Mercury 90 hp overheat alarm at 2700 RPM

[larryd49]

Recently sucked sand into my cooling system. overheat alarm came on. Shutdown motor. backflushed through telltale and was able to get flow through telltale, but the constant overheat warning came on at 2700RPM. Replaced impeller, replaced thermostat and poppet valve, flushed engine prior to installing t-stat and poppet valve. Telltale has great flow. motor runs great, but at 2700 RPM constant overheat warning comes on. Temperature of head is 180F when alarm activates, which is when the alarm should activate. Telltale flow is great. Removed t-stat and poppet valve flushed engine again and didn't see any obstructions. High temp alarm came on again at 2700 RPM (180F). Any ideas why I am getting the alarm

 

[ondarvr]

If you sucked in a bunch sand it's not easy to flush it out without doing more than a simple hose flush, time to dig deeper to get the sand out.

 

[larryd49]

what would you suggest? powerhead disassembly?

 

[Texasmark]

Poppet takes over at 2500, bypassing the tstat. Poppet is there for increased flow at higher engine rpms/load. OT pellet should close at 195F +/-?, setting off the alarm. Pee has nothing to do with it. Follow the tube. Goes to the exhaust manifold water jacket and it gets water regardless of the stat and poppet action.

It'll cost you a water jacket gasket and some time. Remove the water jacket cover over the cylinders.....spark plugs stick through it. That will give you access to the whole block,

Pull the LU. Now wash out the powerhead. Then force water through the water pump by putting the muffs on the LU, water on, N gear, vise grips on the drive shaft, rotating CW looking down at it and watching for clean, clear water coming out of pump outlet. Reinstall LU and water jacket cover. Might want to replace the OT sensor too. You didn't say how long you were at 2700, how hard the engine was working (just on plane???) and what was the ambient water temp....more or less.

 

[larryd49]

thanks, that was very helpful info. I will order the gaskets and once I have them I will remove the water jacket cover. There is a water jacket cover on the spark plug side, I plan to remove it. Should I plan to remove the cover on the port side of the motor also. Thanks again

 

[Chris1956]

Gee, are we discussing an inline 6, 3 cylinder or what?

 

[larryd49]

it is a 3 cylinder. I removed the inspection plate on the power head. it looked very clean. I was very surprised. I figured the water jackets were clogged. Now I am going to remove the drive unit to see if there is any corrosion or clogging that would cause the overheating. Any other suggestions before I remove the drive unit?

 

[larryd49]

Problem solved. removed the power head, lower unit and drive section. Found a very small amount of sand between bottom of power head and drive section. Reassembled motor. Motor runs fine. No overheating issues