Compression Testing
 

i am in the procces of rebuilding a oem603 turbo diesel engine with a replacement head. and before i complete the rebuild i want to compression test the engine. but the FSM manual only gives the hot compression test ratings and my car hasn't run in three years. so i cant do a hot test.

i was wondering if any one could tell me what the reading for a cold compression test would read, and if no one know (which i doubt) then if they could tell me where to look for them.

thanks a lot

cant do a compression test with an engine cold....

So get the motor running then do the test at operating temperature

Why not? Does the engine even run?

Plug in the block heater and get at least *some* heat in the block. Hook the engine up to two batteries if possible, or use heavy (2-gauge) jumper cables to a running vehicle - you'll need lots of power for all the cranking you'll be doing. Crank each cylinder until the gauge stops increasing, usually at least 6-8 compression strokes, maybe more with a cold engine. Repeat the test a second time on all 6 cylinders, average the results.

While the raw numbers will be mostly irrelevant when cold, you're looking for equal pressure between cylinders, within 3 bar from highest to lowest. I would definitely try to get the engine running so it can be hot for the test. Even cold, though, it should be well over 20 bar (spec is 26-32 bar hot).

:boat:

Of course you can do a cold test. How relevant the numbers will be is questionable, probably within 10% of hot, but at least you'll have a base line to compare from cylinder to cylinder. If there are big differences you know you've got a problem. If they're close to each other then you'll have to determine if the numbers are acceptable. Maybe someone that has done a hot & cold test could chime in with the difference. If you've got hydraulic lifters, make sure you crank enough to pump them up before comp test or valves may not be opening enough to give you accurate numbers.
I bought a 300TD (617) with 338K miles and a rod knock. Didn't want to damage the crank/rods by warming it up so I did it cold. 370, 350, 320, 340.320. I deemed that acceptable considering milage to try to save the bottom end, rather than tear it completely down, so cold tests do have some merit. Hot test will always yield higher compression if everything is adjusted right and functioning properly.

I would crank the Engine over with out the Glow Plugs in so that it would turn over faster and easier and I could get some Oil circulated to the parts and maybe this would be enough to spray some Oil up to the Pistons and Cylinders (if your engine has srayers).

However, rebuilt Engines can be tight and not crank nearly as fast so I am hope you will report back on your progress.

During a compression test, either all injectors, or all glow plugs are removed during the test. That will allow the fast/easy cranking. On the early OM603, usually the injectors are removed - make sure you use the injector heat shield (seal to prechamber) under the tester or it won't seal properly. And remove the other five, or they will blow up in the air when you start cranking the engine (don't ask how I know this).

:stuart:

you do a compression test when the engine is warm because the piston rings will be expanded and sealing the best giving you the most accurate results. You can do a compression test with a cold engine but it wont give you really accurate info.

as with anything there are multiple ways of doing something but only one way to do something properly

Why aren't you supposed to test compression cold?

Everything expands when hot - the block, pistons, rings, etc. The expansion makes for a tighter seal in the combustion chamber. When cold, the compression numbers are usually quite a bit lower. The FSM specs assume a hot engine (or at least warm) to get the most accurate number.

:scholar:

Torpedo heater or lower hose heater and aux coolant pump.

Heating torpedos... dang, that sounds dangerous. Don't those things explode?

:joker:

Using an external heat source is going to have little to no effect. Normal combustion chamber temps far exceed anything you can induce with an external source. Many of the internal parts will never reach "normal operating temp".

So it's a few hundred degrees. You park it. Wait an hour till it's cool enough to work on, pull the hard lines, pull the injectors, wire the remote starter . Coffee at work or beer at home, 3-5 minutes / cylinder. Head block and coolant temperature will be more like bath water.

No need to heat it for the test. Just make sure oil is in the cylinder. The readings clearly won't be perfect but should indicate any abnormalities.

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.