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I have a high speed overheat problem with a mercruiser

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Hello - I have a...
Hello - I have a high speed overheat problem with a mercruiser 140 - the thermostat never says overheat but the fuel pump area of the motor gets hot and boils the fuel at high speeds. I've replaced the impeller and housing, thermostat, exhaust manifold and riser but still have the issue. Any help is appreciated!
Submitted: 3 years ago.Category: Boat
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6/23/2015
Marine Mechanic: Jason, Marine Mechanic replied 3 years ago
Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 17,476
Experience: Degree in Marine Technology. Gas and diesel marine mechanic.
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Good morning, my name is ***** ***** I will be helping you today.I've never heard of fuel boiling at high speed before. What exactly is the engine, doing, what kind of symptoms is it having?
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Customer reply replied 3 years ago
Hi Jason- thanks for your reply. It seems that the mechanical fuel pump is getting very hot and vapor locking. It only seems to do this when cruising at higher speeds. The engine breaks up and stalls but if I put a wet towel on the fuel pump to cool it down the engine starts and runs. The thermostat is new and the temp gage works but never shows the engine getting hot. I'm stumped.
Marine Mechanic: Jason, Marine Mechanic replied 3 years ago
A 1986 140 would have from the factory a mechanical fuel pump. Is that is what is on the engine now?
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Customer reply replied 3 years ago
Yes, I replaced it with a new Mercury part while troubleshooting this issue
Marine Mechanic: Jason, Marine Mechanic replied 3 years ago
To be honest in all my years I have never heard of the issue you are describing, not with a mechanical fuel pump. Do you have a temperature gun to take the pumps temp manually?
Can you be as specific as you can about what the engine is doing?
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Customer reply replied 3 years ago
I don't have a temp gun but I do know that cooling the pump allows the engine to run. It's very hot to the touch which isn't right. Wondering if there could be a clogged water port in the engine in that area- is that possible?
Marine Mechanic: Jason, Marine Mechanic replied 3 years ago
No, not possible. I think you need to take this on in.
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Customer reply replied 3 years ago
Ok thanks
Marine Mechanic: K. Kergan, Marine Mechanic replied 3 years ago
K. Kergan
K. Kergan, Marine Mechanic
Category: Boat
Satisfied Customers: 1,031
Experience: 24 yrs exp. Inboard/Outboard mechanic Mercury/ Mercruiser, Volvo Penta and OMC, Johnson/Evin.
Verified
Hi, my name is ***** ***** engine raw water cooled or filled with anti-freeze?
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Customer reply replied 3 years ago
Hi Kerry- thanks thanks for responding. This is a raw water cooled engine.
Marine Mechanic: K. Kergan, Marine Mechanic replied 3 years ago
The water jacket on raw water cooled engines can accumulate rust over time. I'm not sure if part of the water jacket is close enough to the fuel pump to heat it up enough to boil gasoline, but it's the only possibility that comes to mind. Areas that are clogged with rust, usually the bottom of the water jacket, won't cool as much as the rest of the engine.
An infrared thermometer would be a plus to find where the heat is coming from.
You may need to remove the head and manually clean rust from the water jacket.
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Customer reply replied 3 years ago
I'll get an infrared thermometer and report back- what should the temp be on the casing - 160-180?
Marine Mechanic: K. Kergan, Marine Mechanic replied 3 years ago
The thermostat opens at 142 degrees, shouldn't be a lot hotter than that, maybe 160 or so.
The boiling point of benzene is 176 degrees, it has to be cooler than that.
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Customer reply replied 3 years ago
Ok - is there a way to backflush this engine to possibly clear the cooling ports?
Marine Mechanic: K. Kergan, Marine Mechanic replied 3 years ago
That's not likely to work.
The rust flakes off the inside of the water jacket and falls to the bottom of the cavity where it rusts more and turns into a pretty solid cake.
I've seen many engines that won't drain when you remove the drain plug until you reach in with a screwdriver and break up the rust.
Rust accumulates in spots where normal water flow is slowest, if you can force higher than normal pressure through, it may flush out a lot of rust but might still not get the spot you need cleaned out.
You could try removing the thermostat and hooking up a fire hose to the thermostat housing but I can't guarantee it will work.
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Customer reply replied 3 years ago
Ah ok- i had removed the drain plug and the water ran out ok/ i was considering removing the freeze out plugs but they are higher than the drain. I'm trying to avoid removing the head is possible-
Marine Mechanic: K. Kergan, Marine Mechanic replied 3 years ago
Right, don't blame you for that.
I would start with the infrared temp guage and see where exactly the hot spot is, then judge whether the head needs to come off to clean it out.
Removing a freeze plug may work if it's close to the hot-spot and you can loosen rust through the hole, maybe using a coat hanger or something.
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Marine Mechanic: K. Kergan, Marine Mechanic replied 2 years ago
Have you made progress with this problem yet?
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