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Strange compression and leakdown results

Posted this on the general BBS but this is probably a better place.

Just finished checking compression and leakdown.
Cyl 1,2,3: comp ~115, leakdown ~40%, lots of air through oil tank
Cyl 4,5,6: comp ~155, leakdown 3-5%
The testing was done cold, and the results are repeatable. I can do a hot test, but the left bank is so bad that I cannot imagine it getting a whole bunch better hot.
I have been chasing a high temp problem, and eventually noticed a LOT of blowby from the oil tank.
Car has 140,000KM, top-notch rebuild at 75000km (mainly durabilitystuff for possible racing) and about 2o% of the mileage since the rebuild has been at DE events. During the rebuild the inlet manifold was ported, an Autothority maf and chip installed, along with a Al flywheel. Also valves, guides, bolts, etc.

Before I bite the bullet and pull it out, any ideas why the tests would be so lousy yet uniform on one bank, and pretty good on the other?
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Bob D.
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Old 09-05-2005, 02:06 PM
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Most likely piston rings; Compression and second ring damaged allowing prssure to blow down into the crank case.

Possibly that side was running lean due to vacuum leak along intake or fuel delivery/management issue for that bank. This may be some of your high temp issue.
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Old 09-05-2005, 04:22 PM
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Here are a couple of pictures of the spark plugs - all 6 are more or less the same.


I am not an expert but they look OK.

In any case, is there any additional diagnostic work? or is it time to open it up?
Next question, I suppoese, is whether to open up both sides?

Thanks for input
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Bob D.
'84 Carrera - MAF, Wong chip, RSR flywheel, ER bushings and other bits, CTR fiberglass F/R bumpers, 7/9 Fuchs, 22/27 TB, 22/21 SB, bunch of other little stuff
'69 Lotus 7 Series 3; '74 Fiat X1/9
'14 X5 diesel
Old 09-05-2005, 04:37 PM
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A possible cause of the leakdown results could be broken rings. The caust could be compression ratio too high on one bank. This could be due to the case being machined or incorrect piston to head clearance on one bank or something similar.
I would recommend taking the motor out and srtipping down to the case. Make sure that all dimensions are comparred while stripping to compare LH & RH banks.
Old 09-05-2005, 09:58 PM
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I had a similar problem on a turbo motor. Turned out the fuel injectors on 1,2 & 3 were very weak or plugged, and the FI on 4, 5, & 6 were OK. Engine ran lean on 1,2, & 3 and broke the first and second rings. I also like 1meansc thought on different compressions. You never know how the engine was rebuilt. Either way with leak down at 40% its time for a rebuild.
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Old 09-06-2005, 06:17 AM
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Porsche Crest

About ten years ago my SC started running hot. Racing, it would sometimes run strong and qualify well, but the power (measured against other cars) would fall off. Leakdowns were never as bad as yours, and were sprinkled around rather than just in one bank. A teardown revealed some broken rings. It is hard to imagine you don't have broken rings (which probably means you will need new Ps and Cs - in my case this was an excuse to upgrade to Nikasil, but you are already there I think).

I've got no ideas better than those already advanced as to why one side and not the other, but with plugs that look the same (making a restriction in the flow to the fuel rail on that side less likely to be the cause).

I never did figure out why I had the broken rings, but the problem has not reoccurred with the new Ps and Cs. And I did add a center valance front cooler, which really keeps oil temps down.

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Old 09-06-2005, 09:39 AM
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Do the leak test wet and dry. Leak test dry - only air. Put oil in the hose - a squirt from and oil can and do the leak test. Significant differences in reading wet to dry usually indicate rings. No significant difference usually in the valves.
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Old 09-06-2005, 10:48 AM
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Thanks for suggstions.
I know it's rings - a gale of air out the oil filler during the leakdown testing.
As far as possible causes - I have had an opinion that it could be cam timing, possibly triggered by a failed tensioner. I have had oil lines break, and it is entirely possible that a tensioner lost function due to the failed oil line.
Any thoughts on whether this might cause a cam timing change, and then whether a cam timing change could lead to ring-beaking detonation?

I am putting together a list of things which need close checking during tear-down.

