Mercruiser 5.7LX overheating for years

[DaBean]

I have a Mercruiser 5.7LX 1996 that has been overheating intermittently for years. Used to only happen once or twice a season... now seems to happen about every trip. Symptons are that the temp gauge will stay at about 170 solid thoughout a day and then suddenly begin to rise and fall all over the place followed shortly (if I don't see) by overheating. Problem has persisted over about 6 years and through 3 engines. I have had 2 shops look at it 3 times and they have yet to find or fix the issue. Difficult to reproduce since it is intermittent. Any ideas and also where can I find a service manual online? I think it is time for me to try and fix it myself. Thanks

 

[DaBean]

Re: Mercruiser 5.7LX overheating for years

A Little Help Please.

 

[CharlieB]

Re: Mercruiser 5.7LX overheating for years

Let's assume that the gauge and sender are correct and that the engine(s) are overheating.

Look at the COMMON parts to all the engines.

The supply hose from the OD to transom plate
supply hose from the transom plate to the engine
T-stat housing and check balls to the manifolds
and lastly the exhaust risers

The hoses are cheap enough, go ahead and replace them, they are aged enough they could have a tear in the inner lining, intermittently acting like a flapper valve, restricting flow, or particles of impeller vanes stacked up inside, restricting flow.

Inspect the T-stat ports to the exhaust manifold to ensure that no pieces of any previous impeller failure are stuck inside, restricting flow.

Pretty much the same inspection of the exhaust manifolds and risers outlet holes, make sure that they are not clogged with rust or particles of an old impeller.

Lastly, has the engine water pump been replaced, replace it or take the back off and inspect the rotor to pump shaft to be sure that it is not slipping and failing to pump 100 %

 

[DaBean]

Re: Mercruiser 5.7LX overheating for years

Thanks. All that sounds like good advice. Now, I have no idea how to do any of that. Can you tell me where to find a service manual that will guide me through it. I have access to 2 very knowlegeable neighbors which will help... but I'm tired of taking it to the shop. It's time for me to learn something about my boat. Thanks