Cold or warm compression checks..or both?

Got the factory manual CD...200dpi scans...it sucks a little there.

My question is...Mercedes recommends 80 degrees C for compression checks...all of mine were done at stone cold (why we got rid of one car)...So, which one? As is predictable, the stone cold readings are dramatically different from the 80 degree readings.

While I have not yet done a comparison on the cars we still have, on the one we got rid of, the warm one was at 400 as opposed to 190 cold (on the cylinder that was low...the others were over 350). What does this indicate (a leakdown got on #1 got the compression to 400+...would seem to indicate rings.)?

After valve adjustment and all, my '85 is cold 320 on all but #5, at 290. The car starts horribly below 20 degrees F. (Two glow plug cycles are the norm..and even then it's dicey.) My son's car, a '79 non-turbo is at (cold - 40-60 degrees F..tested in spring/summer) 375-340 cold and starts great.

My 300TDs glow plugs are cheap autolites (not me), the injectors do nail (will be able to do that stuff as soon as I order the pump from here..have the nozzles and shim kits from this site already).

Thanks for any help.

-Nathan

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Diesel Engine Specific Aid

Chassis Model Number

W123

Diesel compression tests

Hi Nathan, when Mercedes wrote their service manuals they were directed toward factory technicians working on fairly new diesels in a white coat service facilities. I have a hunch they were not thinking that so many owners 30 years later would be driving and repairing these old diesels with 250,000 plus miles.

For an engine with less than 150,000 miles a hot cranking compression test would be more than adequate since the engine has probably not yet experienced excess cylinder wall wear. For those engines any problems then would show up as broken rings or bad valve seats. (From my experience of working on and tearing these engines down, cylinder wall wear is the killer). When a cylinder has excessive wear it is possible for the rings to still be making a contact seal due to the aid of engine oil on the rings when the engine is running or has recently run. When you let the engine sit overnight the oil will drain away from the rings (due to cylinder wall wear gaps), thus dramatically lowering the piston rings ability the create enough compression to ignite the fuel air mixture.

Of course it ok to do both tests as you have done, but to determine the health of an high mileage engine the cold cranking test Is more important. (Since all of us who have worked around diesels know if you can get it started it will go and probably keep going). If you have cold cranking compression of over 300 pounds on all cylinders and the engine still starts with difficulty when in the 20 degree to 32 degree (F) range, then you should be looking at the possibility of another problem.

These would include:

Weak battery
Weak starter motor
Intake air restriction
Exhaust restriction
Fuel restriction (filters, injection lift pump, injection delivery valves)
Engine timing (timing chain)
Valve clearances (615 616 617 Engines only)
Injection pump timing
Fuel injector release pressure and spray pattern
Glow plugs and Glow relay
Excess carbon in pre-chambers (pencil glow plug engines only)
Quality of fuel