Strange overheating problem, need help

Dinows

Seaman
Joined
May 31, 2003
Messages
65
I have twin 1988, 350 chevys (mercruisers). Port engine runs ok to 2500rpm then as soon as I bring it up to 3K the temperature inches up and alarm goes off. Can run all day at 2500.
Closed system, only risers get salt, impeller is brand new. Will run at 3K all day in the yard with "muffs" on and no load. Any ideas ?
Thanks in advance
Dino
 

jerryjerry05

Supreme Mariner
Joined
May 7, 2008
Messages
18,107
Re: Strange overheating problem, need help

Risers,They rot and restrict flow.If they are original they need to be replaced .J
 

full stringer

Petty Officer 2nd Class
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Nov 8, 2009
Messages
184
Re: Strange overheating problem, need help

also check all belts while your at it the water pump belt maybe slipping at higher rpm happened to me last summer my buddy figured it out
 

full stringer

Petty Officer 2nd Class
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Nov 8, 2009
Messages
184
Re: Strange overheating problem, need help

also feel your risers when engine is running is one engines risers hotter to the touch than the other at 2500 rpm if so replace em at least pull em and check for clogged passages
 

Dinows

Seaman
Joined
May 31, 2003
Messages
65
Re: Strange overheating problem, need help

I replaced them last year. Anything I can do to check the flow ?

Thanks Dino
 

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: Strange overheating problem, need help

Hopefully you have already checked the outdrive raw water pump impeller. But I have seen a warped upper housing pump just fine until the boat gets on 'plane' where the pump is no longer 'submerged' and began to suck air in between the warped upper and the flat lower pump body.

Also have found the water hose from O/D to transom had a small tear inside that flows fine at low RPM but would fold over and restrict flow at higher RPM.

Is there a Power Steering cooler on that side? Is the inlet side restricted with weeds, or debris, allowing enough flow for low speed but not enough flow for greater loads?

Just a couple of the more common things you could check.
 

Dinows

Seaman
Joined
May 31, 2003
Messages
65
Re: Strange overheating problem, need help

Hi Thanks for yor post. There is no power steering cooler and no debris. For what it is worth the engine ran great all last year until i took it up the river in Oct. for the haul out. this came all at once also there is no upper pump and I am bafffiled

Thanks Dino
 

etech2008

Seaman Apprentice
Joined
Jul 4, 2010
Messages
36
Re: Strange overheating problem, need help

Did you use a quality OEM riser when you replaced them? Seen alot of problems lately with the GLM Osco and sierra manifolds. Especialy GLM just because you replaced them does not mean they are not bad.

I would check the w/p in drive first though.
 

jerryjerry05

Supreme Mariner
Joined
May 7, 2008
Messages
18,107
Re: Strange overheating problem, need help

Didn't realize they were sterndrives(lookin for a quick fix).
When you replaced the risers did you check the condition of the exhaust flappers?Were they there? Did they get hot and melt and fall into the Y pipe?
Are they in right?
The old impeller,did it come out in pieces?If so where are the old vanes?
The water pump in the drive:Is it new? We're the plates and housing changed? Lot's of scoring can cause poor suction. J
 

Dinows

Seaman
Joined
May 31, 2003
Messages
65
Re: Strange overheating problem, need help

The flappers had rotted out, never thought about checking the Y pipe. By what everyone is saying it seems to point to a riser or manifold. I have never used one but if I got one of thoes thermometers that you point and get a reading would that send me in the right direction perhaps / It seems there are so many things that could cause this I dont know where to begin. Also no one mentioned the thermostat ?
 

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: Strange overheating problem, need help

If you do not have a Bravo Outdrive then you have a water pump in the lower unit of your drive. This is the water pump that I was explaining can have a warped upper pump housing that once the drive rises in the water when you get 'up on plane' will not be submerged and can start to suck air into the cooling system, causing cooling issues at higher RPM as you described.
 
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