Compression test results

[rubberduck]

First thanks to all you responded to my other post about a good test reading of 140 - 150.

Just completed the compression test on my Volvo AQ125a. The readings were: 105, 100, 75, 125. How bad are these readings?

The boat needs a transom and floor and I don't know if I should go ahead as planned or "patch" it up and try to get a couple of seasons out of it. The patch I'm thinking of would be a transom plate and another layer of plywood over the floor.

thanks i advance for the help.

 

[mikeneal]

Re: Compression test results

What you are looking for a consistency among the cylinders. The 75psi is far away from the others...to far. You can add some motor oil to the cylinder and re-test that one. It will band-aid the rings and the pressure will go up. If stays the same it is in the valves. A leak-down test is better judge but I would worry those numbers say motor has seen better days and needs refreshed.

 

[bjcsc]

Re: Compression test results

OK, is it safe to assume that you did the testing correctly, i.e. ran the engine up to operating temp. first, cranked it for 4-5 compression strokes (16-20 x 360&deg)? If so, then your next step is as Mike described. Volvo specs are that the lowest cylinder is within 75% of the highest. Your 75 is 60% of 125. You want to warm it back up and retest it. If you get the same results, I would skip the oil test and just go ahead and leakdown test that cylinder. You should be able to hook an air chuck to the hose you used for your test when the gauge is removed. Run about 15psi. into the cylinder when it is at TDC on its compression stroke. If you're not sure, you can verify it is by feeling for the piston through the plug hole with a piece of wire. If you can hear air in the valve cover (through the oil fill hole) it's the rings, through the exhaust - the exhaust valve, the carb. - the intake valve. If you don't have an air compressor, then the oil test (about a tablespoon in the low cylinder) would tell you whether it's the valves or rings. The leakdown would tell you whether it was was the intake valve, exhaust valve, or rings.

 

[Don S]

Re: Compression test results

With the boat in that bad of shape, and the engine that bad, I would seriously consider looking for another boat.
Any parts for that engine is expensive, and from your discription, will cost more than the boat is worth.
I would also bet a bad exhaust manifold is partially at fault (They don't last forever) and that alone is over $700 from Volvo, just a head gasket is 60 bucks.

 

[rubberduck]

Re: Compression test results

What would happen if I left as is? Would the lower compression just result in a loss of horsepower or would the engine eventually fail to start/run?

 

[Don S]

Re: Compression test results

What would happen if I left as is?

Here we go again.

Of course it will eventually fail to start and run. They don't heal!