Low engine compression, or is it?
Jonas K.
jkarlsso at metabolex.com
Fri Jun 22 15:59:48 EDT 2001
> From: "Chuck Schott" <schott at abam.com>
> Greetings Listers,
>
> Per repair shop yesterday, the compression on my 4kq is 155psi(40psi
> leakdown) on the #1 and 120psi(20psi leakdown) on the rest.
Well, that's a 22% drop on all but one cylinder. Unusual, as wear is not usually
that even. Oh well, not a big problem. I'm confused by the leakdown numbers.
That seems really high. Usually you'd like to see less than 10% leakdown (10psi
drop with 100 psi in). Also the higher leakdown drop indicates more wear, but
this cyl had the highest compression. You might consider another check.
> Should I crank until pressure stops going up, or just for a certain number
> of cranks?
Huw said it, 5-10 cranks, WOT, go through all the cylinders 2-3 times and
average it out. Don't forget to disconnect power to the coil, and to pull the
fuel pump relay. All plugs should be out for the test. If possible, have someone
else crank it while you watch the gauge. That way you can observe _how_ the
pressure builds up. For instance, if most get up to 90% of pressure after 2
cranks, but one takes 4 cranks to get there, they you know there's an "issue".
> Can I check "leakdown" with a basic compression gage (pressure release
> button on side)? How long do I wait?
A leakdown tester is a slightly more complicated instrument. Basically, you
measure how fast air is leaking out of the cylinder. The way you do this is that
you connect a device with 2 pressure gauges on it. The 2 gauges are separated by
a restrictor to limit air flow, and one end of the device is screwed into the
spark plug hole, while the other is pressurized (usually with 100psi). Because
of the restrictor, the air leaking out of the cylinder will create a pressure
drop. You record the difference in pressure of the two gauges and read this out
as a "percent drop". At the same time, by listening at the intake, exhaust, and
oil filler cap you can better narrow down where the majority of the leak if
coming from.
HTH,
Jonas
90 CQ
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