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
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
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!
Donald Harris is a former mechanic and used car dealer. He
continues to tinker under that ol' shade tree. Drop by his
website at http://www.carstrucksandtractors.com for car repair
tips and links to parts, manuals and classic cars, trucks and
tractors. Also visit http://www.mymodernlife.com for advice
on living a modern life without the modern headaches!
© 2005 - 2007
Free Online Auto Repair Tips & Guide
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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.
The Compression Test
Copyright © 2004, Donald Harris
Engine Repair Topics:
Engine Compression Test