1. Is the gauge accurate? I test my compression testers by plugging them into "shop air" and comparing the two gauges plus the gauge on the air compressor regulator. Most any quick-connect compression tester I've ever seen uses Industrial Interchange couplers, if that's what you're using for compressed air, just plug the tester into the air hose.
You must be registered for see images attach
You must be registered for see images
For the record, I'd like a gauge to be both accurate and repeatable...but repeatable is more important than accurate (within reason). If I know the gauge always reads 20 psi low at an indicated 130 psi, I can add 20 and know that the reality is 150 psi. But if the gauge reads anywhere from true to -20, randomly...I'm never sure what the real number is.
2. Does the engine crank at normal speed? Slow cranking results in low compression numbers.
3. What is your altitude? Cranking compression numbers go down as altitude increases.
4. Throttle open? Some guys make a big deal about having the throttle WFO. Far as I'm concerned, that's not "bad" procedure, but it's not actually needed. If the throttle is opened to "fast idle" position, that's good enough. Anything beyond "fast idle" isn't really needed, but doesn't hurt.
What do you mean "4 different times?" Sounds like your procedure is faulty. I crank the engine ONCE per cylinder test, but long enough for the test cylinders to come up on compression at least five times, and often longer.
Some guys make a big deal about the "first hit", the needle-jump when the cylinder cranks through the first compression stroke of the test. I couldn't care less about "first hit" as long as the ending pressure after several compression strokes is adequate. Around here, I'm disappointed at anything under 150 for typical older V-8s. I've seen cylinders run with 80 psi, but the engine is clearly in trouble. I've seen cylinders with 50 psi cranking compression run, but not at idle. Takes some RPM before they fire.
Last edited:
