4.3 overheating below 2000rpm

[matth121]

Hey guys, got a new problem as of late with my friends boat. seems to be overheating or at the very least temperature begins to quickly climb when boat is at idle. Anywhere from idle to 2000rpm actually. runs between 140 and 160, lets say 150 for arguments sake, all day until you back off the throttle. within about 2 minutes the temp gauge is up to 180. it may go higher than that if left to it but when he notices it climbing, he gives it throttle and anywhere above 2000rpm the temp quickly drops back to normal operating temp.
He's had this problem before and it led him to replace his manifolds and risers. That was 3 seasons ago, but the boat ended up sitting for a season so there really isn't alot of time on these manifolds. That fixed the problem for him. And once boat was returned to service this year, it worked as it should. within the last 3 weeks the issue started to reoccur. Any ideas? Can't possibly need new manifolds again could it? Thanks in advance guys
93 4.3L with alpha one drive

 

[Grub54891]

Change the impeller in the pump,it probaly has a set to it and cannot pump enough water at slower speeds.

 

[matth121]

thats a great idea and it's planned for the next time it comes out of the water, but i don't necessarily believe it to be the cause, unless somethings happened to it in the last 3 weeks since prior to this i could run it on muffs at 1300rpm and it would sit at 140 all day long. i read something somewhere once about water tubes that could be partially blocked by debris, manifold risers maybe? the suggestion was to pull off some hoses and check for debris but im certainly not clear on this or if it applies to this motor. any thoughts on whatelse may be the cause? again, last time this exact problem was fixed by replacing the manifolds but then that was also when the impeller was replaced so who knows.

 

[mkfs9]

When on muffs the water is pushed in by the water pressure. Your impeller has to draw the water in when not on muffs. Temperature will rise briefly after a fast run then suddenly on tick over as not enough water to cool the hot engine, but will go down after a while.

 

[Stamey]

I would agree that the impeller would be the first thing to check, since it has new manifolds on it and unless it is in a highly corrosive environment it isn't likely that the manifolds have a problem. I went though this a couple of years ago with my 1983 Chaparral, 305. Bought it needing a head gasket because of overheating. First found that the exhaust elbows had enough corrosion that they were restricting water movement through them, contributing to the overheat problem. Replaced the elbows and then had the problem you speak of, overheating at idle. It would get to almost 230 in some no-wake zones. In my case I had a new impeller so I knew it wasn't the problem. I eventually found that with my water neck/thermostat housing, the design called for some spring-loaded balls the restrict water flow at an idle. Mine had been removed, probably by the former owner trying to resolve the overheat problem caused by the restricted elbows (he removed the thermostat too). I put those balls back into the water neck and the overheat problem disappeared.
****Please note, water neck designs differ between models and years, so this solution I just spoke about may not apply to your engine at all. It was just to elaborate the troubleshooting procedure.
You need to be methodical and check the more likely suspect first, as others have suggested.

Chris

 

[ihearth2o]

duplicate

 

[ihearth2o]

I would try changing impeller first. If it fixes your problem, it's an easy, cheap fix. If it doesn't, you can check that off your maintenance list. While your at it, check for burrs along the inside of the water pump. Chances are, if your impeller has chipped off any rubber, it may have scored the insides of the pump.

In addition to the pressurized effect of using muffs, the water temp from a tap would likely be colder than wherever you go boating.

 

[Bondo]

thats a great idea and it's planned for the next time it comes out of the water, but i don't necessarily believe it to be the cause, unless somethings happened to it in the last 3 weeks since prior to this i could run it on muffs at 1300rpm and it would sit at 140 all day long. i read something somewhere once about water tubes that could be partially blocked by debris, manifold risers maybe? the suggestion was to pull off some hoses and check for debris but im certainly not clear on this or if it applies to this motor. any thoughts on whatelse may be the cause? again, last time this exact problem was fixed by replacing the manifolds but then that was also when the impeller was replaced so who knows.

Ayuh,.... Change the Impeller, it's way past it's service life,.....