Re: using a compression guage
Posted: Wed Nov 01, 2017 11:55 am
our text book at school says 4 strokes is the industry standard, and they want the throttle open, during testing. You will get less compression with the throttle closed. When I teach compression testing, I show them with the throttle closed, throttle open, running compression test, and a wet compression test. All give different readings, all tell you different things about the engine. Also battery charge and starter condition affect cranking speed which again gives different results. If I do a compression test throttle open and closed and do not see a difference in results. I suspect a flat cam or excessive carbon build up. Running compression can show up worn valve guides, wet compression worn rings. ETC. I tell my students main issue is do the same on each cylinder, or you get in trouble real quick. Newer engines with computer controlled throttles and idle air, make the throttle closed and open test not reliable. Of course performance engines with different overlap camshafts give different readings than a stock engine. The testing I lean to now is using a scope or scan tool. You can measure the amperage draw of the starter and find weak cylinders by simply using an amps clamp on the starter wire. Lots faster than taking plugs out, lots of engines you now have to remove the intake manifold to get to the plugs. Ford for example has Cylinder balance and cylinder contribution tests built into the ECM and scan tool.
I much prefer a cylinder leak tester to diagnose engine mechanical issues. Less things to consider when making sense of the results, Plus I can leak an engine on the engine stand.
I much prefer a cylinder leak tester to diagnose engine mechanical issues. Less things to consider when making sense of the results, Plus I can leak an engine on the engine stand.