I'd do 2 full tests dry (cyl's 1-6, then repeat 1-6 again) before putting oil or any other liquid into the combustion chamber. This can artifically inflate the readings and mask potention problems. Usually the idea is if you have one low cylinder, after the test is done add a few cc of oil and test the cylinder again, to see if the readings increase or not. However on a diesel, the oil can ignite and cause a spike in the reading (I had that happen to me when testing after my head gasket failed... sure does wake up the gauge!)

:nuke:

He rebuilt the engine, I just wanted to make sure some kinda lube was on the cylinder walls. Certainly repeat the test. Wouldn't gasket or valve leak be the most common issue here? Oil wouldn't change that right?

Oooops. I mis-read the original post. I thought he was doing a compression test just prior to a rebuild, not just afterwards. Sorry about that! Hopefully a fair amount of oil was used on the cylinder walls during assembly. He didn't specify if the rebuild included new rings (sometimes people just replace the bearings at the bottom end).

In this context, a compression test is highly questionable... with a fresh rebuild, it needs some run time to seat in new rings (assuming it has them) before the compression numbers will be meaningful. I say fire it up.

:zorro:

My book says that you need 220psi for reliable starting measured cold.

I am not sure why such a big deal is made of testing them hot. When you really need to know how they are is when you want to start them from cold.

well i forgot to mention that i only rebuilt the top end due to a cracked head.
i drained all the oil, so we will be filling her up before cranking. but the bottom half is still original. but we just wanted to know what would be acceptable readings for a cold test. but keep posting.

I would start the car up and warm up the engine to operating temperature so you get the most accurate results and you have to do the test once.

work smarter not harder

I agree 110%. With just a head replacement, there's no reason for a cold test... fire it up, and do a proper compression test with the engine hot. Those specs are in the FSM (26-32 bar, with 3 bar max difference from high to low). In 'Merican terms, that's 375-465 psi, with a max difference of 45psi from highest to lowest. Note that the numbers will be lower if you are significantly above sea level. Check out this link for a correction chart based on your altitude.

:euro:

Low compression on a cold engine is a major cause for hard starts or no starting. Once the engines warm it always starts easier. So wouldn't a cold compression check give you a better idea of the state of the engine.

no.

you want to measure the running performance of the engine and it's efficiency capabilities. ie- warm.

But warm compression doesn't help on a cold start. Cold compression is what counts on a cold start. And I would argue, it is a better indicator of good compression. Good cold compression will translate into great warm compression. When one of us goes to look at a car we might buy, we usually want to start the car cold, and are disappointed when the engine is warm. Just about any warm diesel will start. But a cold diesel is much harder to start. I've experienced it myself. Having a hard time getting the old girl started on those cold mornings, but once she's been run she'll fire up quite easily.

one should seriously consider rebuilding the head if that is the case.

If all one is concerned about is whether an engine will start after a rebuild.. then by all means, do a cold test.

You never now what might happen to an old engine once it is warmed up.

as for the compression test when the engine is cold argument its simply this: If you have trouble starting your car when its cold or it doesnt start at a certain temperature then you can be sure that the engine compression is a factor to consider. Other factors to cause hard starting is problems with the fuel system.

Why waste your time on a cold engine compression test when the car is telling you something?

as for the orginal poster if the car started fine when it was cold out then i almost wouldnt bother with the test. if you had head the shaved then I would bother.