danglerb

Expert 928 mechanic = expert Porsche mechanic + $$$
Best if you can to start with expert 928 mechanic.

fraggle

Is there any other 928 near you that actually runs right? You need to do a side by side comparison and Get this stuff sorted. A spacer washer? You aren't having any luck with mechanics.

MainePorsche

Not a good suggestion.
Says a lot about the 'mechanic'.

svpmx83

Very bad idea, and the mechanic should have warned you against it.

Did you ever get a full list of what work the last mechanic did?
A proper investigation with a borescope to check the pistons tops for marks or foreign matter would be step #1

GregBBRD

I'm fairly critical about people who work on these cars....about 50% of what I do is fixing what someone "fixed" before I got there.

I really didn't have the heart to tell the OP that he needed a replacement mechanic for his replacement mechanic....

Thicker washers under the plugs....seriously?

Chalkboss

Seriously. That was how I rolled when I was 16... a day late and many dollars short.

terry gt

I am still intrigued by the little people with hammers as Greg suggested

danglerb

Some video hacking on a borescope and you could have fun with somebody using the little men with hammers idea.

So much doesn't fit to my thinking, three cylinders, ok for 1500 miles, then repeats in 15 min, and nothing else weird reported like metal clattering around or even knocks.

flyrade

I agree completely. I got this car a year ago, and don't know what oil was used before. However, I put 10K on it with Mobil 1 and no problems. How could I have carbon build up using that oil?

You would think whatever is in there (if something really is) that it would be affecting the valves, and the engine would not run right. But that is not the case - when it runs it's perfect right up to the red line.

I just sent an e-mail to my mechanic containing your solutions. Hopefully that will help him today. I'm not sure why he has not used a borescope to begin with.

Thanks for all your help, and I will certainly keep all of you informed on how things are going.

flyrade

Thanks for clearing up the limp home mode. I'm sure that is what happened. But here are some of my thoughts that are very confusing about this whole thing.

1. How could the engine run perfectly for 10K miles and then go into this failure mode by being shut down for about 60 seconds?
2. If there is a problem with the MAF, or exhaust temp sensors, how would that cause the 3 plugs to lose their gap?
3. When the plugs were regapped the engine ran great - at all usable rpms - for about 15 min. How could the engine run like this if the valves were bent, or with the other possible failure modes mentioned?
4. Is it possible something in the mentioned systems is lose, causing an on or off condition as it makes or breaks contact? If that is a possibility, where to ask my mechanic to look?

Leon Speed

There are a number of possibilities here. Since the plugs show a failure it is logical to start the investigation there. If the gap is closed/wrong there will be no spark/weak/inefficient spark and the engine will run roughly. The fuel protection relay as mentioned will kick in and shut of fuel to the bank with the closed spark plugs (guessing here - as always - since you didn't mention if all three spark plugs are on one side, however I suspect two closed spark plugs on one side will do the trick. You can check btw if the protection relay kicks in, it is in the passenger footwell under the carpet, bolted to the LH, and it will either show a red led or green led, indicating right bank or left bank (I forgot which led is for which bank).

Another possibility is a fault in the protection system. If one of the thermocouplers is faulty (or wiring to it is faulty) it will give a incorrect fault signal triggering the protection system. The thermocouplers can be checked by measuring the resistance. Need to look up the specifics. i cannot think about a direct relationship with the spark plug gaps in this case, those might be unrelated.

davek9

Hi, this is not a MAF issue "Mass air flow" sensor, that was in reference to the term "limp home mode".

Something us hitting the spark plugs, causing ignition failure, which then triggers the CAT protection circuit that cuts fuel to 4 cylinders by distributer bank, not left or right cylinder bank.
Distributor caps feed 4 cylinders each, again (1,4,6,7) and (5,8,2,3). The temp sensors are at 3 and 7.

You need to find out what's hitting the spark plugs.

Edit: Dr Bob gave you a detailed explanation of the CAT protection circuit operation, also agree with knock as possible cause, however im leaning towards debris in cylinders

fraggle

I think the plugs are failing first and tripping the protection circuit. It is a result, not a cause.

"Great" is relative. This is why you need a known well running car for comparison. At one time I thought mine ran great, but after I addressed everything that was wrong, I found that in reality, it ran like **** for the first 4 years I owner it.

Giovanni

There are some people on the internet having the same problem and changing the plugs seems to correct the problem. http://www.thehulltruth.com/boating-...gap-close.html (see update #62)

I don't see how bend valve, carbon built up or foreign material can be the culprit, specially to built just enough to close the gap, not on one but on three spark plugs and without damaging the plugs themselves. Did your mechanic replace the spark plugs or did they just regapped them the 2nd time around? I would start by putting new plugs and drive it some.

flyrade

Great thoughts and encouragement by everyone - thanks again.

I think the way my mechanic left it was to replace the plugs with NGK and see if that fixes the problem. If not, then shim the 3 plugs that have the closed gap and see if that fixes the problem. I don't think we are saying that is a cure, but just something to check to see if that is the right direction to take. If the shims do not work, that is another data point.

But what about gap-less plugs? I thought there were plugs with an outer ring and a center electrode. Fire Ring, or something or other seems to ring a bell. Maybe we should try those just to see if we are on the right track.