1988 3.7 mercruiser engine help

[bucknut408]

You guys were alot of help with my last boat.I have a new one for you
I bought a 1988 Marathon with a 3.7 mercruiser closed cooling system.
I took it out and it overheated.I did what you should do and first thing
change the impeller.Took it out again and now I have a knock, only once
it gets warm.You can't even get it to knock until it gets warm,kind of sounds like a rod knock.I am pretty good at wrenching but would like some opinions
before I start.Also the engine starts as soon as you turn the key with no
hesitation.

 

[achris]

Re: 1988 3.7 mercruiser engine help

That sounds very much like gudgeon knock. It is movement between the piston pin and the piston. It's usually caused by the bore for the pin elongating slightly, usually due to the piston being overheated...

The good news... It's not a major problem... The bad news... If you want to fix it, it means new pistons....

You said the engine overheated, how hot did it get?

Chris............

 

[bucknut408]

Re: 1988 3.7 mercruiser engine help

Gage did not work so not sure how hot.Does this mean pulling the engine.
The knock sounds like it is right under the valve cover.

 

[achris]

Re: 1988 3.7 mercruiser engine help

To get the pistons out you will need to pull the engine, yes. And yes, it can sound like it's near the top of the engine...

 

[langford7]

Re: 1988 3.7 mercruiser engine help

I know this is after the fact, but here is a great product to use after the rebuild. I use it on my exchanger and in the industry I work in. It is safe to use on copper and is totally biodegradable. My boat nevers runs more than 160 after using this product. It's called Rydlyme.


viega-na.com/.../1201716707PI-PR-22006-02-04 (Heat Exchangers).pdf

 

[langford7]

Re: 1988 3.7 mercruiser engine help

Sorry, wrong link..

http://rydlyme.com

 

[abj87]

Re: 1988 3.7 mercruiser engine help

You can also pull then end caps off the heat exhchanger and clean it out the tubes with a wooden dowel.