Low compression GM2.5L 4 CYL OMC

[asbennett]

Hi - My 1982 OMC 2.5L GM 4 CYL has had positive pressure in the valve cover. Open the oil fill cap and you feel puffs of air while it is running. I tested the compression and found 1st and 2nd CYL fine (130 psi). 3rd CYL. was low at 122 psi. 4th CYL was even lower at 97 psi. <br /> Question is this: Being a 21 yeal old 4CYL engine, should I just run the thing and live with a little oil consumption or will there be "unseen" damage that I should have the engine rebuilt now?

 

[WSUDERMAN]

Re: Low compression GM2.5L 4 CYL OMC

Is this a new condtion? Or has it been this way for a while?<br /><br />The 122 cyl is well within the range and 97 is 25% which is questionable. Might be potential valve trouble. A vacuum test and leak down test might help to narrow it down.<br /><br />The only risk you take by continuing to run that engine is from either valve or ring fragments that could break loose fall into the cylinder bore and scor it. If the damage was bad enough it might not be repairable and you would need a new block. That's the worst case i can think of, (other than getting stuck out on the water!)

 

[Boatist]

Re: Low compression GM2.5L 4 CYL OMC

OMC Stringer<br />First thing is to try and find out problem. Squirt some oil in number 3 and 4 cylinders and recheck compression. If you have a bad exhaust manifold or riser leaking water into cylinders then number 4 is the first to go as it is lower and water run to it. If when add oil compression come up to normal then probably bad rings. If only rises a little then I would supect valve problem. If looks like valve then loosen lifters on that cylinder a little 1/4 to 1 turn. If it is just a stuck lifter then can run like a solid or replace lifter. If neither oil or backing off lifter makes no difference then probably a burnt valve. Pull the plugs and check for rust on spark plugs or in cylinder also see if number 4 plug looks cleaner like washed with water. If it is just a stuck lifter preventing valve from closing tight and you run it that way then you will burn the valve right away. With bad or open valve motor should have a low speed miss but be better at higher RPMS.<br /><br />If you have never changed the exhaust manifold and riser then you need to keep a close eye on them. Only last about 6 year in saltwater if you always flush.<br /><br />With oil fill plug out normal to feel puffs, remember this motor has no PCV valve.<br /><br />I have a 1980 OMC 140HP 3.0L 181cubic inch GM stringer engine.