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as a general rule, a cylinder with a leaking lifter/lash adjuster will actually show higher compression than a normally operating one. A leaking lifter cuts down lift and duration and that causes increased compression at cranking speeds.
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I had another thought last night. The car has hydraulic lash adjusters. If one of them is leaking down its prime after shutoff, could that make an intake valve not open enough during the compression test and therefore show low? Or is that not a big factor? Normally, the lifters do not rattle at cold startup. I did run the engine a minute or two before I pulled the plugs and ran the test.
The context is when the engine is not running, and only cranking before starting, as in when doing a compression test. At 100-200 rpm cranking speed, the cylinder will fill up with air just fine if the lifter is not opening all the way. If anything, the bad lifter (bled down, collapsed, having too much lash) will help because that will ensure the valve is closed fully. The biggest thing is to make sure the throttle body is open all the way (wide open throttle). This will hinder air flow the most when doing a compression test. The purpose of a hydraulic lifter is to compensate for temperature and valve-pushrod clearance when the engine is running. Every lifter will bleed down when the engine is off and this should not affect valve opening one bit as long as the lifter is adjusted right to start with, and therefore "not a big factor" and should not affect compression #'s at all when cranking it such a low rpm.
the opposite though, if you're lifter is pumped up from too much preload on the lifter, then the valve won't close all the way and you will see poor compression #'s.
When the engine is running, I don't know if a collapsed lifter would actually cause more compression. It would cause the valve to not open all the way, and/or stay open for as long as its supposed to. In that case, with rpm a big factor, it's not going to pull in as much air resulting in less compression and richer mixture. I've read & heard about adjusting hydraulic lifters, where there is no preload I think, and they operate similar to a solid lifter to help with high rpm performance. You also get into the realm of volumetric efficiency > 1 with inertial ramming, acoustic ramming I think it's called, and a whole other mess of cylinder head & intake design. Then you will certainly see higher compression #'s than at cranking speeds, for the most part the compression # will be variable and dependent on rpm.
I agree with the oil squirt when doing the comression test. That'll definitely pinpoint piston rings, and much easier to do than setting up a leak-down test so do this first before a leakdown.
[ July 11, 2005, 01:36 PM: Message edited by: 1 FMF ]
