Engine compression test

[tude123]

An acquaintance of mine gave me a 1 ltr engine recently, assuring me that it was good. I have no history on it and no idea whether it is worth installing. It is my understanding that to truly trust a compression test the engine should be at normal operating temperature with a normal oil level in the pan. Can I trust a compression test on an engine that’s cold and just getting cranked by a starter? Also, would a leak down test be better?
I would like to have some confidence in this engine before I go though the work of installing it, you guys have any thoughts on this?

[SeanMurphy]

Try taking a compression test at both times. Cold, and at normal operating temps. If it's good at hot, but not at cold, you might be looking at somewhat worn rings that may, or may not, wear out on you in the next few years. To test the compression, just pull the fuel pump relay up by the battery, hook on the compression tester and crank it over a couple times, holding the gas pedal to the floor. She should read 180psi +/- a few on each cylinder for the 3 cyl model, and it should hold that pressure with little or no bleed back once you stop cranking.

[tude123]

This engine has already been pulled from a car. Sorry about that, I should have made that more clear...

[SeanMurphy]

Doesn't make much difference, really. Just no pedal.. Do everything i said, but hold the throttle body wide open when you crank it over instead of holding the pedal down. I usually tested mine cold, and didn't notice much difference in pressure between cold and warm. 180psi is what you should see across all 3 cylinders. Any higher could mean bad carbon deposits and much lower means worn rings or burnt valves.

[Otakon1983]

Sean, you really need to think about it, starting an engine outside of the car is very complicated. But for that matter,

All you need to do is crank the engine with the starter, you should get a pretty accurate reading from that. If you find your compression is low ( below 180 psi ) pour a little oil in the cyl with the low comp, if the pressure jumps then your rings are good.
and a valve is burned. If the pressure doesn't change then you have bad valve seats.

[geozukigti]

Ya got it backwards man, heh. If the pressure jumps when oil is in it, it means the rings are passing compression by. If the pressure doesn't change, that means the valves are shot. Oh, and if the starter is already on the motor, just hook a jumper cable up from the battery of a good car, and hook it to the starter. be VERY CAREFUL. The clutch/flywheel, or serpentine belt rotating can take off fingers. And when that starter cranks, that engine will jump like all hell. Take the spark plugs out before you crank it. It will lessen the load on the starter.

[Otakon1983]

oops, my bad, thanks geozuki, I must have dain bramage!

[tude123]

Anybody have any ideas about the effectiveness of a leak down test on a three banger? Anybody ever tried it?

[chembrad]

Originally Posted by tude123

Anybody have any ideas about the effectiveness of a leak down test on a three banger? Anybody ever tried it?

Can you describe what you mean by this?

[tude123]

I mounted a tranny on the engine, added oil and cranked it over. The cylinders read 180, 160, and 140 PSI. I don't think I will be installing this one...

A leak down test is a test done to each cylinder that has you inject 100PSI of compressed air into each cylinder when at top dead center. You then measure the pressure on a second pressure dial. Supposidly you can tell what is causing the compression loss by listening to where the air is escaping (leaking from one of the manifolds-bad valves, out the oil filler hole-bad rings). If you do a google search on "leak down test" there are several good writeups. The actual leak down testers are available on Ebay.

[chembrad]

Cool! Thanks for the info.