Over heating 40hp

[murrayfish]

1994 Mercury 40hp 4 cylinder, 2 stroke with oil injection.
Overheats and no water at idle to about 4 mph soon as I give it gas I get water flow. I ordered an impler and will put it in before next use.

My main question is: Is this motor good at going slow? I want to troll.

 

[jimmbo]

Well for its HP rating it has 4 small cylinders so it is a very smooth engine at low speed, however all two strokes, with the exception of the DI engines do tend to load up when run at low speeds for extended times. Not a defect, just the nature of the beast. Occasional revving of the engine may needed to reduce the loading up and clear the engine. Will it be slow enough to troll? Maybe, maybe not. Several factors, like size/weight of boat, installed prop(assuming it is the correct prop to allow engine to run in the WOT rpm range), and what rpm the engine is running when you are trolling.

 

[Sea Rider]

Changing a impeller or complete water pump kit is no indication that engine will cool efficiently, should install a new thermo as well. If you find that engine still overheats after changing impeller, thermostat, that horse surely has tons of salt build ups on entire powerhead internal water paths if happens that have been used on salt waters for years. Flushing engine for hours with fresh water won't do a thing. Need to remove cylinder head and exhaust cover and clean all salt build ups by mechanically scraping them off.

Happy Boating

 

[Faztbullet]

Change the impeller as your motor don't have t-stats. Change to synthetic oil and buy a prop 2 pitches under size from what you have. On days your going to troll swap props. Install a tach if it does not have one to watch your RPM.

 

[murrayfish]

Just a follow-up. I changed the impeller, took out my 18' pontoon and went trolling for the first time. Worked like it should!
The old impeller was ridged but all in one piece and I guess that's why it failed to pump water at low RPM.

Thanks for the help!

Murray