up" parameters?

Does the car run a leaner/richer fuel mix when at running/start temp??

Hope you get it sorted.

good luck!!

earossi

So, to be honest I am less of an expert than most folks who have responded to this thread. But, one of the positives to come from my issues, is that I am learning an awful lot about the car........albeit through force feeding! It's great to have others with expertise make suggestions and ask questions. I remain convinced that this process will eventually lead to a solution with my motor.

To answer a few of the questions that have come forward. I sent the ECU to Steve Weiner who sent it out to the company that does his ECU work. Sorry, but I don't know who that company is. They were asked to perform ECU work to allow me the use of the better breathing expected from installing RS cams in the motor, along with a bump up to the RS intake valving. I assumed that this would result in some minor tweaks to raised the red line a bit and perhaps adjust the mapping slightly to capture any power increase potentials made possible with the slightly more aggressive cams.

The firm that performed the programming changes also disabled the immobilizer function of the ECU. I followed that up by replacing the engine starter relay with a jumper relay as recommended on this board by Bill V.

When we suspected the ECU, it was sent off to ECU Doctors, in Florida for evaluation testing. So, they were, indeed, a third party set of cold eyes to look over the ECU. They did not find anything amiss with the ECU. Their suggestion was to consider a reprogram of the ECU. I'm not really interested in that. So, the ECU was reinstalled in the car.

Now, there have been a few new pieces of information determined from playing with the car. My mechanic has determined that at a cold start up, when the aux air pump is activated (for about 30 seconds), that CO levels are fairly balanced between the two banks of the engine. As soon as the air pump shuts down, there is a divergence of the CO levels. One bank shows about 1% CO while the other bank moves up to a value of about 4% CO. I've been told that both values are within normal limits, but there is a question of why the CO levels would remain in close balance to one another as long as the air pump is on, and then diverge once the pump shuts down.

Coincident with the shutdown of the air pump, the engine appears to idle a little more roughly. This may be more noticeable since I replaced the OEM engine mounts with RS mounts when I rebuilt the engine.

So, I may have several things going on. The first is the phenomena of an intermittent stall condition when load is removed from the engine. The stalling does not become evident until the engine has warmed up (15 to 20 minutes after a cold engine start). And, now we have the unexplained divergence of the CO values for the engine banks once the aux air pump is deactivated.

earossi

When they first looked at my engine, the assumption was that all the symptoms pointed to a vacuum leak. A smoke test was performed and was negative on leaks.

Then a test was performed using starter fluid which would get sprayed over suspect fittings or connections with the engine running. Classically, if the fluid hits a vacuum leak, the fluid would get drawn into the engine producing a noticeable rise in engine speed. That test was uneventful.

When I did the rebuild, I did replace all the gasketing and plastic piping and rubber connectors mostly associated with the VariRam system on the motor.

And, not to repeat myself, but we have determined that the engine is apparently, in fact, stalled at the instant that load is removed from the engine. I arrive at that conclusion from observation that when load is suddenly removed from the engine, the decay of engine speed down to idle is usually a "lazy" rundown that takes 3-4 seconds for engine speed to "drift down" to the idle that is controlled by the ISV. When I am experiencing a stall, removal of load on the engine results in an "immediate" plunge of engine speed. Instead of a 3 to 4 second "drift down", I will see an immediate drop from loaded engine speed to zero RPM in less than 2 seconds.

Though I have most noticed the stalling phenomena in the process of slowing down to a stop, I have been able to also trigger a stall running along at 3000 rpm and a road speed of about 40. At that speed, I can simply push in the clutch and the engine will immediately stall. But, not every time.

earossi

Bump to keep the thread fresh.

techman1

Ernie,

Has the idle switch been checked? I was thinking, if it was a little off, the dme would not know you were off the gas, and would not manipulate the ISV signal to hold it at idle.

Maybe it works sometime, sometimes not. Sound plausible?

earossi

You pose an interesting point. As I understand the throttle position switch, it is a potentiometer that tells the ECU where the throttle is at any time. My mechanic has checked the switch and has confirmed that it is working. But, your point is that it could be intermittent in operation. I'll discuss that with him.

techman1

Earlier models, ie tmy '88, had two switches. The throttle switch, which could sense throttle position, and an idle switch, which would tell the DME throttle was closed. Not sure if that has been combined in 1 switch for our 993's.

