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Discussion summary
YOUR RESEARCH996 cylinder height sag - working solutions?
Reported or suspected cause
cylinder height
Reported cause
No successful repair reported for this finding.
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Identified cause
0
Suspected cause
0
Reported fixes
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Reported cause · Original poster
He expressly stated that the issue was the cylinder height on the two middle cylinders were .005" and .003" low relative to the expected flat plane.
Leading evidence: 'cylinder height' (1 discussions report identifying this cause; no attributable repair resolution). Across 1 matching discussions, the most frequently mentioned possible cause is 'cylinder height' (1 discussions; 1 report identifying it and 0 suggest it). The most frequently discussed action is 'Adjust pin height' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
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Performed · outcome unconfirmed · Original poster
My 1999 996 C4 recently experienced a failure of the cooling system that was initially solved with a new water pump, replaced thermostat, etc.
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Performed · outcome unconfirmed · Original poster
Total mileage is 103K; cooling system was replaced ~18K miles ago.)
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Suggestion
We adjust the pin height on the pistons by .010" to accommodate this but in some cases, the blocks are so bad that we need thicker head gaskets.
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Suspected cause
I agree with De Jeeper that cylinder ovaling is most likely the cause of the difference in deck height.
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Greetings to you all. This is my first post after benefitting immensely from this forum over the last few years. I'm trying to learn more about the issue of sagging cylinder height in an 1999 M96 engine and of there is a path to resolution that doesn't entail a full recondition/rebuild of the engine at $15K+.
My 1999 996 C4 recently experienced a failure of the cooling system that was initially solved with a new water pump, replaced thermostat, etc. We were still unable to get the temp to regulate correctly. After exhausting most other possibilities (no intermix, heat exchanger was not the problem), the shop pulled the engine to assess the cylinder heads. There was no cracking or scoring, but.....they observed sag in the two middle cylinders, with one 5/1000ths of an inch low and the other 3/1000ths low (outside the normal range of 1/1000th by a wide margin).
The shop - which I deeply trust and which has done repeatedly good work on this car over the 5 years I've had it - advised that this is a massive issue. If I want to solve this problem, I'll need to send the case out (prob to Flat Six) and put in new cylinders. All in, this is probably ~$20K and many months of wait time.
The "do nothing" approach, which would entail trying to find a thicker head gasket and then putting the engine back in, is effectively a "kiss the dice" short-term approach with a low probability of success. (Also, they can't seem to source a thicker head gasket, which leads me to believe one doesn't exist). Given ~$3K of diagnostic labor to date, this is ~$8K.
(Car is a daily driver, not tracked, and has a factory replacement engine with ~70K miles. Total mileage is 103K; cooling system was replaced ~18K miles ago.)
I'm stuck and can't get a good mental map of my options here. Again, I trust the Porsche-dedicated shop working on the car as one of the best in the Bay Area. I'm trying to find a path to get the car back to working order as I cannot afford $20K to effectively replace/rebuild the engine.
Any ideas? I can't find this issue in any other thread....
Hey Wally. The shop is saying that the cylinder wall lining is sagging? I'm not sure I've ever heard of this one.
If that is the case I'm sorry to say I can't imagine an alternative effort that is better than a rebuild.
Yeah, that's the message. What they told me is that this is a known (albeit low probability) problem that has occurred in the M96 following repeated overheating.
He expressly stated that the issue was the cylinder height on the two middle cylinders were .005" and .003" low relative to the expected flat plane. The tolerance is .001".
I just can't find any notes on this type of issue. Increasingly looks like I sell it as a parts car, which is a massive bummer.
I agree with De Jeeper that cylinder ovaling is most likely the cause of the difference in deck height. You could always park it for a bit and save up some money for the car if this is the one for you. In moments like this I understand why you feel the way you do, however, over time you may reconsider your options.
Sorry to hear Wally.
I would give LN a call. See what they have as options. It doesn't sound like you are in the FSI price range. Assuming your shop would cut you a deal on labor, and your bolt on parts are in good shape, LN may be your only choice versus a short block from a dealer between 7-9k.
I'm with De Jeeper I would suspect ovaling of the bores is your issue, it is unfortunately relatively common. I suppose it is possible you slipped a sleeve, but I have never heard of that happening on an M96. Regardless, it sounds like you are looking at a new case/sleeves.
Edit: Maybe M96 sleeves slip more than I thought. From LN "
When planning to rebuild the flat six Porsche M96 engine or M97 engine in your Boxster, Cayman, or 911 vehicle, there are many considerations when evaluating your options to correct for bore scoring, intermix, cylinder cracking, d-chunk ,
and even slipped sleeve failures.
"
I think what the shop is referring to is a warped deck ( concave) on the cases from overheating. Yes a warped deck or head is common in extreme overheating instances.
Standard practice on most engines with warded deck/head is just to machine them back flat. But with Porsche or any other really high compression ratio engine, just machining the deck/head back flat can incre4ase the compression ratio / decrease the piston to head clearance, too much, and is not recommended by Porsche.
Even though it is not recommended by Porsche, it can still be done to correct the issue, but a Custom head-gasket will need to be made. Cometic or some other Custom Head-gasket maker can make you one any thickness you want, yes they are a little pricey...
Of course this is considering all else is Ok with your cases and not ovaled like others have mentioned. And of course the "while you're in there's" will likely send your cost up near a full rebuild anyway...
To make the decision simple, just sell as a roller, or do the full happy meal...
Short of a total rebuild, I would vote for the shimming the cylinders, or skimming the deck and a custom head gasket. Or find a short block from the factory.
Every one of these blocks are out of round and the decks are warped. We know they aren't like that from the factory as we've bought new blocks in the past and they ship perfectly round and flat.
We deck the blocks at time of machining. We adjust the pin height on the pistons by .010" to accommodate this but in some cases, the blocks are so bad that we need thicker head gaskets. We've had thicker ones made up custom, but only for the our 99-101mm bores.
The two options I'd personally go with is either skim the block 0.005 assuming the cylinders aren't deformed or ovalized or buy a short block from porsche and swap it out.
You've already said you don't have the $20k for an fsi build.
Did you trace the overheating? If it was caused by broken water pump impeller plastic plugging the head water ports it will happen all over again. It's a known problem with these engines.
What's the shop? I'm in the bay area also
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