Temperature on 90HP

[heyblue]

1989 90HP. Noticed recently when ran on muffs getting ready for this season the temperature started raising too high. Ordered all new parts; thermostat, gaskets and all the parts associated with the bypass which is under the same cover. Tested on muffs and temp stabilized, I think about 170 or so. Went on fishing trip in ocean and noticed temp started coming up while idling out of harbor, once underway temp dropped to bottom of gauge. That pretty much was the case for the whole trip, never came up to the 170 that I expected. Since putting a gauge end of last season, can't remember what it did. By design seems like what I got (low temp) is by design. the bypass as I understand happens with increased speed of engine as in while underway. Is that normal.

 

[GA_Boater]

When did you change the impeller?

 

[heyblue]

last season, but what does that got to do with running to cold? And the pea hole starts spitting out water while cranking to start the engine when launched.

 

[Texasmark]

Your thermostat pellet is stamped 143F which is the opening temperature. It will be fully open at another 10-15 degrees. At around 2500 rpms, running in the water on your boat, which should be just on plane, water pressure forces the pop off valve (the other valve under that cover) off it's seat and it takes over engine cooling requirements. If the ambient water is cool enough the stat will close back up until you slow back down and the powerhead can warm back up. If you are running 170F on muffs and everything is ok, you aren't feeding enough water to the engine via your muffs.

The Pee is fed off the exhaust manifold (large plate on port (left looking from the rear of the engine) side of engine which is a separate circuit from engine cooling because you always need exhaust cooling if the engine is running. Getting spurting during cranking speaks well of your impeller....but while running the engine it can only pump what's available and I'd say there isn't enough.

 

[GA_Boater]

last season, but what does that got to do with running to cold? And the pea hole starts spitting out water while cranking to start the engine when launched.

The efficiency of the impeller and pumping characteristics change with motor speed. That's why. Plenty of impeller stories here with very similar operating conditions as you explained - Cured with a new impeller.

 

[bruceb58]

My Mercury 90Hp 4 stroke has a 140 degree thermostat. Spec in the manual is that its fully open at 144. 170 sounds way to warm.

Sounds like your impeller is bad. The reason it drops at speed is that the water forced into the intake is helping you.