5.0 MPI overheating

[Oliver Lewis]

So I’m looking for help diagnosing a long term overheating problem on 2006 Mercruiser 5.0 MPI Bravo 3.


Here is the story so far. Bought the boat earlier this year and it would over heat after about 1 hour of running, after trying a new impeller (old one looked ok though) it still overheated, changed thermostat and cleaned out most of the coolant lines, still overheated, pulled the outdrive and found “bravoitus”, so replaced the water pickup pipe and cleaned out the blockage. The water PSI came up from a maximum of 0.5 to around 7 psi at high rpm. This solved the overheat .... for a while .....


After a few months of running good I was travelling at full throttle for about 5 mins, then slowed to near idle for a no wake zone and the over heated was back..... temp read 220.... I shit down for a few mins then restated at a fast idle and the temp soon returned to below 170 quickly. I then found if I ran the engine over 4000rpm the temps would be rising above 180, then if I slowed they would fall to around 160. I have a 140 stat fitted, so it seems I have a concern with cooling system capacity when running under heavy load.


Is this normal? Or do I still have a flow issue?


The only thing I can think at this point is a partial blockage, worn impeller housing or bad engine circulation pump?

 

[VernJ33]

How old are the manifold and risers? Salt or fresh water?

 

[scoflaw]

Flow issue if your max. pressure is 7, should be at least double that. That bronze pump can score easily

 

[Espos4]

What about the lower unit?
There are holes in it where the raw water is drawn in. Barnacles can grow inside the lower unit and block water flow.

Your 7psi at high rpm is still too low. Can you hold your hands on the exhaust elbows, they should be cool to warm but not hot. The manifolds can be hot to the touch but not your elbows.

 

[Oliver Lewis]

It has been a salt water boy for the last few years, but always flushed and stored out the water. No visual blockage or growth on the lower unit pickup. The brass pump body was scored on the inside, it already has the steel plate and plastic cover installed. I’m thinking I need to replace the whole brass housing?

 

[Oliver Lewis]

I have never touched the manifolds, but the risers are warm but don’t burn when touched. The feed pipes to them are cool, is it possible the flow from the pump is not being correctly distributed to the engine? I may pull the exhaust pipes off the risers to check what the insides look like too.

 

[VernJ33]

 

[Ned]

I'm having a similar problem with a 5.0 I just rebuilt. Replaced impeller. All hoses flush out the back with a water hose. I think I had a wrong head gasket blocking a coolant passage