Mercruiser overheats at low speed only

AndyinWV

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Joined
Jun 22, 2009
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2
HI. I need some advice on a 1985 mercruiser i/o v6. It was running fine when put in storage Nov 07. Didn't get it out in 2008. This year I'm paying the price...had to have the carb rebuilt, and trailer brakes & bearings serviced. My final problem seems to be overheating at low speeds..i.e. when trailering or troller -- RPMs under 1,000. I used to be able to troll at 600-800 rpm for hours, but now it will overheat (over 200 degrees) after a few minutes. Speeding up brings it back down to 130/140 degrees. It seems to run fine, except for the overheating. Is this an impeller issue, or something else?
 

rbh

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Mar 21, 2009
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7,939
Re: Mercruiser overheats at low speed only

try a new thermostat?
rob
 

bustermin

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Apr 13, 2009
Messages
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Re: Mercruiser overheats at low speed only

sounds like an impeller/waterpump issue as lower rpms means less water is being pumped. so cooling is limited to the amount of water passing thru the system. as where if it were the thermostat, once it opens it is open if anything you would have a reverse effect if it were the thermostat. but anytime you work on a cooling system you should always change the thermostat.
 

Don S

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Aug 31, 2004
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Re: Mercruiser overheats at low speed only

Change the impeller. What happens is the impeller is bent over and won't straighten out at low rpms, and doesn't pump much water. At faster rpms,the vanes straighten out and pump more. But you are just on the verge of an overheat at all rpms.
 

AndyinWV

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Jun 22, 2009
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Re: Mercruiser overheats at low speed only

Thanks for everyone's quick replies. I had the boat mechanic replace the impeller and we didn't exceed 130 degrees yesterday on the lake.
 

Lucky Penny

Petty Officer 3rd Class
Joined
May 3, 2003
Messages
82
Re: Mercruiser overheats at low speed only

I really hate to controdict Don S. But I had the same exact problem. it turned out to be the circ pump not the water pump in the lower unit. At low rpm the water pump can not provide cooling on it own, that is were the circ pump comes in to play.
 

patsyjoe

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Jun 28, 2009
Messages
6
Re: Mercruiser overheats at low speed only

had a similar problem on merc 4.3. opened off the pipes at the thermostat manifold and discovered a bit of an airlock. also took out the thermostat and used a cup of boiling water to check the travel.

Combination of both cured the problem. the father in law reckoned it was the airlock, personally I say it was a stuck thermostat.
 

cr2k

Captain
Joined
Mar 19, 2009
Messages
3,730
Re: Mercruiser overheats at low speed only

Water pump impellers should be replaced every 2 years. If you use the boat they wear out, if you don't use the boat the rubber gets a set and won't supply enough flex to pump water.

Fact of boating just like you need to put gas in to to go, you need to do an impeller every 2 years. More if you boat in sandy or silty water due to increased abraison.

And NEVER start the boat with out water to the lower, not even for a second.
It will fry your impeller and pump housing for a repair bill of about $150 PLUS labor.
 
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