Thanks
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'84 Carrera - MAF, Wong chip, RSR flywheel, ER bushings and other bits, CTR fiberglass F/R bumpers, 7/9 Fuchs, 22/27 TB, 22/21 SB, bunch of other little stuff
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'14 X5 diesel
Old 09-06-2005, 12:03 PM
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If your tensioner fails, you hear bad noises. And typically you don't jump cam timing - the noise is bad enough that you at least keep the revs down until you get it fixed. Cam timing changes happened with the really really early 911s with a funky kind of chain rail with rubber parts on it - part of the rail would break off and get stuck between chain and gear, allowing a tooth to get skipped. And when that happens you should be getting air out the exhaust or the intake.

I assume you checked for intake and exhaust air leakage, right? Because you always can hear some leakage past the rings even with close to zero % leakdown figures. Think of a slow puncture leak in a tire - you can hear it, but the tire may take quite a while to lose much air. But if it sounds and feels like a gale, does sound like rings, and 40% is quite a lot (not as bad as, say, a holed piston, which should be 100%).

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Old 09-06-2005, 12:14 PM
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Ok thanks.
Any further suggestions of things to particularly watch for as the engine comes apart, especially given that one bank is especially leaky?
One theory is that the rings have "collapsed" - basically lost their tension. Sounds unlikely that would occur to all cylindters on one bank without some other factor.
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'84 Carrera - MAF, Wong chip, RSR flywheel, ER bushings and other bits, CTR fiberglass F/R bumpers, 7/9 Fuchs, 22/27 TB, 22/21 SB, bunch of other little stuff
'69 Lotus 7 Series 3; '74 Fiat X1/9
'14 X5 diesel
Old 09-11-2005, 03:07 PM
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One thing which may help while doing an autopsy is to clear your head of preconceptions. It is easy to be thinking what on the left side is common to the left side and could get out of whack enough that the best guess is that rings broke (or otherwise failed). It might be something no one has thought about and which is random - somehow the piston oil squirters for that side all plugged up.

I'd suggest that while disassembling things you check the cam timing on both sides. Might as well check valve lash. You could check intake (and exhaust) nuts for tightness (the inakes would be possible sources of air leaks).

Having said that, seems to me the two common elements are the left cam and left fuel rail. Hard to imagine the cam being out of time enough to matter but not so far out that valves aren't smacking pistons, but maybe that could cause a lean mixture. A partially blocked fuel rail or its supply might lead to one-sided leanness. You'd think that would show up in left side plugs looking distinctly different from right siders. Those are great plug pictures, and a guy could imagine that he saw some electrode erosion on one of them.

One reason that chain tensioner failure is not apt to lead to cam timing jumping of teeth is that the tension comes not from the pressure fed oil but from the spring inside the tensioner. That spring is not apt to fail. What the oil does is serve as a shock absorber, so when the chain tries to compress the tensioner it doesn't want to compress very quickly (which is why you usually use a vice to compress it enough to put the installation pin back in, and when you pull the pin the tensioner plunger leaps into position). With just the spring (whose job is only to keep tension on the slack side of the chain) at work, there are undamped oscillations in the chain. But these are constrained by the chain rails, and I think the "chain rattling inside a galvanized garbage can" noise is from the chain slapping the rails.

Walt Fricke
Old 09-12-2005, 09:18 AM
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It is not likely that your cams are far enough out of time to affect compression test or leakdown test.
Let me explain why. When the motor is at TDC coompression stroke, both valves are closed. The intake valve has been closed for 130 crankshaft degrees, (50* ABDC). The exhaust valve is not due to open until 134 crankshaft degrees after TDC (46* BBDC).
So what I'm saying is both valves are closed for a total of 264 crankshaft degrees, roughly centered around TDC compression stroke.
So if you were to do a leakdown with any where near TDC both valves would be closed. Of course adding air pressure much off TDC will force the piston down anyway.
So if your cam chains did jump one or two teeth, it would not affect the leakdown test nor be responsible for a low reading.
Could possibly lower a compression check by a small amount.
Ultimately the leakdown shows the air getting past the rings. You didn't mention any air coming out the intake or exhaust, which would indicate an open, bent or burned valve.
Once you open up the engine, inspection will show whether the pistons, rings or cylinders are the problem.
Based upon your report of the leakdown test I see no reason to believe the problem is related to the valves.
Could be something blocking air flow over that side of the engine.
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Old 09-12-2005, 02:25 PM
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