Compression Test Procedure on Mercruiser 120

[dezmond]

Hey Everyone, Plan to do a compression test on my Merc 120. Right now it is sitting on a skid in my shop at work. Could someone chime in on procedure to do this and what results I should be getting? All plugs removed when doing each cylinder? Thanks. I plan to do it tonight after work.

Ta Ta,

Dezi

 

[Scott Danforth]

all plugs removed
throttle open
install compression gauge in cylinder #1
crank over 5 times
read gauge
install compression gauge in next cylinder
crank over 5 times
read gauge
etc.........

on a cold engine, your readings will be less than they will on a warm engine
you should have about 100-120psi cold or more. however you want all 4 cylinders to be within a 10 psi window.

any higher than 150 psi and either the gauge is off or there is water or oil in the cylinder

 

[dezmond]

And The results are in

Hey Everyone.... Been busy at work and not too much time on the boat. I did however stay after work today so I could do a compression test on the motor to make sure I wasn't doing all this for nothing

Did Dry first and here is what I got...



Then sprayed some oil in all cylinders and did it wet and.....



Not really fond of #2 Cylinder.....

But the boat was never in the water last summer and was out of the water around August the year before. I don't know if that makes much a difference or not?

Would love to get some opinions please.

Thanks Everyone

Ta Ta

Dezi

 

[havoc_squad]

Do a leak down test of #2 and #3. You are looking for the amount of leakage and most importantly where it is leaking.

While you are at it though, might as well do leakdown of all cylinders to gauge engine health to see how long before the cylinders need oversized.

If you hear air from dipstick tube, that's piston rings, if it's air from exhaust or intake, that's valves.

If it's air from the cooling hoses, that's most likely a head gasket issue.

 

[dezmond]

Will do . Thanks Havoc

 

[Bt Doctur]

when doing a leakdown test also put a hose in the adjacent cylinder and stick it in your ear and listen for maybe a bad head gasket