Mercruiser Mag 350 Ski MPI. Low compression on three cylinders.

[Britpilot]

Engine is a Mercruiser Mag 350 Ski MPI with 300 hours since new connected to a v drive in a Tige ski boat. Engine is down on power & idles poorly. Installed a new set of plugs and carried out a compression check. Number 2, 3 & 5 cylinders only show 50 psi all the rest are at 170 psi. Squirted some oil in 2,3 & 5 and tried the compression check again. Number 3 & 5 came up to 120 psi, no change on number 2. Stuck rings on number 3 & 5? Valve problem on #2? What to do next? Any of the ring cleaning additives like Mercury powertune or Yamaha ring free work?

 

[Oshkosh1]

You can use some Sea Foam but I'd do a leak down test. Sounds like it may have overheated and burnt #2 exhaust valve. Ensure #2 is fully closed and add some compressed air to the cylinder and listen to the exhaust. If you hear it hissing and see the pressure dropping you've at least solved that. The ring issue could be more catastrophic and require a rebuild. If it was running well and this just happened I doubt 3 sticking rings...as a matter of fact, that won't happen.

 

[HT32BSX115]

Squirted some oil in 2,3 & 5 and tried the compression check again. Number 3 & 5 came up to 120 psi, no change on number 2. Stuck rings on number 3 & 5? Valve problem on #2? What to do next? Any of the ring cleaning additives like Mercury powertune or Yamaha ring free work?

Howdy,

Welcome aboard!

Oil will almost ALWAYS improve compression in an engine with worn rings.

2-Stroke stuck rings will sometimes free-up with a 2-stroke snake oil but 4-stroke engines don't "stick" rings very often. I am kinda with Oshkosh1, I doubt you have "stuck" rings, and it's unlikely any Mechanic-In-Can will fix your problem.

Stuck valve(s) maybe..........rust in the cyls maybe too..... do any of the old plugs have rust on them?


What kind of vehicle do you "Britpilot"?

Regards,


Rick

 

[Britpilot]

What about head gasket between #3 & #5? No sign of water in the oil though. Thought of carrying out as leak down test on the weak cylinders. I am familiar with the procedure, I do it on aircraft engines. Vehicle = Corporate Jets by the way.