Mercruiser 4.3 Blown Head Gasket

[bmcclean]

Hi All

Just checking in with the experts to see I have found the problem or should check somethign else.

So a 1992 Bayliner 2052 with Mercruiser 4.3 V6. Alpha leg. Sea water cooled.

Was running like a dream for 40 mins then after about 10mins at WOT a small knocking noise started, and got louder and louder and sounded very metallic (first thought big end bearing). I back off and the knock went away below 2000RPM but as soon as I was over 2k it started up and got louder and faster as the RPM increased. I only let it make the noise for 30 secs or so. I noticed it only did it in gear under load, if I rev the engine in neutral there was no knocking, so limped back to the ramp below 2k rpm and started to check things over.

I had done a compression test a few months before and was getting between 150-165psi on all 6 cylinders.
Did another test after and was down to 60psi on cylinders 2 & 4.

Risers and Manifold looked good little bit of rust but nice and black inside.

Pulled the heads and found the metal gasket ring had been pulled into the cylinder. See photo.

Anyway, my logic is the metal ring was heating up red hot and causing detonation in the cylinders hence the loud knocking noise.

I drained the sump and the oil looked clean no metal I could find, no water either.

So before I put it all back together with new gaskets etc what else should I check? I really don't want to get everything back and in the water to find knock is still there etc. I have check the heads and they are still flat no warping so I am not getting them machined.

Thanks in advance for any suggestions.

Regards

 

[achris]

Re: Mercruiser 4.3 Blown Head Gasket

You need to find out why the gasket blew in the first place. If you don't, it will just blow again.

Some of the things that can cause a gasket to go like that are (in no particular order)

Too far advanced timing.
Low octane fuel (stale/old)
Too much load/over propped. (what revs at WOT?)
Engine running too hot (check the obvious, and the not so obvious)

Chris.....

 

[bmcclean]

Re: Mercruiser 4.3 Blown Head Gasket

Thanks for the quick response achris.

I think you hit the nail on the head, a few weeks before all this, I took the boat out with a full load of people, it seemed to sound good but I couldn't get it to rev over 2000RPM at WOT. Since the the boat was full and everythign sounded normal we kept going maybe 1 hours worth at 2k, not WOT thou. Anyway when i had a chance to check it out with a timing light the timing was way out, well advanced. So now I know what caused the blown head gasket, the next question is what caused the timing to go so far out of wack, Its a 2BBL Carb model with Thunderbolt IV ignition, Distributer had not moved???

You need to find out why the gasket blew in the first place. If you don't, it will just blow again.

Some of the things that can cause a gasket to go like that are (in no particular order)

Too far advanced timing.
Low octane fuel (stale/old)
Too much load/over propped. (what revs at WOT?)
Engine running too hot (check the obvious, and the not so obvious)

Chris.....

 

[Don S]

Re: Mercruiser 4.3 Blown Head Gasket

The ignition system is all electronic, so you could have a bad module, a corroded connection, or faulting wiring somewhere.

When you couldn't go past 2000 rpm, that was a sign there was a real problem, and to stop and find out what the problem was before it got worse.
Hopefully others that run into this problem don't just keep running and damage their engine.