Compression Test Results

T

Tom 47

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Feb 10, 2021
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I had the marina do a compression test on my Crusader 220 hp 305 cubic inch 8 cylinder engine. I do a lot of low speed trolling durning the boating season. The results are posted below.

Cylinder 1 160 Cylinder 2 125
Cylinder 3 135 Cylinder 4 130
Cylinder 5 110 Cylinder 6 135
Cylinder 7 135 Cylinder 8 150

The owner manual says that 130 psi with a range of 130 to 170 to be unifrom within 10 psi. between cylinders. The marina has indicated that the normal is approxmatelly 150 psi. The marina has recomended a valve job as they feel there may be burnt valves. What should the is the median compression for this engine.
 
did you run the engine under a load before the comp check? those numbers look like a check of an engine thats been sitting a bit you need to run under load to get a good valve seal then the numbers should be a bit more even
 
Yeah, heat it up and seat them in. I don't see where a lot of trolling is going to burn valves. There's nothing there that would make me sh*t my pants.

Maybe clean the combustion chambers and tell us what the plugs looked like first.
 
Did they run the test with the throttle lever fully down, and on a warm motor.

And do you have a base compression test to compare against, for all you know the valves were never set correctly from brand new.

So say the compression test comes back again after the above procedures are followed, and you post the results we can go from there.

Oh the mechanic who is so quick to recommend a valve job on the heads, should have his motive questioned.
 
I don't think its burned valves from those numbers, carbon build up in a few maybe. Like others suggested, run the engine under load to seat up the valves and do a HOT compression test with throttle wide open (and all plugs out) and see what the numbers are.
 
Thanks for the replys. It has been siting on the hard for the last 3 months. It was a cold engine. I will have to wait a couple of months when the boat goes back in. I will redo the compression test myself. I have more time now.
 
Why not do a vacuum test that will tell you a whole lot. If you don't how to read one google it.
 
I agree those numbers dont look correct, do it again after you run it under load for a while. I'll bet all will be well.
 
to had in. How do clean the combustion chamber with out pulling the heads ?
 
there are couple of ways to do it, some like to pour it in the carb while engine is running at idle speed or spray it in the throttle body if yours is TBI and or connect it to fuel line if yours is EFI, then shut it off and let it sit, others like to remove sparkplugs and spray some in the plug holes then crank it a few turns and let it sit for couple of hours. There will be lots of smoke coming out when you restart the engine.
 
Check plugs first to get an idea of the way the engine is running.

Then (the old fashion way) is to get water and mix tranny fluid or 2 cycle oil to help lubricate the combustion chamber while the water turns to steam under combustion and steam cleans the combustion chamber. Gotta race the engine to keep it running as the water mix is poured in.
 
why bother, lead free gas has significantly reduced deposits. Run the engine hard for a bit to clean out any carbon and dont worry about it.
 
i have an 33 egg harbor thats an ex charter boat with over 18,000hrs on 350 crusaders i,m a mechanic yet won,t do a comp test on her she is banging all 16 and both engines still purr i was amazed when it hit 35mph almost out of fuel with a rope around the prop just before we pulled it...some old boats will just amaze you! the only upgrade i did to them was delco est ignition kits. the carbs are wupped (Qjets) so i might add eldebroc's
 
Performing a compression check on a cold engine is a waste of time. How many hours are on the engine?
 
My engine had similar numbers. I ran it hard with a tank full of Chevron Techlorene. This cleaned it up and my compression was much higher thereafter.
 
A leak down test will tell you much more the a simple compression test.
 
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