1976 mercruiser 188 hp 302 overheats

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Bass4fun

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Hello everyone. I'm new to this site so please be kind. First post. Merc. Cruiser 302 ford 188 hp. Overheats. The boat normally runs 180 temp. New thermostat and no blockage in raw water system that I can see. Temp alarm went off and the boat was at 230. I could smell the exhaust bellows melting. No major damage. Can a weak/worn impeller cause the mechanical water pump on front of engine to starve the entire motor at cruising speeds? At idle forward the boat is fine. Any speed above 30 is a problem. I'm new to boating and I know it's a simple question but to me it is not. Thank you for any help
 

RCAnderson

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Re: 1976 mercruiser 188 hp 302 overheats

With 180 as normal temp, sounds like closed cooling ? Yes a worn impeller will cause an overheat. If closed cooling, when was the last time the heat exchanger was opened up and cleaned out ? Most closed cooling systems have a 160 deg thermostat, not 180.
 

hivoltg

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Re: 1976 mercruiser 188 hp 302 overheats

I recommend you change the water pump impeller.

Remember water boils at 212F. A 230F motor is not good for its life span or parts.
 

Bass4fun

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Re: 1976 mercruiser 188 hp 302 overheats

Wow that was fast and I thank you for it. Here's some more information I just received. The out drive was replaced with one off another boat. I guess a parts boat. No real answer to how old the drive is let alone what the impeller looks like. Someone else I talked with said if the drive is a question, start there. Possible mud dobbers or other blockage in lower unit. I guess with everyone leading in that direction, ill get a maual and tear down Lower unit to see what's inside. The boat is a beauty and well worth repairing. I will keep you posted on progress. Thanks to everyone who answered.
 
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haulnazz15

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Re: 1976 mercruiser 188 hp 302 overheats

Agreed, full water pump housing and impeller kit is the best option, then work your way forward to the engine. You didn't answer whether the engine was open or closed-cooled. (is there a heat exchanger on the front of the engine/antifreeze?) If it's open cooled (meaning no heat exchanger), which most 888's were, then you should have a 160-165 degree thermostat in it, not 180-plus.

If after you replace the drive impeller and show good flow out of the hoses going to the exhaust manifolds, then you might need to look at replacing the exhaust manifolds/risers due to blockage in the water passageways.
 

Bass4fun

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Re: 1976 mercruiser 188 hp 302 overheats

There is no antifreeze or radiator. It's just lake water cooled. Impeller pumps through drive and belt driven water pump on front of engine takes it from there. According to flow chart the exhaust manifolds dump back into lake as well as cool themselves. Thanks for the link and good information.
 

Bondo

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Re: 1976 mercruiser 188 hp 302 overheats

There is no antifreeze or radiator. It's just lake water cooled. Impeller pumps through drive and belt driven water pump on front of engine takes it from there. According to flow chart the exhaust manifolds dump back into lake as well as cool themselves. Thanks for the link and good information.

Ayuh,.... Welcome Aboard,... For future reference, Changin' the Impeller is Always the 1st thing to do in any Overheat situation,...

Relatively speakin', the impeller pump does the work, the circulatin' pump is pretty much along for the ride,...
 

achris

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Re: 1976 mercruiser 188 hp 302 overheats

... At idle forward the boat is fine. Any speed above 30 is a problem. I'm new to boating and I know it's a simple question but to me it is not. Thank you for any help

There's my clue.... As everybody has already said, check/change the drive impeller. That's always the first thing to check when dealing with an overheat. Once that's done, I would look at the exhaust elbows.

Relatively speakin', the impeller pump does the work, the circulatin' pump is pretty much along for the ride,...

I know plenty of people like that. :facepalm:

Chris.......
 

Bass4fun

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Re: 1976 mercruiser 188 hp 302 overheats

Okay here it is. The impeller blades are worn down. They are not completely gone but are not snug in impeller housing and are curved severely. The exhaust manifold on the starboard side of the engine it full of rust and what looks like foreign material that water picked up from the lake. I can save the manifold. Thanks to all that helped on this project. Safe boating to all. Since I am this far I will pull the other manifold and clean as well. Install new 160 temp thermostat. And all should be good for this weekend water test.
 

Papa Grump

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Make sure that the RPM is not slipping in the prop hub or in a bad upper hub off of the crank, at slow speeds it will be fine, put a load on the boat and the RPM will overspeed and overheat the engine. check also that the bottom of the engine block is not packed with sand and corrosion.
 
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