Mercruiser 5.0 bravo 3 2003 chapperal OVERHEAT

[anglosax]

MERCRUISER 5.0 BRAVO3 OUTDRIVE RAW WATER COOLED

OVERHEATING AT IDLE RPM- rapid temp rise from cold to 200deg+ at idle rpm

DIAGNOSTICS PERFORMED

Raw water strainer cleaned and checked for flow
Raw water pump output confirmed strong flow at discharge
Raw water flow confirmed thru fuel cooler module
Raw water flow confirmed at lower drain module
Raw water flow confirmed at thermostat housing inlet

With thermostat closed [cold] raw water flow confirmed at bypass hoses to exhaust headers 5 gal in 30 sec
Raw water flow confirmed thru exhaust headers and exit at transom
To double check headers are not restricted , exhaust bellows were removed and flow thru headers confirmed with pressure hose

As engine warms up , thermostat housing, hoses to manifolds, manifolds ,circulating pump and attached hoses get hot within 5 minutes .

With engine HOT ? exhaust headers remain cold
Check for defective temp gauge/sender ? run up to 210 deg and independent alarm is sounded

Circulating pump was changed to check maximum engine cooling circulation- NO CHANGE
Thermostat first changed? NO CHANGE
Thermostat removed as test ? NO CHANGE

Raw water pump/impeller has NOT been changed as there appears to be strong water flow at idle rpm

All the above has not changed this overheat condition . MY belief is that the raw water is flowing through the bypass to the exhaust headers [keeping them cold] and not being circulated thru the engine ​aLL IDEAS WELCOME AS I HAVE RUN OUT!!!!

 

[tpenfield]

I'd be looking at the exhaust manifold internal chambers at this point to see if they have become clogged.

 

[alldodge]

There should be a plate on the back of your circulating water pump. Suggest removing it and see if the impeller is not loose on the shaft. Might also be able to remove the hose and look along with sticking a screw drive in there and see if you can turn the pulley by hand

 

[anglosax]

Tpenfield - I'm thinking along the same lines - for clarification does the cooling water flow from the thermostat housing thru the manifold and up thru the headers and out th exhaust . there is a hose out and in to the manifold . I will clamp the bypass hose and see if the flow stops

 

[tpenfield]

It sounds like you have the setup where hoses go to the elbows as well as the lower part of the manifolds. The hoses that go to the elbows are intended to provide cooler water to the exhaust, particularly before the thermostat opens and allows water flow through the engine. So, some water goes directly to the elbows, never going through the engine.

The hoses that route to the lower part of the exhaust manifolds are the way that the cooling water that goes through the engine actually gets out of the engine. The water that goes to the exhaust manifolds is supposed to flow upward and join the water that went directly to the elbows. If the exhaust manifolds are clogged, then very little, if any, water can actually get out of the engine. So the water in the engine gets wicked hot and over heats, etc.

 

[anglosax]

Update - I clamped the bypass hose to the headers and the engine ran 30 mins at 160 deg . As soon as I removed the clamp the engine started to rise to 180+.
TPenfield you nailed it - partially plugged manifolds causing restricted flow out the engine

 

[tpenfield]

Glad you got it sorted out.

I gotta do some work on one of my engines as it is running hotter.