Overheat 4.3 raw water HELP please

[Yyachtwork]

Hello- We are dealing with a 1993 4.3lx raw water-cooled MerCruiser. New risers, new impeller, new circ water pump, pulled oil cooler, and checked for restriction. We have also run Barnicle Buster through the engine twice letting it sit for about two hrs each time. The engine will slowly overheat at idle with muffs on. We use a laser temp gauge and both banks seem about equal in temp. We have a hot spot under the laser temp that I will post the photo and some vids of the water flow. We are running out of ideas on what the issue could be. I have read every overheat form on iboats that I can find and we still have not found the issue.
Dang, the system won't let me upload the video of the water flow.
The hotspot is about under the carb. When the engine gets hot we rev it up and it seems to move the hot water along and the temp gauge spikes for a few seconds then goes right back down.
I will post this and try to come back up with photos and see if I can make a link to the videos.
We are looking for some new ideas on how to find the issue.
Scott

 

[Bondo]

Ayuh,..... Welcome Aboard,...... Does yer t-stat housing have the plastic check-balls in the forward top ports,..??
Do they seat properly,..??

 

[QBhoy]

If it had overheated previously (suspecting it might when you say what you’ve changed), there may be a chance the exhaust flappers have melted and ended up blocking the cooling flow a little ?

 

[russvanatter]

Just got done with an extensive overheating on 5.0 MPI. Short story, where the hot spot is, there too is your problem. I had very little water coming out of exhaust ports and low and behold, the water coming out was only luke warm as the engine temp was 200+ degrees. Risers were cool and manifold were ... you guessed it 200+ degrees. Pulled both exhaust manifolds and both were 100% blocked in recirculation ports in the manifolds. Found other blockages along the way, but ended up being located right where the heat was.

 

[Yyachtwork]

Ok, thanks for the ideas. The T stat is the new one without the balls and the flappers are removed. We have the sterndrive off at the moment if there is anything we can check. Thanks again.

 

[Bondo]

Ok, thanks for the ideas. The T stat is the new one without the balls and the flappers are removed. We have the sterndrive off at the moment if there is anything we can check. Thanks again.

Ayuh,...... Did the old one have check-balls,..??..??

If so,.... That's the problem,......

 

[tpenfield]

What Bondo said . . .

You may be able to test that theory by slightly clamping off the hoses to the exhaust elbows and see what that does to the engine temperature. Make sure to keep some flow going to the exhaust though . . .

The issue could be that most of the new cooling water coming into the engine may be going straight out to the exhaust and not circulating through the engine. The check-balls add enough restriction so that a fair amount of water goes through the engine rather than straight out the exhaust.

 

[Yyachtwork]

Ah, much more makes sense now. Thanks for the drawing and ideas.

 

[tpenfield]

Let 'us' know how it works out. It is important for others having a similar issue in the future to reference your cause & solution.