Leon Speed

They didn't do nothing to fix the problem for $700, they spent time diagnosing, although there was no report of what they thought was the problem. Also, it isn't an electrical problem, because the cause is still unknown.

In orde to help diagnosing you will need to post more details:
Was it ever checked if the protection relay kicked in (meaning did somebody check if there was a green or red led on in the passenger footwell)?
What do you mean with new plugs were "no help"? Rough driving after x time? Same plugs were closed?
Which plugs were closed exactly?
Can you post pics of the closed plugs?
What were the compression numbers per cylinder?

flyrade

Thanks for your thoughts and comments. Here is more information.
They replaced the plugs and that did not solve the problem. The engine was still in the limp home mode.
However, the engine was running perfectly for me, and then entered this mode when it was restarted. The engine also ran perfectly for 15 minutes for them after regapping the plugs, but then again entered the limp home mode.
Since the only work performed was on the plugs, and that got the engine to again run perfectly, we can rule out compression since that is either good or bad - it is not a variable that will change instantaneously. Also, it seems we can disregard the fuel system since it also worked correctly. The only area left is the spark, or electronics. It was a waste of time to give it a compression check.
It seems to be the engine must have been designed to go into the limp home mode for a specific number of failures. How many are there, and what are they? One of those must be the problem. Did they ask this question and find out the answer? Evidently not since they were checking compression.
Since they were spending time, and my money, and not getting any closer to solving the immediate problem, I had to take the car else-ware.

Leon Speed

I am having a hard time following your train of thought, but if you want to know about limp home mode, it is covered in earlier posts. Limp home refers to the MAF not working correctly. The other rough running mode (not "limp home") is the ignition protection relay kicking in. Also covered in earlier posts.

OBehave

So are the spark plugs still closing the gaps on them selves after running for 15 minutes as before?
As far as the limp home issue,there is a member out there who spent thousands at a Porsche service business and 6 months while they did top end refresh and tune ups and all kinds of work to fix a very similar sounding "limp Home" issue. It turns out after all that it still did it after short drives and it turned out that all he had to due was remove the exhaust sensors clean them and the connectors and put them back in. Car is running flaweless to this day.
He didn't have the self closing spark plug gap issue though. I think something is a miss there.

Giovanni

Follow Leon's advise and check what has already been covered previously. Don't focus too much on "the uniqueness" of your problem because it sounds like you are ruling out solutions based on it. Find another garage and/or use the search button to narrow down your problem or tear down the engine and end up like the member that Ed S referred too.

Ian928

If I read the earlier information in this thread correct the problem is that the spark plug gaps get closed and the ignition protection relay then kicks in. Do you think that it is the other way around, that the plugs close because of a limp/ignition protection mode? I don't see the mechanism that could cause this, but I understand your reasoning. After all, the problem started after electrical work was done on the car (although some miles so the problem may very well be unrelated). Also, if the problem comes after 15min each time it leads me to think that something is overheating, the engine management system is entering another mode or something. This again could lead to a problem that closes the plugs. I am no expert but who knows, maybe my thoughts trigger someone elses mind...

Is it possible that the engine management suddenly fails and fire the ignition way to early, causing detonation that close the plugs?

Is it possible that the ignition protection relay kicks in, and this immediately causes detonation that close the plugs?

Is it possible that you have a water leak into the intake that goes through the engine as long as the engine is running correct but can accumulate in the cylinders when some of the plugs stop firing and cause hydrolock?

You have tried different plugs so that rules out the plugs as the culprit.

The piston tops have been checked with a boroscope so that rules out carbon/debris.

Courtshark

+1. If so, it's clearly the little dudes with hammers.

How's the coolant looking? Wondering if you might have an internal head gasket failure dumping coolant into these three cylinders. WAG.

davek9

Exactly, he is not following the advice and trouble shooting steps we have offered up, also never posted a pic of the "closed" plugs, if in fact they are getting hit by little hammers.

OP, take your car to a real Porsche shop, preferably one that specializes in 928's, else your just burning ca$h in a barrel, 700 OMG

Alan

I think its time to forget everything you think you know from previous "diagnosis" and just start over afresh.

I have a hard time believing the story so far is completely kosher... too many things that make no sense...

I think you also need to find a mechanic who is completely competent (at least in general - if not on 928's).

What causes the car to run badly? figure that out - don't just assume it has anything to do with the plugs at this point... I suspect that is something of a red herring...

Alan

Giovanni

Dudes with hammers break stuff, they don't close gaps, they hammer down what they touch :-). And this probability is further reduced when it has happened with 3 plugs, repeatedly.

Has the mechanic replaced plug wires from bad plugs to good plugs and see if the problem is specific to cylinders? I doubt this is a mechanical problem but at least it can be ruled out this way.

SteveG

I have to say it cost me more than $700 for re-built brakes (front only) on a Toyota Sienna, not that you should find that soothing. I am wondering about this: . Is Eric the same mech that is dealing with the closed plugs? You do keep adding ingredients to the mix. Apparently, the direction here is toward "electrical" and detonation. If I read this chron correctly, it was running fine til he tried to fix the wiring. What did he do to the wiring?

Mrmerlin

usually solutions to fix 928 or most broken machines is to ask the simple question,
what was the last thing I fixed.
This answer being followed will usually result in a proper remedy being found.

That said you have been given many suggestions At This Point I suggest to listen to Alans Idea and start a fresh with a new mechanic..

but ask the question posted first

ALKada

Here's a cheap fix. Fly SeanR or Stan down and have them fix your car. You will end up with a quicker fix, cheaper than having your car in a shop for three months and an actual fix then the monkeys that have been dry humping your wallet. Seriously, run from those guys.

jeff spahn

That's what I did.

M. Requin

My post #10, reaffirmed by the posts above: "I think you need another mechanic." I wasn't kidding then, and certainly not now. I'll only add, "one who is very familiar with the 928."