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compression check
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Posted by Jim Blake [Email] (more from Jim Blake) on Fri, 15 Feb 2002 08:08:33 Share Post by Email
In Reply to: burlyb: just read your response> Warm Missing, JohnE 92s, Thu, 14 Feb 2002 22:28:27
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Changing plugs is perfect time to do compression check. Inexpensive compression gauges have a rubber cone-shaped thing that you press into the spark plug hole while you crank the starter. Expensive ones have a fitting that screws in. Either way, they look like a dial-type tire pressure gauge.

With a warmed up engine, take out all the spark plugs. I usually pull the fuel injection relay or the fuel pump fuse to prevent injecting gas during the check. Put the compression gauge in a spark plug hole, have someone crank the starter. I'd have to check the book, but I think you want to have the throttle wide-open while you're cranking. If you use the rubber-cone type you'll have to press it hard into the hole, because the compression will try to push it out. Keep cranking till the gauge climbs to some constant reading, then stop & read the pressure. Do all 4 cylinders & compare readings. They should be consistant with each other, but check manual for YOUR year & engine for what the reading should be. Seems to me my '89 900 is around 190 to 200 psi. Typically the spec says they should be between ?? and ?? psi but all should be within ? of each other.

If one is way low, sqirt some oil in & try again. If it goes up dramatically suspect piston rings, otherwise suspect valves. Maybe someone can elaborate on that - I've never had one get low.

Let the engine cool down before you put the new plugs in. Cold plugs in a hot head can get them seized in so they won't come out next time. (That'll make your next tune-up really expensive.) Use anti-seize compound on the threads.

Of course, you will have made enough notes to get the new plug wires on in the right order...


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