How to conduct a compression test?

I have a compression tester but I don't know how to use it. I gather that all spark plugs should be out. Should the engine be hot or cold? Is there a pattern that should be followed with respect to which cylinders are checked first? Any other step by step advice? Thanks!

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E. William Reinecke
1988 Carrera Targa

I have always performed a compression test by removing one spark plug at a time. Screwing in the testing fitting, and cranking the motor over two of three times. Just remember to disconnect the coil wire so the engine gets no spark! As far as a specific order, the compression will not depend on the order the cylinders fire.

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Paul
78SC Targa

I always remove all the plugs, ground the coil, and pull the fuse for the fuel pump (if it's electric, obviously.) A remote starter switch is a big help here-just crank the engine over a few times while observing the guage, and record the max pressure displayed for each clyinder. Some people will say to hold the throttle open, some say closed, others go by firing order, some take a reading, then squirt a little oil in the plug hole to see if it helps the rings to seal and improve the compression reading-it really doesn't matter all that much, as long as you do the same thing all through the test. I use a cranking compression test mainly as a diagnostic tool to compare cylinders in an engine to each other-to see if one or more is much lower than the rest-finding out WHY it's lower is usually the fun part. A leak-down test is a better way to track down where pressure is escaping from a cylinder-hissing heard through the intake is an intake valve, through the exhaust is an exhaust valve, through the dipstick or PCV valve in the valve cover (on a wet-sump motor) is usually rings, and if you hear air pressure escaping at the thermostat housing or the radiator filler neck-well, then you've either got a blown head gasket-or a cracked block. Don't ask me how I know that one-it's a sad story. If a simple compression test shows all the cylinders to be pretty close (10-15% difference or so), then the motor is most likely (mechanically) OK-if the car has power or driveability problems, it's most likely igniton or fuel delivery related. Some of the newer electronic engine control sytems have a "cylinder balance test" as part of thier testing menu-the computer will let the idle stabilize, record the average RPM, then shut off one injector at a time, record the RPM drop, then compare them to each other and to the base idle speed. If one or more cylinders aren't really working well, then the drop in RPM when the injector is shut off won't be as significant as the others, and the computer will display a code showing which cylinder is the weak one. Damn useful-I found a weak injector that way in under 2 minutes, and it didn't cost a dime-nor did I have to remove all the injectors to have them tested.

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Clay Mcguill www.geocities.com/the912guy

[This message has been edited by ClayMcguill (edited 12-11-2000).]

Scopes (digital engine analyzers), in the proper hands, can tell you just about everything. I think they can tell whether the ash tray is too full.

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'83 SC

As a non-smoker, the ashtray is moot. Should the engine be hot or cold? Adjust the valves first? Any other advice?

Thanks for the tips so far!

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E. William Reinecke
1988 Carrera Targa

The engine should be freshly run at full operating temperature. The oil squirters assure plenty of oil on the rings and in the cylinder.

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Warren Hall
1973 911S Targa