How do you do a compression test? - Page 2 - PeachParts Mercedes-Benz Forum

katchung · #16 10-08-2009, 09:23 PM

Quote:

Originally Posted by vstech

there are several different ways to test compression.
as long as it's noted which way it was tested, the results are valid.
a cold engine will have lower compression numbers than a warm oil soaked engine will.
a quick way to see if the motor has excellent compression on a to buy trip is to insist the motor not be started prior to your arrival, and by hand VERIFY the motor is stone cold.
then, unplug the glow plug harness, and crank the motor. if it starts quickly, it's got good compression. if it will NOT start, (unless it's below 30°F outside) the motor has some compression issues!
a cold compression test (done with ALL injectors out, and the IP wired to shut off fuel) will reveal unequal piston compression.
not a valid test to get compression totals though. need warm engine for that.

Thanks. That's really helpful. I didn't realize these engines would start without the glowplugs. Mine won't...but perhaps I have compression issues.

79Mercy · #17 10-08-2009, 09:50 PM

come on now, the compression will be lower after all these years, even if it doesn't start without ghlowplugs doesn't mean its got very low compresssion. My 1976 will not start without glowplugs, it fires in couple cylinders but not enough to get it going.

lonewolftek · #18 10-08-2009, 10:34 PM

I can't imagine why anyone would want to do it through the glow plugs, unless the 4-cylinder engines have far easier access than my 5-cyl.

I tried to do a hot engine test once through the glow plug holes; after the first two were out, the engine had cooled off significantly. It was far easier to pull the injector hardlines, driplines, and the injectors themselves, IMO. This assumes you have a socket that will fit the injectors without damaging them. I put my Mityvac on the fuel shutoff to keep it from feeding fuel.

I need to buy a new compression test kit now. My old one died when I ran the truck over it.