Low Cylinder Compression

M

My Poggy

Member
Sep 12, 2020
44
Boat Info
36 Sedan Bridge (2007)
Raymarine E120, radar, bow/stern thruster
Engines
Mercruiser 8.1
Hello,
I have a 36 Sedan Bridge with two Mercruiser 8.1 engines. One of the cylinders gave a very low compression reading (50 psi). Is this reading more about a broken ring or a valve that doesn't close? I am optimistic that a broken piston ring will not give a very low reading and hope that it is a valve that remains open. If it is a valve, what ballpark figure am I expected to pay? I plan to find a mechanic who can do leak test to confirm. But looking ahead, if it is a broken piston ring, what can I do short of removing the engine from the boat, which is a horrendous scenario? Is it possible to disable that faulty cylinder by turning off fuel supply to it? Or can I still drive the boat with less power but without going into limp mode. Thank you for any advise.
 
A wet compression test and a leak down test are next for you.

If the low compression is solely from worn/broken rings, a few squirts of oil into the cylinder should yield higher compression pressure

A leak down test will tell you even more Info. With the piston properly positioned so that both valves are closed, you’ll be able to hear where your compression loss is
 
Thanks for the comment
Espos4 said:
A wet compression test and a leak down test are next for you.

If the low compression is solely from worn/broken rings, a few squirts of oil into the cylinder should yield higher compression pressure

A leak down test will tell you even more Info. With the piston properly positioned so that both valves are closed, you’ll be able to hear where your compression loss is
Click to expand...

Thanks for the comment. That is why I am looking for a mechanic that can perform those tests. Hopefully, it is something that can be fixed without removing the whole engine.
 
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If it’s a valve you can usually just remove the heads leaving engine in the boat - have them reworked and be back in business pretty quick and cheap - relatively speaking. Rings you are deeper in and are looking at pulling the engine and a lot bigger / expensive repair. It’s important to find out what exactly is wrong at this point before good time and money goes after bad. Also if it is a valve, almost always in a marine engine the cause is failing exhaust risers, so the the repair cost includes new exhaust risers and or manifolds. And, if the risers failed on one engine, the other one is not far behind.
 
You’re not going from 50 psi to 110 or 120 psi by squirting oil in the cylinder. You’ll either have to do your heads or get a shorty.
 
El Capitan said:
You’re not going from 50 psi to 110 or 120 psi by squirting oil in the cylinder. You’ll either have to do your heads or get a shorty.
Click to expand...

Yes, I am prepared for a head work if needed. But what do you mean by “shorty”? Short valve?
 
My Poggy said:
Yes, I am prepared for a head work if needed. But what do you mean by “shorty”? Short valve?
Click to expand...
Short block I'd assume. If you want to see inside the cylinder to see the valves, bowl and top of the piston, you can buy a cheap snake camera from Amazon. They connect to your phone and are pretty awesome diagnostic tools.
 
If the worst case scenario happens: the piston ring is defective, can I still drive the boat with decreased power or at lower RPM (maybe disabling the weak cylinder), or is there some serious consequence in doing so.
 
My Poggy said:
If the worst case scenario happens: the piston ring is defective, can I still drive the boat with decreased power or at lower RPM (maybe disabling the weak cylinder), or is there some serious consequence in doing so.
Click to expand...
Serious consequences if you have a cracked ring etc. You cannot disable any cylinders.
 
scoflaw said:
Broken ring is a rare thing. If the valve breaks and drops, you could toast that whole motor.
Click to expand...
If it is a valve that doesn't close well, can it be adjusted or replaced easily?
 
scoflaw said:
Not easily, the heads need to come off. You probably have other valves not far behind that condition.
Click to expand...
I am hoping that's the worst thing that happened (valve issue). I cannot afford to remove the engine from the boat. It will take a ton of work to remove everything in the salon just to give way for the engine retrieval. Crossing my fingers, hoping to find a competent and honest mechanic in my area.
 
Before declaring the engine dead pull the valve cover and look at the valves and rockers on that cylinder and make sure the valves are closing. You could have a bent pushrod or a broken valve spring which can be replaced without pulling the head. Even if a leak down test confirms a bad valve check the valve train.
 
becnme said:
Before declaring the engine dead pull the valve cover and look at the valves and rockers on that cylinder and make sure the valves are closing. You could have a bent pushrod or a broken valve spring which can be replaced without pulling the head. Even if a leak down test confirms a bad valve check the valve train.
Click to expand...

OK. Thanks for the advise.
 
El Capitan said:
You’re not going from 50 psi to 110 or 120 psi by squirting oil in the cylinder. You’ll either have to do your heads or get a shorty.
Click to expand...

I can’t imagine anyone replacing just the short block.

If it comes to be that there is a broken ring, get a long block.
 
It isn't a Sea Ray, but I bought a boat that had a bad cylinder. I did a compression test, and then a leak down test.
It ended up being a tuliped valve, super easy fix. So that's a possibility as well.
 
How long has it been since the risers where done? I have seen a lot of engine damage done from a bad riser. Is there any other symptoms such as milky oil or excessive blow by or a fouled plug? I had a exhaust valve stuck open from the leaky riser and unfortunately had to do a complete rebuild. Fingers crossed yours is a easy fix. Good luck.
 
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