Question: What will a compression test tell me?
Answer: It will tell you if your engine has good compression.
An engine is essentially a self-powered air pump, so it needs
good compression to run efficiently, cleanly and to start easily.
As a rule, most engines should have 140 to 160 lbs. of cranking
compression with no more than 10% difference between any of the
cylinders.
Low compression in one cylinder usually indicates a bad exhaust
valve. Low compression in two adjacent cylinders typically means
you have a bad head gasket. Low compression in all cylinders
would tell you the rings and cylinders are worn and the engine
needs to be overhauled.
CHECKING COMPRESSION
Compression can be checked two ways: manually with a compression
gauge or electronically with an engine analyzer. The manual
gauge method is the only one available to most do-it-yourselfers.
To check compression, all the spark plugs are removed. The
ignition coil is then disabled or the high tension lead is grounded.
The throttle is also held open. The engine is then cranked for
a few seconds using a remote starter switch or a helper while
a compression gauge is held in a spark plug hole. The maximum
compression reading is noted, then the process is repeated for
each of the remaining cylinders. The individual cylinder readings
are then compared to see if the results are within specs (always
refer to a manual for the exact compression specs for your engine
because they do vary from the ballpark figures we quoted earlier).
If compression is low in one or more cylinders, you can isolate
the problem to the valves or rings by squirting a little 30 weight
motor oil into the cylinder through the spark plug hole and repeating
the compression test. The oil temporarily seals the rings. If
the readings are higher the second time around, it means the rings
and/or cylinder is worn. No change in the compression readings tells you the cylinder has a bad valve.
With electronic testing, a computer analyzer "estimates"
compression in each of the engine's cylinders by measuring slight
variations in engine cranking speed. The results correlate well
with actual gauge readings and can be completed in a matter of
minutes without having to remove any spark plugs. What's more,
the analyzer prints out the results of the compression test making
it easy to see and compare the actual numbers.
