This is gonna' get ugly...you might want to stop right here.
Active compression check at idle is used on modern production engines to identify ring, valve, guide, or valve spring problems. A cranking compression check can provide enough air pressure to help sealing. When the engine is idlng, the air volume available per cylinder is much smaller for a shorter time. We often see 50-80psi across all cylinders, at idle. A weak valve spring, or worn guide , will drop that cylinder to 35 psi or so. Not that much performance issue noticeable, but a check engine light turns "on".
A cranking compression test on this problem engine might show 175-180 across the board. The trick for a proper test is to keep idle air control fixed while testing each cylinder. The more cylinders the engine has, the more useful this test can become. I have seen V8 with a broken inner valve spring, making no noise because the pieces had screwed themselves together, that only this test was able to pinpoint a reason for P30# misfire code. This really gets nerve wracking when you've seen a slow to disengage starter Bendix setting engine misfire codes due to crank dampening during the slow engagement release!
There are some ways to get a diagnosis by looking at the electrical "ringing" following spark event, but this can be caused by a dirty injector. If that is suspected, an oscilloscope must be used on each injector signal. A dirty or damaged injector will have more active back-emf ringing pattern (after injector closing) than other injectors, due to change of range/rate of pintle movement in the coil. That takes more time, and has more potential for harness damage, than starting with a simple active idle compression test.
With the reduced rpm range of the newer designs, allowing very light valve spring pressure, diagnosis judgement can get hard to call. Even the cam lobes are only a little over 1/4" wide, these days. Loads are so light, the chains look like jewelry material...and dont seem to wear at all. It's all about narrowing the rpm range to keep catalyst volume down, by adding more gears in the trans. The bigger the engine, the more gears you need....especially if it is making good power. Electronic throttles allow the ECM to reduce the number of throttle transitions through 15:1 a/f ratios (transition between fuel on and off), which tend to notch up cat temps in steps. At about 1850 degrees, degradation begins. When a/f ratios start leaning beyond 15:1 there is not enough fire left to overheat the cat...its the trip getting there that puts in heat, needing time and air flow to get out.
So now we are back to the delicate little valve stems and springs, and low tension thin rings, which have much less tolerance for "trouble" than the old stuff. We need them to reduce friction, extend oil life, and still have great response in a narrower engine speed range. All modern prodction cars start with the selection of cat size to do the target job... then you build a car around it.
So we teach the techs to do active idle compression testing before they replace a gazillion dollars in warranty parts for naught. Now there is a twisted, tortured trail on the way to one answer to your question!
Welcome to the Brave New World.
.
This was the torture I lived with for 20 years until Social Security tapped me on the shoulder and said,"clock out, buddy, time to go home.". I told you it was ugly.....
JimL
