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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.
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