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Ask K. Kergan Your Own Question
K. Kergan
K. Kergan, Marine Mechanic
Category: Boat
Satisfied Customers: 1031
Experience:  24 yrs exp. Inboard/Outboard mechanic Mercury/ Mercruiser, Volvo Penta and OMC, Johnson/Evin.
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I have a high speed overheat problem with a mercruiser

Customer Question

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: 2 years ago.
Category: Boat
Expert:  Jason replied 2 years ago.
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?
Customer: replied 2 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.
Expert:  Jason replied 2 years ago.
A 1986 140 would have from the factory a mechanical fuel pump. Is that is what is on the engine now?
Customer: replied 2 years ago.
Yes, I replaced it with a new Mercury part while troubleshooting this issue
Expert:  Jason replied 2 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?
Customer: replied 2 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?
Expert:  Jason replied 2 years ago.
No, not possible. I think you need to take this on in.
Customer: replied 2 years ago.
Ok thanks
Expert:  K. Kergan replied 2 years ago.
Hi, my name is ***** ***** engine raw water cooled or filled with anti-freeze?
Customer: replied 2 years ago.
Hi Kerry- thanks thanks for responding. This is a raw water cooled engine.
Expert:  K. Kergan replied 2 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.
Customer: replied 2 years ago.
I'll get an infrared thermometer and report back- what should the temp be on the casing - 160-180?
Expert:  K. Kergan replied 2 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.
Customer: replied 2 years ago.
Ok - is there a way to backflush this engine to possibly clear the cooling ports?
Expert:  K. Kergan replied 2 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.
Customer: replied 2 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-
Expert:  K. Kergan replied 2 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.
Expert:  K. Kergan replied 2 years ago.
Have you made progress with this problem yet?

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