Overheating can't figure it out

[jr_gray]

I have a 1998 DA 240 with a 5.0 mpi engine. It has a closed cooling system on it. Engine will over heat, to 200+ degrees when at higher speeds. It has new impeller on alpha outdrive, new t stat, new sending units. Heat exchanger is clear and risers, manifolds and elbows all run cool. It seems as though engine coolant (anti freeze) is not flowing efficiently enough through engine. Any help would be helpful, cause I'm pulling my hair out trying to figure this out! TIA.

 

[Espos4]

Could be poor water flow from the outdrive up into the heat exchanger. Any chance you have a water pressure sensor? I don't think a 1998 would have one.

Also, what is the antifreeze concentration? Anything more than 60% antifreeze and you will have poor heat exchange.

 

[jr_gray]

No water pressure sensor...flow is great. As manifolds, risers and elbows remain cold. Was thinking of draining and flushing antifreeze, but its in the water and don't know procedure. Thinking bad water pump on engine?? Thanks

 

[Bt Doctur]

In most cases a outdrive impeller cannot supply enough water to the heat exchanger , thats why closed cooled systems will use the engine mounted pump and 1 -1/4 hose with a thru hull

 

[JVM225]

Circulater (what us car guys call a water pump) pump healthy?
Some kind of restriction in raw water side of exhaust?

 

[jr_gray]

In most cases a outdrive impeller cannot supply enough water to the heat exchanger , thats why closed cooled systems will use the engine mounted pump and 1 -1/4 hose with a thru hull

Alpha drives don't use engine mounted pump, just outdrive pump.

 

[Bt Doctur]

And thats why they all have a overheating problem.
It`s not a factory mercruiser install so it could be the heat exchanger to too small the capacity. The flow willl cool the manifolds and riser but cant remove the excess heat from the exchanger

 

[jr_gray]

And thats why they all have a overheating problem.
It`s not a factory mercruiser install so it could be the heat exchanger to too small the capacity. The flow willl cool the manifolds and riser but cant remove the excess heat from the exchanger

But so long into the boats life? I have owned for 5 years, just started happening last couple weeks. There is plenty of flow from outdrive through exchanger,manifolds risers and out exhaust. Just has me baffled. Do you know of a way to drain antifreeze and flush engine itself?
Thanks

 

[jr_gray]

Using laser temp gun at tstat housing it is running hot at 195...risers and manifolds running like 105.

 

[Bt Doctur]

only other reason may be a bad head gasket allowing hot gas into the cooling system

 

[dmorin]

Back in 1993 When I installed close cooling on a new 5.7 mercruiser with alpha 1 second generation you had to install 2 kits 1 for closed cooling and one with a pump to add on motor pully system for thru hull water said drive pump wouldn't supply enough water to cool heat exchanger. It was a mercury kit.

 

[Loyd Dinneen]

Did you inspect the outdrive intake to see if you may of picked up some debris or mud inside? Had that happen to a outboard.

 

[Westie]

When was the thermostat last replaced or tested?

 

[Espos4]

What is the antifreeze concentration? Anything more than 60% and you will have poor heat transfer which could be a cause for your issues.

 

[ZZ13]

My turn: T-stat installed upside down?

 

[Thornton69]

It sounds like a raw water circulation problem as previously mentioned. There is more than likely a restriction or blockage not allowing the required flow increase with rpm's. Don't overlook anything.
Is your system full (incl manifolds) or 1/2 fresh water cooled?
Are you able to hit wide open throttle before it heats up?
Marine growth or similar restriction in the out drive?
Possible failing cooling hose in gimbal assembly?
Corrosion build up at the transom plate restricting the raw water flow?
Debris in power steering oil cooler or main heat exchanger?
Beyond that is exhaust as also previously mentioned. If your system is 1/2 FWC then manifolds as well as just the risers as with full FWC can create enough restriction to limit the amount of flow at higher rpm's (loads).
Mercruisers also have exhaust flappers in their Y pipes to reduce the chances of raw water entering exhaust from rear of boat. If they fail and come apart they can get stuck in the outdrive and potentially create enough back pressure to reduce the raw water flow. This scenario would more than likely lead to a low WOT or horsepower problem long before heating but a possibility.
Although a high coolant concentration will lead to poor heat transfer among other problems, it can only be a contributor with other issues. Cooling systems are normally over designed by 20% or better of full rated horse power/load.

 

[SCORPIO]

Since this just started happening, i would be thinking a clogged heat exchanger or poor raw water flow. Can you pull the end off the exchanger and see if it is clogged? A flush with a hose could remove debris or pull it and take it to a radiator shop. I have aftermarket fwc systems on my new 5.7s and they work great, half systems btw.

 

[kap2]

What temperature thermostat did you install?

 

[Soulshine]

Someone mentioned a problem at the gimbal housing, a problem I’ve had myself that created the same sudden thing you’re experiencing.

While on plane, air can be drawn in through a leak that is otherwise submerged at slower speeds.

A simple way to check this is to splice a short section of clear hose in the hose coming from the gimbal housing to the thermostat housing. While running at high speed, have someone look at that clear hose. If there are ANY bubbles visible, you’re sucking air from outside the boat.

If your incoming water supply is good, you have a partial obstruction.

Good luck.