Does anyone have any ideas about...

[Jim14560]

Does anyone have any ideas about an overheating engine? 8.1L Mercruiser on a 340 Sundancer. So far, I’ve replaced the temperature sensor, the seawater pump, and the heat exchanger. The engine overheats after 20-30 minutes. Local techs haven’t been able to pinpoint the problem. Any new ideas?

[wingless]

Welcome to the forum.

Start w/ the basics, is the raw water flow acceptable, has the raw water intake been inspected, is the antifreeze flow acceptable, have all the hoses been removed for inspection for internal obstructions, has the thermostat been replaced?

Has the engine been inspected for a head gasket failure?

[wingless]

Is it possible to continuously hand hold the exterior of the exhausts on both sides of both engines of the operating engines at operating temperature?

[Jim14560]

Thermostat was replaced with the heat exchanger. The water flow looks fine, about the same amounts from both engines and the port engine has no overheating issue. I checked the strainer, no problem there. I haven’t tried holding my hand over the exhaust. I did replace the intake hoses but haven’t checked the head gasket. I guess I’ll go there next; good idea. Thanks.

[KevinC]

I think @wingless is referring to touching the elbows on the exhaust. If they are too hot to the touch then raw water is not properly flowing to cool them. Next, do you have smartcraft onboard? If so what is it showing for raw water pressure? Another couple of things to look at are slipping belts and a possible blockage upstream from the raw water pump. If the impeller broke apart parts could be blocking flow.

If the exhaust elbows are cool to the touch then pull the cap on the antifreeze coolant when cool - look for sludge. If the wrong AF type was mixed with Dexcool it creates a nasty gel that blocks cooling.

-Kevin

[wai_phung]

Risers are clogged

[cbruck]

I just went through this whole process with both my 8.1's. Have the manifolds and risers been replaced and how old is your engine? The bottom line is you will get 6-10 years before they have to be replaced. Raw water flows through both the manifolds and risers and over time simply rust up. Your only option is to replace them. If you have smartcraft the water pressure should be around 3-5 psi at idle and 15-20 at full power. If it reads zero at idle then your raw water pump is not working and your smartcraft won't allow the engine to run. If it reads above 20 psi at speed then you have restricted water flow through the rest of the system and the plugged manifolds and risers are your most likely problem. Mine did the same thing and would heat up over a 30 min period. I made sure everything was flowing to the manifolds with no change in the problem. Then I replaced the manifolds and risers and resolved the issue. Not a cheap fix, but when you buy them, I believe they have complete kits that will save some money.

[d9ever]

I had a plugged hose between the straner and the raw water pump. As the engine was running, I shook the hose along the length. Dirt appeared on the outside of the boat by raw water exit. No problems after that.

[Gaylenjeff]

check the discharge tee that screws into the bottom of the manifold

[Goodtimes]

Check to see if the oil cooler is plugged