compression test help

[gparadiso]

hi i have an omc gm 305 5.0l and checked compression . results were 8- 145# 110#-7<br /> 6- 155# 35# -5<br /> 4- 0# 145#-3<br /> 2- 150# 120#-1<br /><br />Seems strange, engine idles and sounds great; problem i get is slight backfire thru quadrajet at high rpm and max rpm are at 3100 which i think is about 1000rpm shy of recommended WOT rpm. Wouldn't the engine run really rough with 1 cylynder having no compression and one really low. I'm trying to do diagnostics myself and any ideas or suggestions would be great.

 

[DJP]

Re: compression test help

Are you absolutely positive those readings are correct ? Are you turning the engine over by hand or with the starter ?

 

[iwldntli2u]

Re: compression test help

Do a wet test and tell us your results. This does not sounds right?

 

[flashback]

Re: compression test help

sounds like you have problems in 4 and 5 and yes it would backfire.................pull the heads and get a valve job, then tell us what kind of boat you have before you put it back on the water.......good luck.............

 

[gparadiso]

Re: compression test help

i'm sure readings are correct, in the low readings i re-tested three times with same result. Was told to try adding some oil into spark plug hole of the cylynder with no compression, did that and cranked engine several times but still guage didn't move. When doing test i am warming up engine, remove all plugs, remove coil wire and have throttle to wide open position. is this correct? What is a wet test?

 

[gparadiso]

Re: compression test help

sorry...am turning engine over with starter....boat is a 21' Galaxy

 

[akriverrat]

Re: compression test help

center rise manifolds?

 

[gparadiso]

Re: compression test help

no...end risers with log style manifolds

 

[akriverrat]

Re: compression test help

have you pulled valve covers to check valve lash and lift and if any rods bent?

 

[jlshields]

Re: compression test help

gems, when you added oil thru the plug hole then checked comp you did a wet test. With this test if you have bad rings you will see an improvement in readings. Since you saw none sounds as if you have a valve problem. If you have access to a vaccuum gauge run that test. Between the two tests you can pretty well identify the problem. John

 

[gparadiso]

Re: compression test help

no haven't pulled valve covers yet...<br />how do i do a vacuum gauge test.. i have a gauge but there are no vacuum ports on engine...

 

[jlshields]

Re: compression test help

gems, my 5.7 has a plug in the intake manifold runner that goes to #8 cylinder. Remove the plug and install the adapter that came with the gauge. John

 

[flashback]

Re: compression test help

come on guys if he is showing 0 on a compression check he has a problem

 

[jlshields]

Re: compression test help

flashback, no question about that! We're just trying to help identify it. Why pull the heads when it may be just a stuck valve? john

 

[akriverrat]

Re: compression test help

1 and 7 are also too low. pull the heads.

 

[gparadiso]

Re: compression test help

am going to pull heads next week , hopefully if it ever stops raining......but have been told that it could be my camshaft is worn or lifters are concave...is it still best to just pull heads and get them redone and check lifters

 

[akriverrat]

Re: compression test help

manifold comes off befor the heads. you can at least inspect lifters befor continuing. intake has to come off to do anything with cam. but really, if they were that bad, there would be a lot of valve train noise that you have not mentioned. yet.

 

[gparadiso]

Re: compression test help

took heads off and found valve on #4 had small crack on it, put pair off reconditioned heads on and torqued to specs...now i need to do the valve lash adjustment ...is this relatively easy or should i hire someone to do it.. any suggestions or methods of doing it would be great...Thanks