Re: cyl. pressure test, what to results mean? CJ
I wonder if your results are accurate on such a new engine?
To get accurate meaningful results from a compression test
you have to do several things first.
Make sure your battery and starter are in good shape - cranking
speed does affect your readings.
1. Remove ALL spark plugs so the engine turns freely.
2. Block the throttle open so air can get in the intake.
3. When spinning it over, you can feel the pulses, do each
cylinnder the same number of pulses - usually 3 or 4.
Watch the gauge as you spin it - Somewhere around 80% of the final reading should happen on
the first pulse.
The second pulse should come very close to the final
- That's on a fairly new engine.
- An older worn one may build up a little slower but should
still reach max in about 5 pulses.
Then, after you have good readings but you have some low ones,
then do the "with oil" test:
If you are not 100% sure of your reading, do it again, and
even again 'till you are.
One cylinder at a time - the low ones - squirt a shot of oil
in the chamber - 30 wt preferably. Just one or 2 shots, not a
whole bunch. (Oil takes up space, too much will artificially
raise the compression leading you astray.)
Do not squirt oil in the other cylinders until it's their
turn to be tested.
Spin the engine over 5 or 6 times to distribute the oil around
Do the compression test on that cylinder, using the same number
of pulses as you did before, throttle open etc.
Now do the next one. Do it one cylinder at a time.
Doing it that way will give you meaningful results.
There's been lots of engines "rebuilt" or worked on when the
only problem was the test wasn't done right.
And - if you have been having a misfire for any reason, like
defective ignition or mixture problems, even the above procedure
Reason being the misfire washes the natural amount of oil off
the rings, lowering sealing, causing your first test to be
inaccurate. Again, lots of engines have been rebuilt due to a
bad plug wire.
The cheapie "push in and hope it holds" type compression
gauges are really difficult to use, hard to hold in, hard to
watch while cranking, and the leakage around the rubber can
Best kind is a screw-in type with a hose.