The Compression Test
Copyright © 2004, Donald Harris
Cars, Trucks and Tractors
http://www.carstrucksandtractors.com
Has your favorite mode of transportation lost some of its "zip"?
Maybe you think its time for a tune up and maybe you have had your engine
tuned up but it still doesn't have the performance it used to have.
Or maybe its just a little "nervous" when idling.
There are several things that can be the cause: bad spark plugs, bad
plug wires, worn distributor, leaking valves, or worn or broken piston
rings and the list goes on.
Its time for a compression test to determine if you have bad piston
rings or leaky valves. The pounds per square inch of compression varies
with each engine. If you want to know the compression specs for your
vehicle, consult your shop manual.
To run a compression test, you need a few basic tools.
First and foremost is a compression tester. With your tester, check
each cyclinder and record the results. There shouldnt be over 20% variance
between the highest and lowest reading. If there is a greater variance,
you may have internal engine problems.
To further isolate the problem, run a "wet compression check".
This is done by squirting a some oil in each cylinder (I use about a
teaspoon per cylinder). If the compression comes up to specs with the
wet check, you have worn or damaged rings and are looking at engine
rebuild. If the compression doesnt change, you have valve problems and
usually can solve your issues with a valve job. However, bad valves
and worn or damaged rings can be partners. So, when you pull the cylinder
heads, check for ring grooves and other signs of cylinder/piston damage/wear.
You should also consider the mileage on the engine. If it has high mileage,
many times a valve job will cause the engine to start using oil. If
there is evidence of wear in the cylinders you might want to re-ring
the engine as well.
The compression test may not help you pinpoint exactly what the problem
is, but it will help you narrow down the possibilities. And for that
alone, it's worth doing!
