Luca Signore

Hey everyone -

So I bought a leak-down tester recently (Proto brand, got good reviews) to chase a potential oil burning issue on my 91 22re. Compression test and vacuum test were both good (165ish across all 4 cylinders, vacuum normal) so I went ahead and did the leak-down too.

My main issue is that my air compressor sucks. I don't have one myself, so I borrowed my dad's and its terrible. It takes forever to get to 100 psi, and when it does and you shut it off, it leaks air. I'm able to get the tester hooked up to it, but I find the pressure on the tester drops pretty quickly, making reading the results difficult. Also, the Proto unit I bought suggests opening a bunch of stuff (oil cap, dip stick, air intake, radiator cap) before actually connecting the test. Is this the right way to do it? Or it only necessary to remove the spark plugs and then remove each of those things individually to listen for air escaping. I did do the test the best I could on all 4 cylinders with each at TDC (a minor question here, is having the distributor rotor pointing at the corresponding cycling enough to ensuring that each cylinder is at TDC?) Should I invest in a better compressor in order to perform this properly?

Second question: how much air is normal blowing through the crankcase? I know some is normal as no engine will be 100% sealed, and the rings will seal better when running, but what is a normal amount? I could definitely hear a "whoosing/rushing" from the oil cap and dipstick, but not necessarily super loud. Again, per above, I'm not sure if I got accurate results given my compressor sucking, but all 4 cylinders seemed to have minimal leakage when comparing the two gauges (one on the left would stay static for a bit until the compressor leaked, and the one on the right was usually only about 5 psi lower when at 80ish psi).

Any help would be greatly appreciated!

scope103

Hmm. I think a compressor is supposed to "blow," not "suck." Could you have it hooked up backwards? :^)

The numbers you get with a leak down test are really only useful to compare to a prior test. The "precision leak" in a leak down tester just isn't that precise, so you'll get different numbers on the same engine with different testers.

What a leak down tester is useful for is just what you're doing: trying to figure out where your (big) leak is. Removing the oil cap, etc., isn't necessary for the numbers part, it's just to get your ear closer to where you're listening. I haven't done many of these tests, but clearly audible "whooshing" doesn't sound too good to me. On the other hand, consistent 165 on a compression test sure doesn't sound like a sealing issue.

And gee whiz, get a spray bottle with dilute (5:1) dish soap, and a roll of teflon tape. Find the leak in your Dad's compressor, and fix it. He's done so much for you ....