A friend of mine insists that I need to do a cylinder leakage test on my engine. He says that a compression test is not good enough, that just because the engine passes a compression test means nothing; that it is possible for an engine to have good compression, but be 'leaking' or something. He says there is a special gauge for this cylinder leakage test, and it will read a percentage. Now. Can someone please explain better what this is, and if it is necessary?

 

I thought it was the same as a compression test. Guess I was wrong.

 

A leakage test is when you pressurize the chamber with compressed air to find where the excesive pressure loss is

a Compression test will only tell you that you have a compression loss but, the leakage test is a useful tool to find where the leak is coming from/going to

Hope that helps

 

Ok Hecubus. That's what he was telling me. But he said that I could have a leakage problem even though I have 180 psi in every cylinder. I guess that is what I'm not understanding...

 

You read 180 on the compression tester right? Well that thing will read the highest it see's and STAYS there. You may have a leak but the tester won't see it. It reads max and that's it. In other works it can read 180psi then drop to zero as soon as you stop cranking and the gauge will read 180 till you remove it.

A leak down test is more then just pumping air into the chamber, it requires dual gauges to watch entry psi and chamber psi.
This is done without cranking I believe.

 

A leak down tester does have two gauges. One for incoming air pressure, and the other reads cylinder pressure. This is a static test, (not cranking the engine) While the cylinder you are testing is in firing position (TDC, both valves closed) the tester will pressurize the cylinder. Leak down is read as a percentage of incoming air pressure vs. cylinder pressure. (if you have 100 PSI incoming pressure, and 80 lbs cylinder pressure, you have 20% leakdown.) According to GM, anything under 25% leakdown is acceptable. I personally feel that is a little high, but, hey, I am not an engineer....... In any case, all cylinders should be within about 10% of each other. If you have one or more that are significantly lower than the rest, time for a rebuild.

The beauty of this test is that not only does it give you a real good idea of the internal condition of your engine, it will also let you know where the problem is. You will be able to hear where the air is leaking to. This is a much more informative test than a simple compression test.

 

ViciousZ,

What these guys are describing is something like this:



They are useful in determining the leakage rates and source of any leakage, but so it a compression tester in the hands of a skilled technician. A little oil in the cylinders is a good diagnostic aid with a compression tester.

BTW - If you are holding 180 PSI on all cylinders with a cranking compression test, I would presume you have no real compression nor cylinder leakage problems. You can't make that kind of cranking compression on a basically stock engine with any appreciable leakege - certainly not with anything near the "allowable" 25% leakage. If you have a power balance issue, stop looking at compression as a problem.

 

Ok, thanks guys. That's exactly what I was wanting to know.