Low engine compression, or is it?

[rob hod]

    I'm not sure whether this would be any help but i'll quickly relate my
experiences with a jag I bought a few years ago.

    I bought it with a damaged body from an old guy who only ever pottered
around in town , but it was an easy fix so I put on the road for a few
months before scrapping it. The engine ran ok but was low on compression and
smoky too. All the signs of terminal bore wear. Under the circumstances -
i.e. imminent scrapping, I removed the plugs, squirted WD40 into the bores
and cranked a couple of times, then left a couple of days , then repeated
the process a second time. Within a couple of weeks normal driving and with
a few excursions into the 4-5k region the engine had stopped smoking,
comperssion was back and
oil consumption down.

    The problem had of course been gummed piston rings, possibly due to
missed oil changes or the fact that the car never saw more than 1.5k revs,
or to the dreaded AED enrichment device fitted to those cars.

    The point I'm making is that an engine can give all the indications of
being knackered when in fact there is nothing *mechanically* wrong with it.
In particular its always been my experience that general *condition* of
piston rings/bore is more critical for good running than valve/seat
condition. I wouldn't know how much of a good idea using wd40 is on an
engine you intend to keep though.

    HTH

rob
----- Original Message ----- > Subject: Re: Low engine compression, or is
it?

> Subject: Re: Low engine compression, or is it?
>
> > 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
>