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Old 10-25-2020, 07:58 AM
v8volvo v8volvo is offline
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Join Date: Jun 2009
Location: Montana, USA
Vehicle: '86 745, '83 764
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Checking the rest of the valve clearances is a good idea, but as long as there is some amount of clearance, the exact amount should not affect the compression pressure (when measuring at cranking speed on a cold engine, at least).

With valve clearance, it's pretty much a black or white question: either the valve is closing all the way, or it isn't. There's not much middle ground to cause "low" compression -- typically you either have full compression with a sealing valve, or zero if a valve is not fully closing.

Also, it would be rare for valve clearance problems to affect the compression in a way that is highly consistent from cylinder to cylinder. You said an average of 17 bar (=250psi for those us reading in the US) -- were the cylinders all pretty close to 17 bar, or was that an average taken from a lot of variation (e.g. some cylinders very low, but others higher like close to normal spec?)

When you had the head off, did you spray any kind of solvent on the cylinder walls that could have washed them down and made it hard for the rings to seal?

Have you tried putting a little oil down the cylinders and seeing if that changes the compression readings? That is usually one easy test to show if the loss of compression is from the rings or the valve train. If the pressure increases significantly with oil added, then the ring to cylinder seal is what you're looking at. If the oil causes no change, then it's more likely the valve train.

A leakdown test, or even a simple seat-of-pants audible leakdown test involving just putting some air pressure to the cylinder and listening for where you hear it leaking out (listen to crankcase thru oil fill cap, and also listen at intake manifold and at the tailpipe) will tell you where the pressure is escaping. So that's another way to narrow it down.

I'd start with a "wet" compression test (with a few ounces of engine oil down each cylinder before testing) and see what that does. If you can report both your original dry compression test pressures and the new wet pressures for each individual cylinder here, not as an average of all 6, that will then help us get a lot closer to understanding what may be going on. Hopefully something simple.
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86 745 D24T/ZF 345k lifted 2.5"
83 764 D24T/M46 155k
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