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Re: Compression Testing...

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03-05-2013 10:50:19

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Tom in MO said: (quoted from post at 09:21:29 03/05/13) Is there a thread on this site, or on another website, or a book, that has detailed, step-by-step instructions for testing compression? (On a Ferguson TO-20, in my case.)

I"ve never done it, but I can read and follow instructions, and it doesn"t sound too difficult.

Are gauges usually available for rent/borrowing from auto parts stores? I have a local parts store, and an O"Reilly"s nearby.


1. Remove all spark plugs

2. Be sure fuel is shut off

3. Attach battery charger to battery (the test can be skewed by lower battery voltage, esp on the last cylinders you test)

4. Set throttle lever to full open

5. Screw the gauge in and crank the motor over

A compression test is more of a comparison test between cylinders rather than a pass/fail kind of deal. The maximum pressure is a determined by things like compression ratio, speed of the starter motor, ambient temperature, and other factors. You want to compare the readings of the cylinders to each other rather than looking for a specific number, although anything less than 90 psi does indicate a problem. A 10% or less variation between cylinders is ideal, but the motor will still be OK with anything less than 20%.

If you have two cylinders with lower compression that are side-by-side, that usually indicates a blown head gasket between those two cylinders.

If you have a much lower reading with a dry cylinder than a wet one, you probably have a ring seal problem.

You can use a two-gauge cylinder leak down tester for more meaningful results.
Remove the plug for the cylinder you're testing and bring the piston to the top of the compression stroke. Install the tester and shoot air into it. Don't put too much air in or the piston will be pushed down by the pressure, but get at least 50 psi. You may have to hold the crank nut to keep the piston up.
Compare the readings between the two gauges. One gauge will read the air pressure you're applying to the cylinder, the other gauge shows how much the cylinder is holding. I.E., if you're putting 50 psi in and the other gauge reads 40 psi, you have 20% leak down. Another benefit of this test is you can tell where the cylinder is leaking. If you feel air coming from the exhaust pipe, the exhaust valve isn't sealing. Same for an intake valve if you feel air coning out of the carb. Pull the dipstick and, if air's coming out of it, the ring seal is suspect. If the head gasket is bad, you can feel the air coming out of the adjacent spark plug hole if you pull the plug out of that cylinder. Sometimes you can find a cracked head using this tool. If the crack's large enough, you'll see air bubbles coming up in the coolant when you remove the radiator cap and look in the radiator.
Hope this helped,


EDIT: Jason types faster than me. :D Good info from him, too.
This post was edited by Ark68SS at 10:52:37 03/05/13.

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03-05-2013 14:35:37

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 Re: Compression Testing... in reply to Ark68SS, 03-05-2013 10:50:19  
I always disconnect the air cleaner at the carb so that a partially clogged air cleaner does not affect the results. I would also conduct the test with the engine at operating temperature and would also conduct the tests both dry nd "wet"(add several ounces of motor oil to each cylinder and repeat the test) recording both numbers.

Very low compression readings are generally indicative of a stuck open or burned valve. Low pressures that don"t improve from dry to wet are generally indicative of leaking valves.

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