Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

[Fordiesel69]

This boat ran great for 2 full seasons, prior to this it is unknown when the engine was last opened up. But anyways the head and block surface looks good. Bolts were definatly torqued good as they were hard to break loose.

What would cause this to blow? I would hate to put $46 down the tubes for it to blow again. I do plan to use a tool and die straight edge to check for overall straightness of the deck and head.

 

[sutor623]

Re: Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

Did it blow under a load, or did you figure it out after it set in the off-season? Did you have bad compression only between the two middle cylinders?

 

[Fordiesel69]

Re: Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

It blew under load last year. We assumed it was bad fuel as it was noticed after we filled up at the marina, it was raining, then when we left and tried to get on plane it didn;t sound right. After diagnosing, the center 2 were at 30 psi and the outer cyls were normal.

 

[sutor623]

Re: Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

Yea it def. sounds like a blown gasket. How do the pistons look? What year is this motor? I have had carbon build up in 1 cylinder causing increased cylinder pressure, and it blew a gasket out on me. Check for excess carbon in the rings. (Possible through exhaust plate)

 

[Fordiesel69]

Re: Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

Head is off, and there is no carbon at all. Pistons look good and like I said no visual defects in the surfaces. I will of course check with a straig edge to verify.

 

[sutor623]

Re: Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

Hmmm, sounds like just a gasket failure to me. Waterpump never failed did it?

 

[Fordiesel69]

Re: Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

The raw water pump failed in the drive when the prior owner had it. Hence why they sold it. But it has been maintained frequently since and had good cooling when the gasket blew. Like I said it ran for 2 season without issue.

The cylinders do not have abnormal wear to indicate an overheat condition and the rubber rubber parts were just old & dry rotted, again not indicating a melting point.

 

[sutor623]

Re: Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

Ok, well as long as you didnt warp your head I think you will be fine. Put the new gasket in, check compression, and check temperature while running. If all seems okay give it the full test run.

 

[Fordiesel69]

Re: Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

Well it checks out nice and level. I cannot slip a 0.003" guage under my straight block anywhere on the head or deck. We shall see today how it turns out.

Now I just need to remeber how to set the lifters with the engine not running. I have always did it in the past with them running.

 

[sutor623]

Re: Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

Cool man, let us know how she runs after you get the gasket on.

 

[Fordiesel69]

Re: Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

Runs like a top now. The merc book worked well for setting the valves with it not running. I just skipped the "add an additioanl 3/4 turn" and did that part with it running to make certain they were quiet. I did not want to chance them being too tight and have a nightmare starting it.

There is no massive amounts of blow by smoke out the vent hole, but I can feel pulsing coming out that I do not remember feeling on my inline 6. How much should be normal coming out.

The whole bummer with this job is I did not have the time to take pics / videos. My hands were a greasy wreck and I knew I would be pressed for time.

 

[achris]

Re: Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

The raw water pump failed in the drive when the prior owner had it. Hence why they sold it. ...

There's your clue....

That said, I would also be checking lots of other things too... In particular the water pocket cover in the bottom of the top box in the drive... When an engine runs without water (as in 'raw water pump failed') the water pocket cover, which is only made of plastic, get melted and the gasket leaks. Easy to check.. run a section of clear hose between the transom water feed, from the drive, to the thermostat housing. Take the boat out and run 'normally'. Particularly at WOT there should be no bubbles in the water stream as seen in the clear hose....

Chris......

 

[Fordiesel69]

Re: Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

I owned the boat back in 2009 and sold it in june of 2010. All that was taken care of back then (Sept 09'). It cooled perfect, and has lasted all that time with the HG letting loose in the tail end of the 2011 season.

Since my friend bought it, there has been approx 80-90 hrs added to it. So whatever happened to the HG it lasted quite a while which is puzzles me. Usually they blow right after an overheat. And like I said earlier the cylinders are not all scored up like it ever had a severe overheat. Maybe just a mild one. Who knows.......

 

[sutor623]

Re: Merc 120HP 4 Cyl. Blown Headgasket between center cylinders

Glad to hear it is back up and running.