Compression Test

[dirt1bike]

I have my 4.3L OMC Chev. engine out of my boat. I was told I had poor compression in cylinder 3. I had previously blown a head gasket due to a faulty water pump. When I removed the heads I had to untighten the rocker arm bolts to remove the push rods. If these were not reinstalled correctely, could that have given me a poor compression reading? Like a dummy, I did not due a compression check myself. I removed the heads & the valves look OK. Not open or burnt. The cylinders look OK, and the rings appear in tact. Can any one offer me some insight or suggestions?<br /> Thanks, Bart

 

[Trent]

Re: Compression Test

Bart. You are correct. Improperly adjusted rocker arm will give you no compression if they are too tight. <br />Before you ever remove a head do a wet and dry compression test. After you do a compression test and its low or none. Remove the sparkplugs and squirt some oil into the cylinder and rotate the crankshaft a little and then redo the test..If the compression readings go up its the rings (oil is sealing any gaps). If still low compression its the valves. As you stated check the compression with the rocker arm loosened before you pull the heads.<br /><br />Another good way if you have access to the gage is a differental compression check. You put the engine on Top Dead Center.. And inject air into the cylinder. If it leaks more than 20 percent (from the injected pressure) you have a problem. The good thing about this type of test is you can tell where the air is going. Out the exhaust, or pressurizing the crankcase..Which tells you what your problem is..rings/valves.

 

[dirt1bike]

Re: Compression Test

It appears that everything looks OK when I inspected the cylinders. Everything looked uniform, no scuffs on the walls, rings intact, valves OK. I now think that the mis adusted valves were responsible for the low comp. & start problems after the head gasket was replaced. I will re-install the heads & perform a comp. test before I set the valves. Does this make sense. Thanks in advance for any comments.