Lower compression in one cyclinder- Deal killer?
 

Hi Folks, I'll be test driving a beautiful 2004 Mariah with 259 hours on the 4.3 tomorrow. At my request, Seller took the boat to a local boat mechanic to have compression tested. The results are giving me pause.
Compression of 190 in all cylinders except for one cylinder with a compression of 125.
Certainly outside of the +/- 10% rule that allows us to have warm fuzzies. Since the boat has been run for only 1 hour since coming out of winterization, his mechanic said the issue could very well be a compressed ring, which "should" work itself out after being run some more. I've also read that lower compression in only one cylinder could be indicative of a stuck exhaust valve. My local boat mechanic suggested that I still go for my test drive and keep a close eye on all gauges and be mindful of how well it starts, idles, shifts, and performs. Surprisingly, He did not say that I should run away.
I'm looking for some advice here. Should I run some Ring Free through the gas for an hour and retest, just in case it's just some carbon deposit? I'll be taking my friend with me who is a aircraft mechanic, but he is certainly capable of running compression numbers. The price and condition of the one owner boat make this a deal that I feel comfortable with otherwise. But if retesting of the compression after the test drive don't show improvement, should I walk away even if the engine performs well?
**Good news, see follow-up post**

Re: Lower compression in one cyclinder- Deal killer?
 

Welcome!

This is just my opinion, but I would save the headache of trying to chase the low compression issue and look for another boat. The only exception would be if you could get it very very cheap and had the time to fool with it.

You could spend countless time and money trying to fix it. Usually low compression on one cylinder could indicate either a valve seating issue or a ring issue, like you said. It could be other things but those would be the most likely causes. One easy way to test would be to squirt a little oil in the problem cylinder then perform compression test again, if compression jumps up then you'd know there's a ring issue.

If you could get it cheap enough you could just throw another long block in it and be done. But just my opinion, save yourself the headache... there's other nice boats around.

Re: Lower compression in one cyclinder- Deal killer?
 

Thanks MTH. Owner will try to have a wet test performed today with oil added to bad cylinder in order to see if issue is simply a stuck ring. If that fixes the issue, I will test drive the boat and then recheck the compression. I've been advised to walk away if compression stays bad, but to proceed cautiously with purchasing if compression becomes uniform. Advice has been mixed, some say if ring was "compressed" by off season storage, it will happen again, and some saying it could be a one-time issue. I will keep you updated

Re: Lower compression in one cyclinder- Deal killer?
 

Wet tested,. Then taken to lake and allowed to run for a "while" then retested. Low cylinder came up to 165 (13 % difference) so tentative plan is to drive for an hour tomorrow and then check comp again. If it rises to 171 or higher (10%) I will buy. Thoughts?

Re: Lower compression in one cyclinder- Deal killer?
 

Just remember compression will be better with a warm or hot engine vs a cold compression test, so make sure you're still comparing the problem cylinder PSI when hot vs the good cylinders PSI when hot. Sounds like you guys are pinpointing it, that's good!

Re: Lower compression in one cyclinder- Deal killer?
 

If the reading does come up to spec, I would test it again after the engine has sat overnight, just in case the improvement is due to a crack in the cylinder or head that has temporarily closed up when hot.

Re: Lower compression in one cyclinder- Deal killer?
 

The boat did fine! I bought it.
Started quickly, with an even rpm sounds. Idle was a bit loping, but sounded good. Once warmed up and out of the no-wake zone, "holeshot" was amazing...AMAZING. Had to be on plane within what seemed like 2 seconds. Boat rode well and accelerated quickly, trimmed down it cornered well and ride was smooth and even. RPMS with a 19p prop stayed right at 4800 (boat had more in it, and I may consider a 21p prop, as I never left her WFO and trimmed up for long) Temp was steady at 160* , oil pressure was at 40. Obviously the PO took good care of the boat, as everything was in good working order. He seemed quite nervous as I and a buddy drove the boat, we are both Bass Boat guys and used to running at 75mph. I don't think he liked how we put her through her paces during the test drive. It will certainly take a while for me to get used to the "pleasure/cruise" state of mind where 40mph is considered to be too fast lol. But, I'm learning! Now for the compression test results:
Cylinder #1 153 psi
Cylinder #2 155 psi
Cylinder #3 150 psi
Cylinder #4 153 psi
Cylinder #5 158 psi
Cy;inder #6 150 psi
(Max spread of 8 psi or 6%) The compression test results were performed by myself and an aircraft mechanic using my mechanics Cornwell gauges. How the previous "mechanic" got such skewed numbers is beyond me, it almost cost his client a sale. Perhaps the numbers improved due to us burning out excess carbon which caused a valve to stick? I'm doubtful since he showed a 190 psi on five of the cylinders. Regardless, I'm proud to be joining your ranks. The SX9 is now parked in my barn.

Re: Lower compression in one cyclinder- Deal killer?
 

I assume you checked the condition of the oil right? Nothing muddy in there? The 4.3 has a reputation for leaking intake manifold gaskets that can cause water in the cylinders. I can't think of any other reason for temporarily having a higher than normal compression ratio. But in 5 cylinders ... maybe not. Still, I would check, as the fix is cheap and fairly easy but must be done before it gets hydro-locked and causes serious damage. Again... just a thought.

p.s. Was the engine run before the initial compression test? It's possible that the normal cylinder was the only one with no open valves while in storage.

Re: Lower compression in one cyclinder- Deal killer?
 

Thanks GaryDoug, yes we did in fact check the oil, but the PO had only put about 3 hours on the engine since having it serviced last fall. The oil was clear and clean.

Re: Lower compression in one cyclinder- Deal killer?
 

I guess I would check the appearance of the oil frequently, certainly before cranking the engine, for a while. If it starts to look "milky" then more inspection may be needed.