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Valv

How to check compression on motors

2 posts in this topic

You can check compression yourself or have a shop do it for you.

It's a very simple procedure, go to Walmart (or any auto part store) buy a compression tester for $ 30.00 (automotive section), take all plugs out, thread tester in and crank motor keeping WOT (Wide Open Throttle), repeat process for every cylinder and record the results, you don't want anything more than 10% difference in between. The compression can vary from motor to motor, you can see from 85lbs to 175lbs depending on motor and year, but below 85 it's very suspicious. Now, if you find a cylinder that is almost flat, try to pour a little of motor oil in the hole, then test again, if it reads much more than previously (almost like the others) you MIGHT have stuck ring(s), caused by carbon, they will loosen up when you start motor.

This is normal, but very uncommon, 95% of the cases you have a bad cylinder...walk away regardless of how good is the deal (unless it's free) or how good loooking is the rig.

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Three things

1. Make sure to disable the fuel and/or ignition systems.

2. Be consistant. Usually a motor will make a distinct sound once every revolution. I usually give each cylinder at least three revolutions to show me what it has.

3. If you have two cylinders right next to each other with low compression and the oil trick doesn't work, theres a good chance the head gasket has failed between the two cylinders.

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