Compression testing needs to be carried out with the engine warmed up and with all plugs out... but... ...the plugs should only be removed when the heads are cold, or I risk damaging the threads.
Maybe I'm being overcautious here but what's the correct procedure?
not according to vw-resource....
"Note: The compression test should be done with the engine cold."
Ha! You found it; d'ya know that was the ONLY place I found that suggested that compression testing should be done cold. And its normally such an excellent resource, hence my confusion.
I couldn't find any other place that describes the procedure, and my manuals are not accessible at the moment.
Anton
Dangermouse wrote:
Cheers Anton. Google is indeed my friend and tossed up
Just wondering what applies to avvws?
Both.
A cold compression test may have slightly lower readings due to reduced battery strength and heavier oil.
A warm compression test can be easily performed with no danger to the sparkplug threads if you bunt them all slightly loose when cold, snug them back up lightly, run engine briefly and then perform your test.
There is a great forensic advantage to knowing both cold and hot results, a reduction in hot compression results can signal a tight valve. . . as can the lousy idle that crops up warm but is not present when cold.
I use a dab of copper anti-seize on new plugs (keep clear of internal combustion chamber surfaces of plugs) and have never galled a thread. Colin
hm, now I am intrigued by this topic. Perhaps Jim or Jan can chine in on why the discrepancy? I can only guess cold is due to the fact that you can strip sparkplug threads.
Ant> >
The pistons, rings and cylinders seal properly only after they have warmed up and EXPANDED to the size and shape they are supposed to be run at. When cold, pistons are not round... you'd get more blowby. Also the tolerance between the aluminum piston and steel cylinder gets tighter as the engine warms up. Aluminum expands much more than steel when heated.
Jan
ant> hm, now I am intrigued by this topic. Perhaps Jim or Jan can chine in
very nice that Jim and Jan are so highly respected...they are both very knowledgable....
now a comment from a peon... the warm compression test is the most accurate due to the engine tollerances (cold) being set so they will have optimum clearances HOT..... so cold is a bit lower.....more akin to an engine with some use on it....(IE wear)....
most folks do not do a compression test properly anyway, so results vary.... most(including myself) don't prop the throttle wide open during the test. You should only crank the engine three or so compression cycles, but most crank to the "highest" reading on the gauge...which is somewhat fine, however if it is reported (for instance to RAMVA) that you have 125psi on all cylinder, we might conclude that the engine is "fine"..... however adding that two of the cylinders built that pressure in a few cycles while the other two took fifteen cycles might lead us to possible worn rings..... enough of me...back to lurk mode...
damn....i should retrieve new posts more often if i'm gonna lolly-gag when reading...
Agree... I let the engine turn as many cycles as it takes to reach maximum pressure, but I count how many it takes for each cylinder to reach it. Sometimes one or two may be slower to build pressure, indicating a problem.
Jan
My $0.02 Possible rumor but.................. When removing a spark plug sometimes a piece of carbon may fall off and possibly get lodged inbetween the valve and it's seat. This also may affect the compression readings. ;-)
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