Mercruiser 496 (8.1L) overheating

[Luigi Morales]

When you did impeller, dud you inspect and correct any scoring issyes?

yes. The kit came with a new wear plate.

 

[PlayDate]

Just to eliminate the pump.....swap it to the other engine. I have seen new impellers spin on their hubs and vanes from a destroyed impeller blocking the water flow from the pump. Be sure to baseline how much water is coming out the exhaust over a 30 second period.

 

[Luigi Morales]

Ended up being that fitting that goes at the bottom of the exhaust manifold. At the lowest part of the manifold where the water goes in was blocked by a thin layer of rust that was blocking water flow.
I took out the fitting, cleaned it. Then cleaned out the water entry into the manifold, with a round wire brush. Put it all together and the flow is much better and appears to be running cooler on port side. water test next weekend but I’m sure this solved the issue.

I think someone here had the same issue when swapping the manifolds and found rust build up at the same spot. Thank all for your replies.

 

[PlayDate]

Ended up being that fitting that goes at the bottom of the exhaust manifold. At the lowest part of the manifold where the water goes in was blocked by a thin layer of rust that was blocking water flow.
I took out the fitting, cleaned it. Then cleaned out the water entry into the manifold, with a round wire brush. Put it all together and the flow is much better and appears to be running cooler on port side. water test next weekend but I’m sure this solved the issue.

I think someone here had the same issue when swapping the manifolds and found rust build up at the same spot. Thank all for your replies.

I'm not sure how that would have worked if your manifolds are Fresh Water Cooled (filled with Antifreeze). Raw water cooled makes sense but I thought your boat came with Fresh Water Cooled manifolds.

 

[Luigi Morales]

I'm not sure how that would have worked if your manifolds are Fresh Water Cooled (filled with Antifreeze). Raw water cooled makes sense but I thought your boat came with Fresh Water Cooled manifolds.

Exhaust manifolds are cooled by raw water. Which then gets expelled through the risers and out the back.

 

[Spark600]

Exhaust manifolds are cooled by raw water. Which then gets expelled through the risers and out the back.

Not all of them do your research.
Some small blocks do cool with Antifreeze, All big blocks use raw water.
I called MerCruiser on this and they told me that some small blocks do this as they do not run that hot and all big block run to hot to use Antifreeze.

 

[Luigi Morales]

Not all of them do your research.
Some small blocks do cool with Antifreeze, All big blocks use raw water.
I called MerCruiser on this and they told me that some small blocks do this as they do not run that hot and all big block run to hot to use Antifreeze.

My response is for my engine, I never said all engines are designed that way. I have a 8.1 496 big block, same as the original motor on this post

 

[James T Kirk]

Luigi, so this was the culprit on your engine as well as the original poster?

 

[Luigi Morales]

Luigi, so this was the culprit on your engine as well as the original poster?

It was debris sitting on top of that fitting in the manifold (right above the threaded area) So water could not flow into the manifold. The fitting pictured had a light rust residue which I cleaned out. There appears to be a moving mechanism inside of this which was stuck (not sure if open or closed or what it really does) After I cleaned the drain fitting and obstruction removed from manifold it appears to work now. Thanks to the picture you have up there I decided to look in there.