Anybody?

earossi

I believe that my 993 has only the one throttle position switch. It's physically attached to the end of the throttle butterfly shaft.

techman1

If there is another one to swap in for a test, it may be productive.
I recall being able to see the throttle position in the scantool software. The mechanic should have something to watch as you drive, especially as you said you can trigger the event.

earossi

DME relay has been mentioned by several others. So, even though my situation does not appear to suggest a bad DME......the relays are cheap, so I'll change it out.

I'm also considering changing out the flywheel position sensor since that is a relatively inexpensive part.

Thanks for your thoughts on this issue.

Ernie

nine9six

As I previously stated back in June (post #5), it could very possibly be an intermittent DME relay issue.
This is a cheap part, that is known for failing and creating such symptoms as you describe. Because of this, it should have been one of your first courses of action But as the saying goes, you can lead a horse to water...

Another culprit could be as Vorsicht describes.

If it were me; and I realize I am wasting my keystrokes here, I would replace each component, one at a time to see if it resolves the issue... ...Or at least swap them out with known good units. Any 993 Porsche pals close by?

P.S. For the reasons stated above, I question any wrench who did not automatically suspect the DME relay as a potential first course of action...
How many checks has he cashed to date?

PRSWILL

this is crazy.. can't wait to see what's really happening here. Especially the stalling at 3000 rpm- very weird. I had a DME issue on my 85 targa years ago- it would literally cut off the electrical an stall the car at ANY speed for a split second- then work fine! the tach would go from 3000 to zero and back in the blink of an eye. It finally gave up completely ( on the highway outside of Sacramento) and had the car towed in as I didn't have a spare.
Your issues doesn't sound like a DME issue, but who knows! I carry an extra in the glove now at all times. $ 45 dollar insurance policy.

earossi

Well, this has been a learning experience for me. My formal training is as an engineer, so I know the importance of testing one thing at a time. And, in defense of my indie, he has followed that protocol exclusively. He has replaced items one at a time to determine if there was a change.

He changed out the CHT probe. He changed out injectors, one at a time. Not sure what he did to prove out the MAF and the TPS, through he did tell me that they checked out electrically.

As noted in my threads, we thought that we had eliminated everything other than the ECU; so, he finally pulled that and sent it out the Bosch ECU guy in Florida who has been recommended by others. And, as noted, they did not find anything wrong with the ECU. I still kind of suspect that unit, but finding one to swap out is easier said than done. The unit is almost $2000 from Porsche, and they are year specific. And, they are set up with immobilizers that are specific to the car. So, you just don't take one off the shelf and put it in the car. To date, I have not been able to locate one that is correct for my year car. If I could find a used one at a reasonable price, I would send it off to have it configured to work in my car. But, so far, I have struck out in that camp.

As far as cost, as noted in my earlier postings, he stopped the meter on the tab quite a while ago and is working this car "pro bono" when he has open time in his shop. He has 3 mechanics in the business. He assigned the car to one of them, who has been working on it as he has time. Now that he has done everything they thought possible, he is about to assign the car to another one of his mechanics. Hoping that the second guy will do something differently.

So, he is being very methodical in the manner in which he is testing on the car. I will travel up to his shop next week and go over everything that he has done since he does not debrief me as he would if there was cost involved in changing something.

The only handicap is that he does not have some of the test equipment that the dealers have. He does not have a Hammer, bust has a tool that is similar (sorry I don't remember what it is). He has been monitoring CO levels, so he has the capability of doing that. He does not have an oscilloscope, but has access to one; so, not sure what he will do there. So, since he is doing this work without charge, I have stayed out of his business; but, feel that it is time to go over the details of what he has done.

If he has not replaced the DME relay, I will have him do that.

nine9six

Ernie,
I'm no expert, but all I'm saying is the DME relay is one of the FIRST culprits in the issue you describe. To have gone through and done all that has been done to rectify the problem, I cant believe at this point, the DME relay has not yet been approached!

Its a $30 item, and when a solder joint in the relay cracks due to vibration, and heat cycling, it can produce the very same symptoms you have described. A wrench worth his salt should know this...

Hang in there, brother

vincer77

I am assuming these values are pre-cat, or no cat. You clearly have one bank running richer than the other. When the air pump is activated, there is enough O2 to complete the burning of the fuel, when off, the imbalance is evident. This could point to a manifold or vacuum leak on the bank showing 1%, but 4% to me sounds very high, so I would more likely suspect a problem on the rich bank.

I am not sure if the ECU controls fuel injection by bank or not - I would think it does. I do not recall if you mentioned if O2 sensors have been replaced. I would also guess that a cam not timed correctly could also lead to a rich condition.

I found an interesting site trying to find a reference for CO levels http://www.systemsc.com/problems.htm

This is interesting:

Also,