Correct way to do a compression check?? - Mercedes-Benz Forum

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post #1 of 3 (permalink) Old 02-01-2016, 04:34 PM Thread Starter
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Correct way to do a compression check??

I just want to confirm the correct way to do a compression check on a 1970 M130 280 sohc injected motor.

first off does the engine need to be warm or can it be cold?

I take out all of the plugs. Is this correct?

Should I take out the fuse for the fuel pump so fuel is not flowing? Especially with all of the cranking.

I usually crank each cylinder about 6 times. Is this enough or too much?

Anything else that I need to do?

Thanks in advance
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post #2 of 3 (permalink) Old 02-02-2016, 09:28 AM
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Preferably at operating temperature or there about.

Plugs removed, injector power, coil wire or coil removed/disabled.

Spin engine using starting motor.

Once the compression hits maximum stop and note gauge reading.

Compression readings are approximate. Specifications listed are optimal with perfect conditions (engine at 180 degrees F, sea level, no miles on engine, yada, yada..). The main thing is you have the lowest cylinder within 80% of the highest reading cylinder and no adjacent cylinders reading the same low.

Example 1: Cylinder #1 150 psi, #2 145 psi, #3 147 psi, #4 130 psi.

Engine is weak but still functional (150 X .80 = 120). Your reading of 130 keeps you in the "usable" range.

Example 2: Cylinder #1 150 psi, #2 130 psi, #3 130 psi, #4 147 psi.

The low compression reading, even though it is within the minimum specs, on the adjacent cylinders could be an indication of a bad head gasket, warped head, etc.
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post #3 of 3 (permalink) Old 02-02-2016, 11:49 PM
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You also want to have the throttle wide open while cranking. It will raise the numbers across the board.
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