Compression test help

[Dut Dut]

I have a 1986 Bayliner Trophy, with an OMC chevy 350, and Volvo outdrive. It was having problems getting past 9 or 10 knots. Last night I did a compression test and all the cylinders, except 1 and 3, were in the 120 range. 1 and 3 each read 25 psi. There couple be several problems, block, manifold, GASKET?, any ideas of what it may be. Baffled since it is two side by side cylinders.

My plan is to pull the boat out of the water, and to take the manifold off and take a look, but thought I would ask first.

 

[bsh21wash]

Re: Compression test help

Could be a couple of small holes in your pistons?

 

[Haut Medoc]

Re: Compression test help

Welcome to iboats......
You need to pull the head & look......​

 

[Don S]

Re: Compression test help

Before you pull the head, Do a leak down test. That will tell you if it's a valve, rings/piston, head gasket, etc. If you just take it apart, you won't be able to see bad rings.

 

[MikDee]

Re: Compression test help

Sounds like it could be a head gasket.

 

[Dut Dut]

Re: Compression test help

Don, how do I interpt the leak down results to tell what's what?

 

[thrasher]

Re: Compression test help

As its two cylinders side by side, chances are it's the head gasket that has failed between the two cylinders, but Don is correct that a leak down test will give you more information than just removing the head. Do you have the facilities to run a leak down check?
As its an old engine, you may find the rings are worn, although the 120PSI on the other cylinders seems to indicate the engine is in reasonable condition (a little bit low!). If you can't do a leak down test, then you have no choice but to remove the heads and see what you find..
You mentioned the manifold in your first mail, the manifold cannot cause low compression, you have either broken rings (or holed piston!), leaky/burnt valves, failed head gasket or cracked head, nothing else will cause low compression.

Gary

 

[Haut Medoc]

Re: Compression test help

Do the leak down test like Don says to isolate where the leak is....http://www.aa1car.com/library/leakdown.htm
Google "leak down test" & you will get a boatlaod of info on how to do it & how to interpret the results......​

 

[Don S]

Re: Compression test help

Basically you don't need a complete leakdown test. Just the air pressure in the cylinders. 50 to 100 psi should be plenty if all you want to find is why the 2 cylinders are seriously low. Just get #1 piston at #1 TDC (0?) on the timing pointer. Then add pressure to #1 cylinder using the compression gauge hose.
Then all you have to do is listen for the air leaking. If you hear it at the spark plug hole on #3, you can bet the head gasket is blown at least. If you hear the biggest leak at the carb, the intake valve is leaking, from the exhaust, the exhaust valve. From the oil fill cap, the rings or piston.

NOTE: A proper leakdown test will tell you if the rings are also bad on the cylinders with 120 psi compression. That is on the low side, so take that into consideration on your future repairs after finding the problem with 1 and 3

 

[rndcomp]

Re: Compression test help

When you are all saying that 120 psi is 'low', is that for this particular engine, or is there a standard for all cylinders? I have an 87 Merc 170 that had 125, 130, 125, 125 readings, and I was told that was great. Can 5 psi really make a difference?

Thanks in advance!

 

[mikeneal]

Re: Compression test help

Every motor type is different based on cam timing overlap etc., what you are looking for is correlation between cylinders, some say 10% some other places I have read 15%. Either way you are <5% difference so are in good shape.

When you are all saying that 120 psi is 'low', is that for this particular engine, or is there a standard for all cylinders? I have an 87 Merc 170 that had 125, 130, 125, 125 readings, and I was told that was great. Can 5 psi really make a difference?

Thanks in advance!