[Cloud9...68]

paulzebo,



I agree, the compression and leakdown numbers don't mesh at all. Also, just from my own personal experience with my recent engine rebuild (at 115K miles), here's what the machine shops told me:



With regard to the head, the shop owner said the 968's are the hardest valve seats he'd ever run across, and showed virtually zero wear. I believe the same was the case with the valves themselves. The guides were somewhat worn, though, so he placed inserts in them, which he told me he has had much better success with than with replacing the guides altogether. But despite the measurable valve guide wear, my engine wasn't using much oil before the rebuild. So, based on this single data point, leaking valve seats doesn't seem like a very likely cause of a high leak rate.



I agree that the test ought to be repeated. I'm guessing the OP has a combination of work valve guides and a leaky head gasket.

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[paulzebo]

Forgot to say a leak down test is to be performed at top dead center. Both valves are supposedly closed then, so compressed air cannot escape into the crankcase except by the rings. I suppose if the fire ring between cylinders was blown, it could escape into the adjoining cylinder and past the valve guides into crankcase. Probably not though. The compressed air would take the easiest way out which would be through that cylinder’s intake or exhaust, one of which may be open. That’s where a static compression test is helpful. Two adjoining cylinders close in low compression numbers are a bad thing. We don’t really have that here. Compression is low but not low enough to indicate a head gasket failure. So, if you hear a hissing noise on <acronym title='top dead center'>TDC</acronym> out of the intake or exhaust side, more than likely it’s a bad valve(s). No hiss out of the intake or exhaust but a louder hiss in the crankcase, ring issues. The trick is to know how much is a bad thing. That’s done by comparing each cylinder to the others and yes, you’re going to hear some hiss on an older engine. That being said, the numbers still don't make sense to me.

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[mrayner]

Thanks for the diagnosis. Yes I find the numbers suspect as well. The car drives well with new plugs, and there is no indication of issues other then the following: Car sometime requires and extra revolution or two on cold startup, but always starts first time and additional oil consumption. I re-checked my oil consumption. I do still need to add a quart when I drive from Vancouver to Seattle (~150 miles), but in regular driving I only need to add a quart every (~500 miles).



The compression numbers were taken by me. I am pretty sure I did them correctly.

The leak-down was done at the shop. I asked them to look for the source of the oil.

Here is what they wrote up:

• ### leakdown #1-95%, #2-20%, #3-8%, #4-8%
  
  
• ### oil leaks at lower balance shaft and oilpan, possible rear main seal
  
  

The shop indicated they saw scouring in the #1 cylinder and the leak was coming from there. They said they would take some pictures with a special camera. I'll see if I can get them, and ask for clarification on the source of the leak.



They indicated that with the pressure during normal engine operation and the leak in #1 that additional pressure may be pushing oil out through other parts of the engine. They see sources of that from the lower balance shaft and oilpan. They are a reputable shop that does a fair amount of work on our cars...so no reason to suspect they do not know how to do a proper leak down, but certainly possible that the 95% is too high.



I have never seen blue smoke come out of my tail pipe, but have not been looking for it. I will check.

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[paulzebo]

First get all the leaks fixed. Then ask the shop to add and engine oil dye if they haven't already. High pressure wash the engine and run it for 100 or so miles. The shop should be familiar with oil dye. Under a black light if glows, regular oil does not. It's used to pinpoint hard to find leaks which are usually the case of high oil use. Get the engine sealed up prior to starting any laborious work like oversize pistons.



I've seen my share of cylinder photos with a bore scope and can tell you they alone are not a good reason to begin a piston replacement. Basically, it's not a 3d photo where you see the depth of the scoring. What has looked like a score turned out so minor you fingernail could not feel the area. Even a slightly score block still can get over 1000 miles to a quart.



Valve guides are a different cat altogether. Long ago in the 70's I had the same condition as you with a domestic car. Turned out the previous owner rebuilt the heads and installed the valve guides wrong.

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