Confusing cooling issue...

[Ferson206]

Inaugural launch yesterday of '89 SeaRay with Merc 100hp outboard. After starting, I was running the motor for a while to warm it up. As I pulled slowly away from the dock, I realized the temperature (on dash gauge) was getting high, approaching 220. As I increased the RPM a bit, the gauge started to drop, and as I sped up to cruising speed it went way down to the bottom left of the gauge (pic attached). This was the pattern for the whole outing: sitting at idle or very low speed caused temp to rise toward overheating levels, speeding seemed to keep the engine cold, and moseying along right around the no wake threshold kept temps in the middle of the gauge. Is this something funky about the thermostat or maybe my impeller system or ... ?
Thanks for any suggestions.

 

[Scott Danforth]

Start with the impeller

 

[Ferson206]

Start with the impeller

Thanks. Based on my experience with prior boat, an inboard, an impeller problem would cause continuous temp rising to overheating, and wouldn’t get cooler with engine or boat speed. Am I missing an obvious difference with outboards?

 

[Scott Danforth]

Forward movement is forcing water up the pickups.

Start with the impeller

 

[Chris1956]

Mercury Impellers can do that. You will likely find damage along the bottom of the impeller, causing it to pump inefficiently at low ROM.

Replace the impeller, wearplate and 2 gaskets at a minimum.

If the SS cup is scratched replace the water pump cover.

If the water pump base is melted, replace that as well.

There are impeller kits, impeller+ pump cover kits and full kits avail.

 

[Ferson206]

Forward movement is forcing water up the pickups.

Start with the impeller

Understood, thanks!

 

[Ferson206]

Mercury Impellers can do that. You will likely find damage along the bottom of the impeller, causing it to pump inefficiently at low ROM.

Replace the impeller, wearplate and 2 gaskets at a minimum.

If the SS cup is scratched replace the water pump cover.

If the water pump base is melted, replace that as well.

There are impeller kits, impeller+ pump cover kits and full kits avail.

Great thanks Chris!

 

[Chris1956]

Thermostats also do not last forever. if it has been 8 years or more, you might check/change it as well. Most modern Mercs have a thermostat for low speed cooling and a poppit valve for high speed cooling. Since you motor runs cool at high speed, the poppit is likely fine.

 

[racerone]

Outboards need a new impeller about every 5 years.

 

[QBhoy]

Very standard symptoms of a worn impeller