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.