Mercruiser MAG 350 MPI 2005 -No compression on one cylinder

[gcarmich]

I have a 2005 MAG 350 MPI engine with a Bravo 3 outdrive both with about 350 hours on them. This is on a Bayliner 275. Late this summer season I started experiencing performance issues. I found the boat would not get on plane and WOT was about 3200RPM. At first I thought the outdrive was not trimming properly but I did an compression test and discovered one cylinder has no compression and another cylinder on the same bank only has 65 PSI - the remaining cylinders have 150-190PSI. I took a look in the 0 compression cylinder with an inexpensive endoscope (pictures attached). There seems to be heavy carbon buildup - especially on the back of the valves. There also appears to be some rust? on the piston head. Fortunately the cylinder walls looked ok. I'm wondering if one or both of the valves are no longer seating properly? I also wonder if I have a gasket leak where cooling water is getting into the cylinder?

I'm curious as to what my next steps should be - I want to determine if there are any issues with the block or if the issue is just in the heads. Should I do a leak down test and try to determine where the issue(s) is?
Thanks,
Gil

 

[alldodge]

There is water drops on the piston and the rust all indicate a blown head gasket. Not to mention the 190 psi means water in those cylinders

 

[todhunter]

Yes, do a leak down test to determine where the compression is being lost to.

 

[Lou C]

Yep blown HG can let water in cyls and that will eventually cause valves to stick. If you fix it promptly you might be able to save it. Do not let it sit like that all winter though or you will be buying a reman or new engine in the spring!
Had the same problem 5 years ago and wound up installing reman cyl heads, new head bolts, new gaskets etc. Not a bad job actually. In these pix you can see if you look close I had a sticking valve on each head, they would loosen up after starting, it ran rough for a few min then smoothed out. Last pic of the head in the back of my Jeep you can see some of the valve stems are lower than the rest, stuck valves.

 

[racerone]

Remove cylinder head ---ASAP.

 

[Lou C]

yep you may need to break out the 1/2 drive electric impact gun, that's how I got mine off and didn't break even one bolt.

 

[Bt Doctur]

Inspect the tops of the pistons, I`m sure the one with no compression is missing the top edge of the piston.exposing the top compression ring That is caused by detonation

 

[QBhoy]

Got to be either as suggested earlier about cylinder head blown…or also as likely would be if it’s had water ingress from rotten manifolds…you can end up with sticking valves. For it to have 0 compression…it’s likely to be valve related, you’d think. Or total failure at the head.

 

[gcarmich]

Remove cylinder head ---ASAP.

I have a 2005 MAG 350 MPI engine with a Bravo 3 outdrive both with about 350 hours on them. This is on a Bayliner 275. Late this summer season I started experiencing performance issues. I found the boat would not get on plane and WOT was about 3200RPM. At first I thought the outdrive was not trimming properly but I did an compression test and discovered one cylinder has no compression and another cylinder on the same bank only has 65 PSI - the remaining cylinders have 150-190PSI. I took a look in the 0 compression cylinder with an inexpensive endoscope (pictures attached). There seems to be heavy carbon buildup - especially on the back of the valves. There also appears to be some rust? on the piston head. Fortunately the cylinder walls looked ok. I'm wondering if one or both of the valves are no longer seating properly? I also wonder if I have a gasket leak where cooling water is getting into the cylinder?

I'm curious as to what my next steps should be - I want to determine if there are any issues with the block or if the issue is just in the heads. Should I do a leak down test and try to determine where the issue(s) is?
Thanks,
Gil

Thank you for all the insight. It is currently wet slipped but I'd like to bring it home to work on the engine. The ramp is about 1/4 mile from the slip - can I start the engine and motor to the ramp or should I get a tow instead? I don't want to make things worse.

 

[alldodge]

Check the oil, and if its milky get it towed, or change oil and move it

 

[gcarmich]

Oil still looks good