Overheating in 5.7L Mercruiser at higher RPM

[TracyT]

I recently purchased a 305 Sedan Bridge and the starboard 5.7L Mercruiser is heating up at higher RPMs, I can run around at 2K-2.3K and it hold below 160. Push it higher and it climbs fast. I am looking for suggestions where to start as this is my first twin inboard.

The other side runs about 145-155 degrees.

I am thinking to start at the impeller, thoughts?:huh::smt100

 

[Boater420]

I would get a IR Temp gun at Harbor Freight for $30. It's a great tool to have when diagnosing any of these overheating issues. You can point the gun at the engine while it's running and locate where the heat begins to build. Basically you're tracing the flow of water through the engine.

Although it might be an impeller, it could also be a riser/manifold issue as well. When the water flow through these parts becomes restricted they are no longer able to cool properly and you'll see the temperatures rise when putting more of a load on the engine.

If you don't know when the impellers were last changed that is the easiest & least expensive place to start. If that doesn't fix it chances are the risers and manifolds are in need of some attention as well.

 

[TracyT]

Thank you for the suggestion I will follow through with impeller swaps and belts and hoses just so I know they are new and get a gun to watch the motor temps

 

[JVM225]

If it turns out to be the impeller, change the other side too. They are usually replaced together.
I would also try to get an idea about when the manifolds and risers were last changed, even if they aren't your problem right now. They have a life span of about 5 years in a saltwater environment, and failure can lead to water ingestion in to a cylinder through the exhaust port of the cylinder head. Since water doesn't compress, it results in hydro-lock, and is a death sentence for the motor.
I only used Mercruiser brand exhaust parts, and changed them as soon as they were 5 years old. Consider it to be life insurance for your motors.

 

[FINAL DRAW]

Thank you for the suggestion I will follow through with impeller swaps and belts and hoses just so I know they are new and get a gun to watch the motor temps

Never posted before,but consider this. The lining of suction hose collapses between thru hull and strainer and restricts water flow at higher rpm. Have seen this on 1988 boat with gas inboards.

 

[SCORPIO]

Is the sea strainer clogged? Is the sea cock fully open? Is marine growth blocking the thru hull pick up. Start at the source and work down stream.

 

[TracyT]

Found a loose belt on the fresh water pump and alternator, still have a 15 degree ( gauges maybe) differential between port and starboard. Making progress and getting bruises:smt021