Logo
FULL SITE MENU

Mercury 9.9/15 hp 4 Stroke back pressure related cooling problems

L

LeoRie

New member
Hello guys,

I'm seeking advice on this Mercury (Motor code is 66M)
The Motor has good leakdown values and runs very well.

The strange thing is that it would only have an intermittend telltale at idle, which leads to overheating eventually.
If you give it some revs the telltale gets stronger and the motor doesn't overheat any more.

-Thermostat checked fine
-Impeller and water pump is brand new
-Copper tube is connected correctly and doesn't leak

Here is what I found. If I take off the muffler plate at the back (behind it is the exhaust chest where water and exhaust mix) the telltale steadies and the motor doesn't overheat anymore. Problem is now it sounds like a top fuel dragster :)

I'm suspecting an exhaust leak into the water jacket now. Theory being that the leak gets better when exhaust backpressure is reduced.

Any ideas? Thanks alot in advance, I'm curious what will turn out to be the problem.
 
Thanks for your reply racerone, the motor is in an actual testtank. The waterlevel is well over the water pump, so that shouldn't be an issue.
 
Good morning,

that is a good question. I'm not sure, as the previous owner told me, that everything was fine until he changed the waterpump as part of the routine maintenance.
I doublechecked his work with the new impeller twice, but everything was okay. What gets me is that the tell tale is really strong once you take the baffle/exhaust plate off.
I'm thinking that if the waterflow is interrupted by an old impeller vane, the waterflow would be bad regardless of what I do with the baffle plate.

Thanks for your help :)
 
leorie I am having the same exact issues right now on my 2004 mercury 15hp 4 stroke. Were you able to have any luck with yours? As soon as I pull the exhaust plate off I have perfect water flow… please let me know what you found! Thanks!



LeoRie said:
Hello guys,

I'm seeking advice on this Mercury (Motor code is 66M)
The Motor has good leakdown values and runs very well.

The strange thing is that it would only have an intermittend telltale at idle, which leads to overheating eventually.
If you give it some revs the telltale gets stronger and the motor doesn't overheat any more.

-Thermostat checked fine
-Impeller and water pump is brand new
-Copper tube is connected correctly and doesn't leak

Here is what I found. If I take off the muffler plate at the back (behind it is the exhaust chest where water and exhaust mix) the telltale steadies and the motor doesn't overheat anymore. Problem is now it sounds like a top fuel dragster :)

I'm suspecting an exhaust leak into the water jacket now. Theory being that the leak gets better when exhaust backpressure is reduced.

Any ideas? Thanks alot in advance, I'm curious what will turn out to be the problem.
Click to expand...
 
Back
Top