Mercruiser 4.3 Overheating (Raw Water)

[HillCountryJeep]

Last season, I purchased the boat and I was having some minor overheating issues, with the boat operating between 180 and 190 degrees. This temperature did not different between idle or WOT. Because of the overheating issue, I did not run the boat much at all last season.

It's a 2005, and the risers and manifolds appeared to have been stock. So I changed those out with a direct replacement. I was able to clean out quite a bit of debris from the hoses.

https://marineengineparts.com/mercu...kit-for-v-6-dry-joint-4-3-liter-with-7-riser/

I went through and installed a new thermostat (160 degrees - not backwards - double checked), cleaned out the housing and removed any debris, and installed a new gasket. I also installed a new impeller.

I took it out for a test run this season and the temperature climbed fairly quickly. After about 5 minutes of idling, the temperature climbed to 200 degrees. I ran it for a minute at about 2,500 rpm to see if the temperature would drop, but it held 200 steady at idle and with rpm. After about 10 minutes, I loaded it up to go back to the drawing board.

The risers were both cool to the touch. I've attached a sketch of the hoses I checked.

This leads me to believe the circulation pump may be going bad, but I am reading they rarely fail. What am I missing?

 

[scoflaw]

If the risers were cool I would assume it's not overheating. What would make you come to that conclusion? Have you had an overheat alarm? Did you check the temp at the t-stat housing with an IR gun?

 

[HillCountryJeep]

If the risers were cool I would assume it's not overheating. What would make you come to that conclusion? Have you had an overheat alarm? Did you check the temp at the t-stat housing with an IR gun?

I'm buying an IR gun this week and I'll check to see if the temp gauge may be malfunctioning.

Maybe it's bad logic on my part, but I was assuming the risers could also be cool due to poor circulation within the system?

 

[Lazy Daze]

Scof is right - if the top of the elbows are cool (or moderatly warm, even), then you're doing fine. It's possible that the problem, all along, may have just been be a faulty temp sensor, gauge or the wiring.

No, risers will only get hotter and hotter with poor circulation.

When you replaced the impeller, what did the housing and wear plate look like? Were any vanes missing?

 

[joeyleggz]

When I had my raw water 4.3’s the thermostat mercruiser called for is 140 the freshwater cooled calls for the 160

 

[Bill Curtis]

Is your drive a Bravo or an Alpha? Sounds like you have a raw water restriction somewhere. Since raw water cools the power steering and oil before the engine, have you checked power steering / oil coolers (if you have them) for and debris in there?