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1994 Mercury 40HP 2 stroke outboard overheating

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ROBJABLONSK

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I've replaced my water pump twice and engine is still overheating. I get tons of smoke/steam out of the pee hole and when I shut down the engine it puffs out a huge cloud of steam/smoke. See photo. I suspect the exhaust plate gasket is damaged right where the copper tube slips into it's home, the sleeve at the base of the exhaust plate. I do get a little water out the pee hole when I start the engine and run at low RPM's. But if i run at a decent speed, it turns to a spray of fine mist, probably steam.

I do not have any leakage from the power head. It is dry and solid and looks like new.

To test the water flow through the powerhead i connected a garden hose to the copper tube with clear tube and hose clamps. I turned the water on low flow and immediately I get tons of water coming back at me down through the cavity in the mid section where the drive shaft is located. I also get a little water instantly coming directly down through the exhaust tube. About 80% of the water comes down the main cavity, 10-15% through the exhaust pipe and 5% out the pee hole. See pictures. Eventually a stream of water starts to come out of the pee hole. However most of the water is coming back at me down through the cavity in the mid section of the outboard and exhaust tube. Seems like the water is leaking back from the exhaust plate directly into the exhaust tube and the drive shaft cavity. Which would mean the gasket on the exhaust plate is bad?
 
That could be. I know the exhaust cooling is a separate circuit from the engine and the outside cover plate should be cool to warm to the touch, not hot if it is flowing properly. I don't know the percentage of the water pumps output is directed to the exhaust cooling.

Have you tested your thermostat? The pellet should be stamped with the opening temp (125-145 F range usually depending on the engine) and a pot of water on the stove with a candy thermometer would tell you if it is functioning properly.
 
Thanks Tejas. Appreciate your input.

I pulled the powerhead Sunday. The fitting that the copper cooling water pipe fit into was loose and the gasket was 70% missing. Screws were loose. I fixed all that, used some good gasket sealant on the new gasket and Loctite on the screws. Working good now. Too bad it took 6 hours and I had to buy a third new water pump and exhaust plate gaskets and it turned out to be two cheap loose screws on a poorly designed fitting as the root cause. Oh well, se la vie!

No thermostat on this model. Not sure why, but I've verified it in the service manual.

Other lesson I learned is only buy the Quicksilver water pumps. The Sierra I bought was missing a seal on the cooling pipe and the actual elastomeric pump wasn't in the package. Used a Chinese one instead before this fix and that one sucked. The metal pump housing didn't sit flush and the hole in it was off center. After one outing I replaced it with quicksilver because the metal had contacted the drive shaft and flared upward. Chinese crap.
 
Pictures for reference
 

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Too bad the Sierra pump was sub-standard. They have been an after market supplier for a long time and I have used their products without problems.
 
Too bad the Sierra pump was sub-standard. They have been an after market supplier for a long time and I have used their products without problems. Glad you got it fixed.

On no thermostat on some engines, I don't know why they are omitted......what difference does horsepower make on an engine requiring a controlled environment for an engine to operate at top condition especially in cold weather? I do know that not only are thermostats used on some engines, so are higher RPM pressure regulated pop-off valves used for increased flow at the higher speeds. I guess the dimensions considered when designing the engine requires reduced cooling passages and such requiring additional cooling capacity. Both my 90 and 115 HP Mercs. had both.
 
ROBJABLONSK said:
Thanks Tejas. Appreciate your input.

I pulled the powerhead Sunday. The fitting that the copper cooling water pipe fit into was loose and the gasket was 70% missing. Screws were loose. I fixed all that, used some good gasket sealant on the new gasket and Loctite on the screws. Working good now. Too bad it took 6 hours and I had to buy a third new water pump and exhaust plate gaskets and it turned out to be two cheap loose screws on a poorly designed fitting as the root cause. Oh well, se la vie!

No thermostat on this model. Not sure why, but I've verified it in the service manual.

Other lesson I learned is only buy the Quicksilver water pumps. The Sierra I bought was missing a seal on the cooling pipe and the actual elastomeric pump wasn't in the package. Used a Chinese one instead before this fix and that one sucked. The metal pump housing didn't sit flush and the hole in it was off center. After one outing I replaced it with quicksilver because the metal had contacted the drive shaft and flared upward. Chinese crap.
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@ROBJABLONSK, I have a 1994 Merc, S/N 0G058636, and have the same issue. Starts and runs perfectly (mostly), but intermittently cools just fine, or overheats like crazy. Mostly, it's the overheat scenario.
Did you replace the entire exhaust plate, or just the gasket? Can you share a part number perhaps?
While there's value in teaching the kids how to troubleshoot engine issues, I really want to get them out on the water!
 
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