: Hmmm. . .Interesting read. Running Compression Test

08-22-10, 01:21 AM
Someone had talked of doing a running compression test in an earlier post. I was reading about compression testing (proper techniques) and firstly found that I did my compression test wrong. I only let the engine do 3 revolutions. The site I read stated at least 5 to 10 to get an accurate reading for a "static" compression test. It also says to hold the throttle wide open while doing this so the engine gets the air it needs for the test. So I'll be redoing my compression test needless to say when I get home. This could be a couple of the reasons for my low compression readings.

Next, I read of the running compression test to test how well the engine breaths. Heres the site that tells all about it. Seems like valuable info. . .


I plan on trying this on my unhealthy engine.

08-22-10, 03:45 AM
Interesting, I think I've seen that before but never tried it.

I've had a hard time diagnosing much from compression tests because by the time problems show clearly on a CT it's pretty obvious where or what the problem is. A dead cyl with good spark means pull the head (check the cam when you pull the intake!). By that time a thumb on the spark plug hole shows the difference in pressure.

The N* is a little different. We are trying to pin down a problem in a cylinder that's still firing with good valves and cams. I've found the first compression stroke is the most important. You will catch a difference there sooner than 4,5,6 strokes later. Even the bad cyl will quickly run the reading up near the others.

The problem on the N* is the HG's still have some sealing ability at the early failure stage. The first one I worked on (my brother's) had good compression the first time we ran the test. It was just starting to fail. A couple months later we got about a 25lb difference on the first stroke on 1 cyl. It almost matched the others after several strokes. There was actually a 2nd cyl failing but that one showed fine on the CT.

Another issue with diagnosing HG's is even a cyl pressure test will show negative with early failure. ( This is 100-150 air pressure in the cyl)

There are a lot of ways to diagnose problems, it's good to know as many ways as you can.:thumbsup:

08-22-10, 12:22 PM
This is true. I figured it was a good reading. I'm going to try it anyways when I get home and see what happens. It can't hurt anything, just maybe clear things up some hopefully.