Overheating 79 Mercruiser 5.7 260.

[redneck joe]

Yes I've read the stickies and think I know what to look for but want to know 'normal' progression of what ones to start with.

35 year old boat, bought cheap as it was abandoned for slip rent. Sat inside for three years. Put new SEI on it first year. Maintenance unknown, form the look of the spark plugs when I replaced could have been 5-10 years since anything done.

Slow overheating under load this year, fine last year. Took to the shop to winterize and was talking to him. Asked him to pop off one side of the riser/manifolds to check condition as my first guess. He just called and told me no water coming from exhaust he thinks water pump but could be the usual suspects. He pulled the petcocks completely so not clogged there. I'm going to hold off until spring when we de-winterize and or this winter if I decide to do new transom. If I do transom will just do everything but if not I'll do the work in the spring.

So what order would you put these in to check: risers/manifolds, water pump, impeller (maybe 75 hours in two seasons), exhaust flappers, ?? (anything i've not listed.

 

[alldodge]

Yes I've read the stickies and think I know what to look for but want to know 'normal' progression of what ones to start with.

35 year old boat, bought cheap as it was abandoned for slip rent. Sat inside for three years. Put new SEI on it first year. Maintenance unknown, form the look of the spark plugs when I replaced could have been 5-10 years since anything done.

Slow overheating under load this year, fine last year. Took to the shop to winterize and was talking to him. Asked him to pop off one side of the riser/manifolds to check condition as my first guess. He just called and told me no water coming from exhaust he thinks water pump but could be the usual suspects. He pulled the petcocks completely so not clogged there. I'm going to hold off until spring when we de-winterize and or this winter if I decide to do new transom. If I do transom will just do everything but if not I'll do the work in the spring.

So what order would you put these in to check: risers/manifolds, water pump, impeller (maybe 75 hours in two seasons), exhaust flappers, ?? (anything i've not listed.

My first check would be the impeller and water pump body if were talking about water coming out of the exhaust on the boat. Now if the water coming out of the riser/manifold then my first check would be the thermostat and housing.

Then go to risers(elbows)/manifolds, impeller and pump body

 

[redneck joe]

dodge sorry if being obtuse, I've been more of an outboard guy so hang with me.


where would water come out differently than the exhaust from the risers v the thermostat? And I took the t-stat out mid summer to eliminate that probability.

 

[alldodge]

dodge sorry if being obtuse, I've been more of an outboard guy so hang with me.


where would water come out differently than the exhaust from the risers v the thermostat? And I took the t-stat out mid summer to eliminate that probability.

Well taking the thermostat out can cause some over heat issues. The thermostat helps keep the block full of water when running. With it removed air pockets can form in the block. You have the what I consider an octopus thermostat housing. Thing has ports going everywhere
http://www.mercruiserparts.com/Show...png&inbr=990&bnbr=70&bdesc=THERMOSTAT+HOUSING

The water would still come out the exhaust but the engine could still over heat from trapped air or have hot spots. So your saying no water is coming out the exhaust "at the prop". If this is correct then start with the impeller and pump housing.

 

[Bondo]

Well taking the thermostat out can cause some over heat issues. The thermostat helps keep the block full of water when running. With it removed air pockets can form in the block. You have the what I consider an octopus thermostat housing. Thing has ports going everywhere
http://www.mercruiserparts.com/Show...png&inbr=990&bnbr=70&bdesc=THERMOSTAT+HOUSING

The water would still come out the exhaust but the engine could still over heat from trapped air or have hot spots. So your saying no water is coming out the exhaust "at the prop". If this is correct then start with the impeller and pump housing.

Ayuh,.... Agreed,... The T-stat is an important piece of the t-stat housin', 'n how it works on a boat motor,....

That said, I also agree the impeller is the 1st place to look in any overheat situation,.....

 

[redneck joe]

Yeah aware of the need for the t-stat but it was overheating with it in, same with it out.

so impeller at sterndrive or the water pump?

dodge - I've never run on muff but the guy said the exhaust comes out the housing, not the prop?

 

[Bondo]

Ayuh,.... The Raw water pump,.....

Atleast test it's flow,....
Run it on the muffs, 'n pull the hose from it at the t-stat housin',...

 

[alldodge]

Yeah aware of the need for the t-stat but it was overheating with it in, same with it out.

so impeller at sterndrive or the water pump?

dodge - I've never run on muff but the guy said the exhaust comes out the housing, not the prop?

Agree with Bondo, stern drive pump
If the exhaust is coming out the transom housing holes and no water is there at the housing or prop, check water flow. After that start checking hoses, exhaust manifolds, elbows and Y-pipe. Water is going in but not out, there is an issue

 

[thumpar]

Without a thermostat you can get hot spots in the block. It is what regulates the flow. I would go with the impeller and thermostat first. The flappers wouldn't cause an overheat but can be damaged by one with no water.

 

[redneck joe]

thanks folks. I'll report back in the spring....

 

[DJ Jaws]

I'd start with the outdrive impeller. They're cheap and relatively easy to fix. It sounds like you have a supply problem, not so much a circulation issue.