Premature Deal Breaker

Posted: May 1, 2013  |  Tag: Engines

By: Grid Michal

We were getting ready to buy a Grady-White with a 5.7 MerCruiser. A friend suggested we get a compression test done before we buy, so my husband bought a compression gauge at the auto store and performed the test with the owner present. I won’t go into his getting the plug wires mixed up afterward, but what he did find was that the compression was well under 100 pounds per cylinder, which killed the deal. The owner had showed us that the engine ran well, and it did again, after the correct wires were on the correct plugs. How is this possible?
Well, it could run well but still be worn internally, or have carboned/leaky valves and/or seats. But, for the uninitiated, in order to get an accurate reading, you have to warm up the engine, ground the firing system, open the throttle all the way and test the compression in that manner. You may have canceled the deal prematurely, but that’s an answer from afar, without seeing the whole shebang.

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