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It started out as such a nice day...

 
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jazz guy  



Joined: 26 Nov 2002
Posts: 411
Location: Colorado

PostPosted: Sat Jun 27, 2020 3:59 am    Post subject: It started out as such a nice day... Reply with quote

I was actually going to give some attention to the '82. I'd just picked up a new battery and was stoked to spend a little time getting the CIS dialed in for some nice Summer mountain cruises. The car had been running pretty well, firing right up & idling nicely, but part throttle performance was a little sluggish. I started by pulling the plugs and all looked good, except for a slight oily residue around the thread body of #1. I decided to run a quick compression test and that's when things took a turn. Results:

cyl #1 - 120psi
cyl #2 - 134psi
cyl #3 - 136psi
cyl #4 - 140psi

I'm not sure what's considered good compression for an old 931 but cyl #1 is outside of the 10% variance window. So I pulled the head and BUMMER, head gasket carnage between cyl's #1 & #2. I think the block and cylinder bores look good, though.

Pics (click to enlarge):

-------H.G. Cyl 1&2-----------H.G. Cyl 1&2 flip side----H.G. (top) 4,3,2,1 left to right


Block: 4,3,2,1 left to right ------------Cyl #4---------------------Cyl #3--------------------------Cyl #2---------------------Cyl#1


Head: 4,3,2,1 left to right ------------Cyl #4---------------------Cyl #3--------------------------Cyl #2---------------------Cyl#1


Where to go from here? I'll freshen the head and get some good head studs, but to me it doesn't look like valve sealing is the main issue and there weren't any other symptoms of a failing H.G. No leaks, no water/condensation in the oil, or excess pressure in the cooling system. Would the H.G. damage be the only reason for the low compression? When I pulled the head, I expected to find broken piston land or ring bits, but no. I guess the only way to confirm piston & ring integrity is to pull the block, remove the pistons, and see what falls out?

What do you think? Observations? -Cheers
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Fifty50Plus  



Joined: 28 Feb 2008
Posts: 1102
Location: Washington DC area

PostPosted: Sat Jun 27, 2020 4:08 am    Post subject: Reply with quote

I'd do a light skim on the head, valve job, new Cometic head gasket and bolt it back on.
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 2040
Location: MI

PostPosted: Sat Jun 27, 2020 5:54 am    Post subject: Reply with quote

I'd probably just put it back together with a new hg and see what it does..

I wouldn't be pulling the pistons because the new rod bolts needed for that job are big bucks..
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winddude  



Joined: 15 Jan 2020
Posts: 54
Location: Vancouver Island

PostPosted: Sat Jun 27, 2020 8:07 am    Post subject: Reply with quote

not an expert by any means, but it looks like the intake valve on #1 isn't seated as much as the others, guess it depends where in the stroke it is or maybe carbon build up. I'd inspect further and maybe lap the valves, and check lash.
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jazz guy  



Joined: 26 Nov 2002
Posts: 411
Location: Colorado

PostPosted: Sat Jun 27, 2020 1:46 pm    Post subject: Reply with quote

Thanks for the replies.

winddude wrote:
...#1 isn't seated as much as the others, guess it depends where in the stroke it is or maybe carbon build up.

Yeah, the cam is still in and #1 intake is slightly open. The back of the valves are squeaky clean, probably helped by the water/meth injection. On that note, I was pleasantly surprised I didn't find blatant evidence of detonation. I've run significantly increased boost since the early 90's.

I'm definitely going to fully rebuild of the head with a skim to make sure it's flat. Hopefully it won't be too far out of shape. As for the rest of the engine, I'm still debating whether to go full monty on a rebuild. I don't want to spend money needlessly but it's almost one of those "while your in there" moments.
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924RACR  



Joined: 29 Jul 2001
Posts: 8334
Location: Royal Oak, MI, USA

PostPosted: Sat Jun 27, 2020 10:44 pm    Post subject: Reply with quote

Full clean and valve lapping, for sure. I'd not skim the head unless measurements show any lack of flatness; just be sure (learn from my mistake) to check flatness of the top of the block too!
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emoore924  



Joined: 13 Apr 2004
Posts: 2801

PostPosted: Sun Jun 28, 2020 12:24 pm    Post subject: Reply with quote

I dunno. I see some funkiness right between 8:30 and 9:00 on that #1 piston. The head looks pretty oily for #1. That burn-through in the head gasket is ugly. There's no real explanation for the oiliness other than a broken land or failed ring. The block/cylinder looks good, which is good news but I'd be worried about the side of the piston and/or some failure with the rings.

All that said, put it back together and see what happens. But you might have to do it again (and in a turbo, that's a pain...).
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Mike9311  



Joined: 14 Dec 2004
Posts: 878
Location: Chicago-ish

PostPosted: Mon Jun 29, 2020 2:19 am    Post subject: Reply with quote

jazz guy wrote:
...... "while your in there" moments.


This is the proverbial rabbit hole

I too, think I would try a head gasket first. Then maybe go crazy over the winter if it rears its ugly head again
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jazz guy  



Joined: 26 Nov 2002
Posts: 411
Location: Colorado

PostPosted: Tue Jun 30, 2020 3:41 am    Post subject: Reply with quote

emoore924 wrote:
...I'd be worried about the side of the piston and/or some failure with the rings. All that said, put it back together and see what happens. But you might have to do it again (and in a turbo, that's a pain...).

Mike9311 wrote:
This is the proverbial rabbit hole

These pretty much sum up the two main arguments rolling around in my head...
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safe  



Joined: 18 Mar 2017
Posts: 171
Location: Sweden

PostPosted: Wed Jul 01, 2020 12:10 am    Post subject: Reply with quote

I would skim the head if fitting an MLS gasket, they like a really clean, flat and smooth surface. Stock gaskets are more forgiving.
Pour some ethanol into the head combustion space to see if the valves leak or not. But you might just lap the valves and replace the valve stem seals if you have the head off.
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Raize  



Joined: 18 Sep 2013
Posts: 136
Location: Scotland

PostPosted: Wed Jul 01, 2020 3:05 am    Post subject: Reply with quote

The head gasket is damaged but not to the extent where pressure could escape - the #2 fire ring is intact.

I think therefore that the only explanation for a lower compression on that cylinder is indeed a damaged piston ring.

Pity you didn't do a leakdown test as well before pulling the head. It could tell you more. I bet you could make some kind of adapter plate which bolts over a cylinder using four head bolts, a big O-ring to seal it, and a threaded hole in the middle for the leakdown tester to screw into. That would give you a direct comparison of ring leakage in each cylinder since the valves would be out of the equation.
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