i read that the way to test compression is with a warmed up engine. What if the engine is out of the car or non-running? Any ways around that? Would a leak down test be just as good, without the need to warm the engine up?
I prefer leak down test’s. Especially on an engine that’s out. If percentage is off it’s super easy to hear where the air is leaking by.
Sun Electric who sold both compression testers and leak testers said in their instructions that the engine should be warm when doing either test. That is the most accurate way to test since that is how you operate the engine. However, either test will point out a problem cylinder even on a cold engine.
Tests performed under normal operating conditions best reflect how the engine works under normal operating conditions, but I'd expect any significant problems to show up cold too.
Pretty tough to do a test on all of them when hot, by the time you get to the last one, temp is going to be quite different. If asked I’d say do it hot. In reality I never have. A bad valve shows up hot or cold. Guess a ring might expand more warm/hot. Maybe wouldn’t expand and seal as the others? I dunno. But an issue with a running engine that’s do to compression, would most likely show up when cold as well. Just my thoughts and what I’ve done.
Well... There are some more modern options that makes a big difference. There's something called a relative compression test, based on how much current the starter draws (as that's based on how hard it has to work against the compression), done using a digital storage oscilloscope. One cylinder with lower compression will have a lower current peak, or if really bad, a valley. This test only identifies differences between cylinders, hence relative compression, so it doesn't find a cam out of timing or anything else affecting all cylinders, but you'd only have to make a conventional test of one cylinder to verify that if you need to - then you know the sate of all of them.
But I’d bet there’s just a handful of guys here who have a scope at all, even less a storage scope. I mean I know a way to really extend a “hot” compression test, but dang. Who has the time, besides it would be best suited to someone who has no issues in a DD and is looking for a 3 psi difference between cylinders
It is better to test the engine warm, but a cold test still reveals inconsistencies between cylinders, so it's viable. And if the compression is a bit lower and it's cold, you know it's going to be higher once warmed. So doing the compression test cold is still better than just throwing an engine in your car and hoping it's OK.
You may be right, and using modern digital equipment may not be completely in the traditional spirit of the HAMB (is it strange that it feels slightly ironic to write that online using a computer?) but they do offer some amazing possibilities when diagnosing vehicle issues of many kinds, in some cases making tests quicker, and in other cases diagnosing problems that may be hard or impossible to find without just swapping parts and see if the problem goes away. Just a few hundred dollars spent on a suitable storage oscilloscope and some accessories can solve a lot of problems and speed up the work diagnosing others - they've been common for ignition work at least 60 years and there's so much more to use them for if you know how to. Might not make sense for the average cl***ic car owner, but for someone spending a lot of time working on cars it can be money well spent.
The only exception to this might be an engine with valves or piston rings that tend to stick or free-up depending on the engine's temperature.
Not too long ago I rebuilt the SBC in my T. After 100 miles I took a leak-down and compression test with the engine warm and recorded it in my notebook as a baseline. It is so easy to do in the T. I then took leak and compression at about 100 mile intervals, just to check progress. Remembering that I had heard hot testing was the way to do it. I took some tests cold and some hot. I didn't find any appreciable difference. YMRV.
I think a guy needs to buy himself a digital storage oscilloscope. Not sure how to set it up or use it but hey, if I can paint a car, run a bead, how hard can it be? Looking on line, they range from down right cheap to holy smokes that's a lot of money, just like most things these days so why was I surprised. Only thing left is to find the drill and a bit to go in it for the hole I must need in my head. LOL. I'm kidding. I'd buy one in a blink if I wasn't so broke. Hell yea, all kinds of things you can do with one of those if your smart enough to attach the wires. I'm not but hey, I like wires. Taking things apart. Now and again I solder them. This electronic stuff. Isn't it BS. This Snap On is DOA...anyone tell me what's wrong? Don't say battery cause I'm going hungry having bought a fresh 9V. Anyhow, as far as the compression test goes, we are mostly old guys here, who hasn't gone for a morning blood test because that's the way to get the best results. If it's a close accurate result your looking for, go in the morning. Close enough to see if there's a problem, check for drugs in the system, any time works. I'm saving for a new automotive diagnostic tool that has those digital storage oscilloscope abilities built in. The future is in electronics. Electronics...to complicated to take apart or fix, but hey, as long as the doors to trade with Asia are available, it won't be bought cheaper elsewhere when it ****s out.
Yes, the best thing is to do the compression test after the engine is warm. But doing a test on a cold engine will also tell why it won't start....if it's really bad....
Sun was selling scopes in the late 50's and Champion used to have a spark plug scope in the same era. Using a scope on an engine either confirms or eliminates an ignition problem very quickly.
Remember on a compression test to open the carburetor ****erflies. I remove the spring and pull the bottom of the linkage.
Engine is out of car, no cranking motor, no compression test. That leaves a leak down test. If you have a good compressor, you can do the leak down test. Easy to do, no special tools needed. Look at plugs, if they are still in engine, do they look like engine has problems?, like oil fouled. Leak down warm would be ideal, but it sounds like you're unsure of engine condition, what are your concerns about using it as is?
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