[josephsc]

This is update to my earlier thread where I was "sure" that I blew a head gasket on my 968-powered 944. I pulled the top off yesterday, and it looks like the head gasket is fine! Two of the cylinder walls on the other hand looks scraped up.... [img]style_emoticons/<#EMO_DIR#>/mad.gif[/img]

The initial compression test was 180-90-185-85 (#1->#4) so I knew I had two trouble spots, but pulling the head revealed what looks to my eye like perfectly fine head gasket (bad news...) and two cylinder walls that looks like the piston scraped them. You can see gaps near the bottom of the pictures between the pistons and the walls. #4 (in the enclosed picture) looks like there's some indentation on the piston on top of the cylinder. It's as if the engine sucked in something -- or had some serious detonation going on in there.

What would cause such damage?

BTW, someone was taking pictures of my car when it happened. The picture is BEFORE smoke got really bad and started entering the cabin (turn #3 of Thunderhill -- about two miles left to go to the pits).

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[flash]

i think you might have broken a ring, but it seems unlikely that it happened in 2 cylinders

either way that sucks, because you still have to pull the pistons

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[banditsc]

Perfect time to go to 3.2 liters.....

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[rustech]

^second that [img]style_emoticons/<#EMO_DIR#>/smile.gif[/img]

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[xrad]

looks like antifreeze bloewing out the back...did you smell oil burning?

That first pick crown edge looks melted ..overheated? Also, looks like burned oil on crown. how do the plugs look.

Have to agree w/ Flash...need to pull pistons.

Cracked cylinder also a consideration...

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[S_Cal968]

Bummer...
I'd like to guess a blocked oil galley which in turn resulted in a piston skirt failure, [then the piston will rock in the cylinder]. The hint is the damage at the top and bottom of the piston. However, the fact it happen to 2 cylinders that aren't next to each other makes it more of a conundrum.. Did you see/notice any fluctuation of the oil pressure?
Piston ring failures are typically concentrated in one side/location, but can be anywhere around the piston.

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[josephsc]

Welp.... I spent the better part of today trying to get the short block off, but it won't slide off the torque tube..... [img]style_emoticons/<#EMO_DIR#>/mad.gif[/img] So the bidding is still open for any guesses as to what would kill two non-adjacent cylinders at once.

For future reference, the symptoms I experienced were
* slightly (but not overly) high running temperature. This was on a track day, and I was constantly comparing my temperature to other people's before this happened.
* reduced oil pressure near the end of the day -- this always happened to my car at the track, so I didn't think much of it.
* when it did happen, white smoke everywhere (including inside the cabin, and some seeping out the cat). Later when I was trying to diagnose, more bluish tint.
* huge pressure in the coolant reservoir, and milkshake for oil -- so we had coolant and oil mixing.

And don't tempt me with thoughts about over-boring this block! (Though the thought did cross my mind). This is a 944 that I kept alive longer than it was supposed to go (I've already blown the original engine and tranny). I think it's time to let her go, so I can focus on my two 968 cabs and two 944 turbo's.

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[xrad]

also consider head gasket failure or warped head...even if you don;t see a gasket problem..

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[josephsc]

Well, after much thinking and discussion (on another forum), I'm thinking of three potential culprits that might have led to two non-adjacent pistons to fail.

The first (and I think the most likely candidate) is the lack of any harmonic dampener on this car. I had neither the S2 harmonic dampener (on the front pulley) nor the 968 dual-mass flywheel. So I had nothing to absorb crankshaft vibrations -- and I might not have felt it because the motor mount was absorbing it.

Second is a faulty oil level sender masking any oil level problems -- like an oil leak or a sloppy oil change. There was no known leaks, so I keep wondering if I was careful enough in my last oil change. Improper lubrication might have caused multiple piston rings to fail.

Third is the use of an aftermarket chip causing detonation. The top of the piston and the head looked fine, so I think this is less likely.

I'll post more post-mortem as I think of stuff....

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[flash]

vibrations - you'll know the minute you pull off a bearing cap - any harmonic disturbance will result in bearing wear - it is possible that this would only be on those 2 cylinders - if it were to be on 2 cylinders and not the others, it would be on 2 that were in the same orientation on the crank shaft, not opposite sides - since it is 1 and 4, this may be the case - you will see this at the bearings for those cylinders - i have seen this before when a clutch/flywheel setup is not balanced, or the engine was grossly out of balance (like somebody stuffed in a used piston from a different engine)

oil level - not likely - you would see a much more even issue - the oiling of those 2 cylinders is not all that different than the others, and if the level were an issue, you should have seen other problems pop up - it would have to be very low for something like that to happen - these things are pretty tough - i have run mine completely out of oil a couple of times now, and not seen any signs of damage yet - i tend to think that this is not the issue

chip - not likely - it would not limit itself to 2 cylinders, and you would see signs on the plugs

still, i tend to think it is a foreign body issue, as if a valve seal or retainer let go, got pushed down into one cylinder, then sucked up and pushed down into the opposite fire cylinder

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[RS Barn]

#1 Pistons look very washed from coolant. There should be a carbon crust on pistons
#2 Scores in block may be from valve guide wear/crumbling. This also shows up as pitting on top of piston.
#3 Carbon from intake can score cylinders .
#4 How many miles on 968 motor?

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[xrad]

Agree with Pete and myself: I'm sticking with coolant in the wrong place and the secondary effects....

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[Eric_K]

<!--quoteo(post=77117:date=Aug 7 2009, 08:56 AM:name=josephsc)-->QUOTE (josephsc @ Aug 7 2009, 08:56 AM) <{POST_SNAPBACK}><!--quotec-->The first (and I think the most likely candidate) is the lack of any harmonic dampener on this car. I had neither the S2 harmonic dampener (on the front pulley) nor the 968 dual-mass flywheel. So I had nothing to absorb crankshaft vibrations -- and I might not have felt it because the motor mount was absorbing it.<!--QuoteEnd--><!--QuoteEEnd-->

I've raced lots and lots of hours with neither and never had a problem.

Cracked head?

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