1997 k1500 5.7 engine has a knock did a compression test at 280k today.
Cyl 1-170, cyl 3-170, cyl 5-90, cyl7-160
Cyl2-160,Cyl4-150 , Cyl6-140, cyl8-170
Any reason for low compression
There's
always a reason. Sometimes there's more than one reason.
Rarely--but possible--is a compression tester that is not "repeatable". In other words, the compression tester tool is broken. I always re-test the highest and the lowest cylinders if there seems to be a problem.
Here’s the page from the service manual which covers additional diagnostics, maybe it will help...
I acknowledge that this is "GM OFFICIAL" procedure, but I still believe that a good share of it is
total crap. Others will disagree.
1. The GM spec of 30% between highest and lowest compression is excessive. The only reason to allow 30% difference is to have a reason to deny warranty repairs. 20% for "daily drivers" is PLENTY, and less is clearly better. 10% is a better tolerance for modern engines.
2. Adding oil to a cylinder is supposed to temporarily "seal the rings" which improves compression pressure if the rings are leaking. How does the oil get to the "high side" of the piston on any engine where the cylinders aren't upright? This advice MIGHT have been valid in an era of upright, in-line engines. I don't believe it's valid on any slanted, Vee, or horizontally-opposed engine. The oil does take up room in the combustion chamber, so compression pressure is likely to increase based on that fact alone.
MY advice is to go directly to a cylinder-leakdown test. Add compressed air to the cylinder when the piston is at TDC, compression stroke; listen for leakage at the carb, (intake valve leaks) the tailpipe (exhaust valve leaks) the oil-fill cap (ring leakage) and the radiator (cracked casting(s) or failed head gasket)
ANY leakage at the intake or exhaust valve, or seen/heard at the radiator fill is cause for alarm. Some leakage past the rings is virtually guaranteed; the amount of leakage can be "normal" or "excessive"; and the only way to know is by using a leakdown tester that allows measuring the leakage. (As opposed to just connecting the compressed air directly to the cylinder without a "leakdown tester" in place.)
Be aware that if you use a leakdown tester, you MUST test some "known-good" engines of approximately the same bore diameter as the one giving trouble, to know how YOUR tester responds. The orifice built into the tester has as much effect on the indicated leakage as the actual cylinder leakage. There is no industry standard or legal regulation for the orifice size of
automotive leakdown testers. (There is Federal Aviation Administration (FAA) regulation for
aircraft piston engines.)
In other words, it's not possible to say that "10%" leakage is "ok", or that "20%" leakage is too much; because
different testers will report different results on the same cylinder, due to their internal construction
.
Stay the hell away from Harbor Freight leakdown testers. At least one of them "tests" at 15 psi (too low) and they tell you to put the piston at the bottom of the stroke (Liability bullshit, completely wrong procedure.) This tester must have been designed by dirtbag lawyers instead of engineers.
This is the tester I bought, several years ago. Suitable for aircraft (and automotive) engines with a 5" bore or less. When I got mine, the plastic case wasn't included, and I asked for a 14mm adapter instead of the 18mm adapter it normally shipped with. You could contact them and ask about buying the tester and the "automotive" 14mm adapter just as I did in 2012.
https://www.amazon.com/Aircraft-Tool-Supply-Differential-Pressure/dp/B005VR9H24?ref_=ast_bbp_dp
/ref=sr_1_fkmr0_1?keywords=at+leakdown+tester&qid=1580242136&sr=8-1-fkmr0
