1998 Merc 200hp Outboard Overheating at High Speed

I have a 1998 Mercury 200hp which is overheating at cruising speed. At idle, it operates at normal temp, but if I take off, the overheat alarm will come on within about 2-3 minutes, at which time I immediately pull back to power to idle, overheat alarm turns off within 1 minute once engine cools down. The impeller was replaced 1 year ago and the water pump seems to be pushing out a constant stream. Engine runs very well except for this. I have done the following so far:

1)Replaced the Thermostats
2)Pulled off the heads, looked for damage (cylinders, pistons, head gaskets looked ok), replaced head gaskets, reassembled
3)Opened poppet valve housing, poppet valve seemed to actuate ok, replaced poppet valve rubber diaphram, reassembled

Engine is still overheating at cruising speed. Not really sure what to try next. Any thoughts?

Re: 1998 Merc 200hp Outboard Overheating at High Speed

Since you replaced the diphram it iself, I would sugget that you replace the spring and the carrier and the grommet on the poppet assy. It sure sounds like the poppet valve from what you describe. The only other possibility is a clogged water passage in the block itself.
Do you have a water pressure and temp guage? What is the water pressure? If you don't have a pressure guage, I would sugget you install one as the pee stream indicator doesn't tell you anything other than water if flowing.
Maybe an exhaust leak, but for a 98 engine, I would not think that would happen.

Re: 1998 Merc 200hp Outboard Overheating at High Speed

Agreed. Replace all the components of the poppet assembly.

Have you replaced the water pump? The hub of the impeller could be slightly spun and at higher RPMs may not be spinning at all.

Re: 1998 Merc 200hp Outboard Overheating at High Speed

i have a 1987 60hp 3cyl merc 2stroke that over heats at high speed in WARM
WATER? WONT OVERHEAT IN 60-70 DEGREE WATER AT ALL???????
BEEN GOING ON FOR 4 YRS NOW AND I HAVE A BUDDY WHO ASKED ME IF I CHECKED AND REPLACED ALL THERMOSTAT/POPPET VALVE ASSEMBLY YET??
NO...........IT DOESNT HAVE ANY OF THE 6 PARTS THAT I SHOW IN MY SHOP
MANUAL ON THE MOTOR?? DONT KNOW WHY THEY ARE NOT IN ENGINE?
BUT I HAVE GONE TO 4 DIFFERENT SHOPS HERE IN RICHMOND,VA AND NONE
OF THE SO CALLED" EXPERTS" CAN FIX THIS TROUBLE???
SO I WILL LET YOU KNOW WHAT MY FIX IS GOING TO BE...PLEASE LET ME KNOW YOURS.


RICK
804-276-2806