I only have 50psi in one cylinder and the book I have calls for 120, so I'm gonna start with that, but also go through everything else
"120" isn't going to be a hard-and-fast rule. Cylinder compression varies with dozens of factors; chief among them being altitude (air density), cranking speed, cam timing, and compression ratio.
Seems to me that GM specs are "minimum 100 psi", and no more than 25% variation among cylinders. To me, that's way too generous; I think GM is cutting warranty claims by allowing too much variation, and too-little pressure.
A "typical" compression pressure where I am, for older V-8 engines would be "around" 150 psi. I get concerned at 130-ish psi even though I've seen cylinders run at 80 psi; and run
above idle at 50 psi. (that cylinder was dead at idle, though.) Low compression pressure leads to a "lazy" engine.
Spray some oil in that cylinder, if compression goes up, you have bad rings. If not, time to tear into it like said above.
I quit putting oil in cylinders to verify rings decades ago. No one could tell me how the oil was supposed to seal the intake-manifold-side of the cylinder, given that the cylinder slopes at a 45 degree angle on a V-8 or 90 degree V-6. Similar for "slant-4" or "Slant-6" engines.
OTOH, a
cylinder leakdown test reveals intake valve leakage vs. exhaust valve leakage vs. ring leakage vs. compression loss into the cooling system. The downside of course is that you need a decent source of compressed air, and ~$100 worth of specialized equipment.
Decades ago, the shop I worked for did not have a leakdown tester. We'd just pump unregulated air into the cylinder and see where it leaked. This is fine for testing valves, castings, and gaskets; where "zero" leakage is the only acceptable result. It doesn't work for testing rings. There's always some leakage past the rings, without gauges to tell you "how much" leakage, there's no way to know if what you're hearing is normal or excessive. After doing this on two or three engines, I bought a leakdown tester so I'd have actual answers for the ring seal.
