2004 Merc 150hp

[waterking]

I have an overheat problem. Changed out the waterpump and thermos last June. Runs fine--130 degrees at cruising speed, 170 degrees at idle or trolling. This past weekend i tried to tow a guy that was in a 19 ft alum, boat that had taken on water. I was at 3200rpm and 6 knots--overheated in 3 mins. Stopped cooled down and started back up--same result. Had to cut him loose. Within 100yds i dropped from 200 + degrees to 140 degrees. Any thoughts??

 

[CharlieB]

Re: 2004 Merc 150hp

Depends how much throttle was used to maintain that 3200 RPM?

If WOT, then that was a LOT of SUSTAINED load and not enough water pump flow for the load.

Accelerating for a brief period the engine will produce substantially more heat than while running at sustained high RPM at WOT, but it is only a few seconds and you're up to speed, the water pump is flowing properly and the engine cools before the system has time to register the 'thermal shock'.

Think of it as 'dwell time', low rpm WOT = highest possible load, atmospheric pressure has more time and pushes more fuel and air into each cyl than high rpm WOT. More fuel and air = more heat.

Never tow another 'full size' craft at half throttle or above.