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Mis-fire Problem


It is difficult to make it out from the photo. If Dave is correct that it is bearing material then it's a worry. Shell bearings are steel backed, coated with a bronze-based bearing material with a lead-based surface. Usually the steel is bright on the outside, and the coating is dull on the bearing surface, see http://en.wikipedia.org/wiki/Babbitt_metal. I assume that the fragment is largely steel. Unless you can see something out of the ordinary through the sump opening, then I can't see any alternative to stripping it down to find out where it has come from. Personally, I wouldn't want to risk putting it back together as it is and firing it up.

That's probably the cheapest option, but you have to bear in mind that Dave is likely to find one or two other items that might need attention after 100,000+ miles. Piston rings, valve guides, perhaps. Maybe you should look on the exercise as a rejuvenation program if you intend to keep the car for a year or two longer. It would also add something to the saleability when you do decide to part with it. Just my 2p-worth.
 
Thanks Richard,

I appreciate your further comments. It's probably a good job that the fragment was found in the course of the investigations. I anticipated the the engine lasting for more than 118,000 before needing a major overhaul though, bearing in mind that it's never been abused perfomance-wise, the vast majority of the mileage is motorway cruising and it's serviced every 3-4 months or so, or was I being over-optimistic/trusting in the technology?

Regards


John
 
There doesn't seem to be much information about the longevity of these engines (apart from the failures, which are rare but too well documented). There was a chap on this forum who had 140,000 on the clock without a scrap of trouble. The old air-cooled engines usually need a top-end rebuild at 100,000, but they use a different cylinder bore material. Trouble is that there aren't too many 996's with 100,000+ miles on the clock to make a comparison. Normally, you would just keep going until it starts to burn oil, but most 996's seem to use less oil as they get older! I agree that you should expect to get more than 118,000 before a major overhaul, but I'm talking more about a minor overhaul really. If you HAVE to strip the engine, it makes sense to spend a bit more and make sure it is tip-top when it goes back together.
 
Hi Richard,

Couldn't agree more. Whilst the engine's out it would be worth while taking the extra effort to ensure you maximise the opportunity for greater longevity. Less costly than a major "let go" for sure.


Rgds

John
 
Hi JC, Dont waste time thinking of a corrupted ECU this is just "cant find the fault so blame it on the ECU syndrome". You mention that the coil packs were changed and one spark plug ,this is not really the way to do things, all plugs must be changed.Sounds a bit silly but check that all of the connectors for the coil packs are really REALLY well connected as these are a real pain to clip together sometimes.Cylinder No 1 connector is also just below the coolant header tank drain pipe which can also cause issues.What where the readings of the compression and Cylinder leak tests as these would be good to look at. Cheers.
 
Thanks for your thoughts. All leak tests came back with good results.

Spoke to OPC earlier this week and they confirmed that they had experienced a mis-fire moving to different cylinders. The cause was swarf getting trapped in the tappets. This however was on a new car and they wouldn't expect the same on mine. However if the intermediate shaft bearing has lost its coating in some places this may have resulted in similar symptoms. Won't know until the ISB is inspected fully.

Rgds

JC
 
Have you checked the lambda sensors. I did suggest this some time ago - not sure if this has been done.
 
Thanks Jonathan,

I'm reviewing the situation with DGR tomorrow. Having had diags done at 2 OPCs and the analysis done at DGR, I'm sure the possibilities have been discussed and checked. The timing will be checked and reset tomorrow and then the engine run to see what prevails.

Would a Lambda problem show up on the PIWIS test?

A hydrocarbon test was done and the one exhaust showed a reading of 42 but the mis-firing one was up at 860. Does this give you any clues?

Rgds

JC
 
JC

I'm afraid I have no idea what a PWIS test is. I had a simialr problem some years ago and it turned out to be a lambda sensor malfunction. As Eccles would say " only a thought" ( if you're old enough to remember the Goons).

 
PIWIS is the Porsche Integrated Workshop Information System. It's a diagnostic computer which also has the workshop manuals, wiring diagrams etc. It replaced the PST2 (Porsche System Tester 2) which was purely for diagnostics and control unit programming. A diagnostic check with a PIWIS or PST2 should show up any faulty sensors, so it is surprising that the readings from the two exhausts were so different.
 
Hi Jonathan,

Yes. Old enough to remember the Goons, even as far back as "The Interlude" on TV before ads came along!!

As far as the car goes (or more to the point, isn't at the moment!!) one of the challenges is still to figure out why the mis-fire moved from one bank of cylinders to the other. The diag at OPC Swindon didn't point to a Lambda problem, their suggestion was potentaially a broken valve spring but not definite until the engine came out.

I'll be in touch with DGR today regarding getting the timing reset and then see where we go from there.

Rgds

John
 
Hi Richard,

It wasn't until the mis-fire switched to the other side (which was after the PIWIS at OPC Swindon) and became a lot worse, that a hydrocarbon test was done at DGR. Before that, it was just the evidence that the nearside exhaust outlet was blacker than the offside and was "popping".

Rgds

John
 
Ignition timing cannot be adjusted on a 996. Sensors can be checked with many types of tester ( Bosch Hammer / Porsche System Tester 9288 etc ) but the readings can only be used as a guide they are not reliable enough to go ahead and order parts for as some people do.More important than anything is the person who is using the tester whatever it is. Car can be run with Lambdas disconnected to see if missfire stops but I have a feeling it wont. Dont worry you will get it sorted some of these things just take time.
 
Engine's coming out on Wednesday

Spoke to DGR this afternoon. Decision made to take engine out on Wednesday for fuller investigation. Likely to be off the road for 14 days (minimum?)

I have also been reading the posts on www.renntech.org about various misfire and CEL situations. Seems like it's a fairly common occurrence with many different diagnoses.

Rgds


John
 
John, just an update for you about the C4S we were discussing. A new battery went in early last week. Problem seemed cured.

The car behaved impeccably for a 150 mile run at the weekend, but on return to London, light came back on and fault is now back! [:(]

Next step is to read the codes again when the car comes back next week.

Rick.
 
Rick,

Thanks for the update. Looks very similar to my problem i.e. Clearing and then coming back again. Are you still getting a misfire/vibration/lumpy tickover?
If yes, is it coming from the same bank of cylinders or has it moved and has it got worse?

Good luck. Engine's due to come out of mine today.

Rgds

John
 
Quick update.

Engine's out. Intermediate Shaft Bearing looks OK. Will update topic with deeper-level investigation results as I get them.

Rgds

JC
 
I've come into this late. But just read the entire thread. Surprised that nobody has mentioned cracked (or broken) cylinder liner.
My bets are that's exactly what your problem is. This has been discussed at length in the mags....specifically the autofarm writeup.
The engine seems to run fine, then it suddenly runs rough. When the engine is opened up a cracked cylinder is the cause and often a
piee (roughly the size you show) is cracked off the top of the liner.

I would call autofarm and discuss with them. Bear in mind that the cost of investigation and then refurbish may be more pricy that
just replaicing the engine with an already refurbished one......with guarantee too. I would defo consider taling to autofarm.
 
I somehow doubt it Berny, as there isn't any oil in the water, or vice-versa, and a compression test didn't reveal anything unusual. If it was a piece from a cracked liner, how would it get into the sump? Also, the metal piece was identified as bearing material. I guess they will find out more as it comes to pieces.
 
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