Idle Temp HIGH?

[Bucknbass01]

mercury 175 Pro XS…(SN-2B119052)
I Have had this engine on boat a few years. November(ish) 2024 I decided to install Smartcraft so I could get solid engine readouts vs gauges I had on dash. After install did not use boat till about mid March 2025, and with digital readouts got concerned with couple items (water PSi/Temp). at that point I change L/unit oil; water pump (47-89984-3); cleaned compressor filter; flushed out water line; changed belt (57-828507Q 4); thermostat (833072003)x2; thermostat sensor (8M0148668)x2. Took boat out past weekend. Water pressure seems okay around 10lbs at not quite WOT. But TEMP still scares me. When left ramp had to idle out to lake temp got 165-170 but started to get on pad with increased throttle and temp went down instantly to around 118-120. Stayed around 120-122 for pretty good run but when came off pad to idle to first spot temp jumped way back up in high 160s-170s. Not sure what other details needed to help resolve this issue, but will try to fill in blanks if I have left info out. Any and all feed back is GREATLY appreciated. Thanks..Andy

[JOE54]

were all 3 gaskets replaced in water pump ????

[SilverFox579]

were all 3 gaskets replaced in water pump ????

+1!

Key water pressure value is at idle not above it. Need to see 1 - 3psi at idle, if not, you'll need to find out why. Joe asked a good question, all 3 gaskets must be replaced during water pump service.

I posted a water pump service topic, complete with pics, you can find in the FAQ at top of this forum - Water Pump Replacement

There's a topic covering thermostats there too.

[Bucknbass01]

Mr. Silver fox, I do have above mentioned pressure at idle, however I did not change the 3 gaskets. I will also rewatch that number next time out.

[EuropeanAM]

Mr. Silver fox, I do have above mentioned pressure at idle, however I did not change the 3 gaskets. I will also rewatch that number next time out.

If you did not replace ALL THREE gaskets: Stop there. Go back and do it right.

Can't stress this enough: I've found "Casper the ghost" hidden under the impeller plate or the water pump base MANY, MANY times. And Casper has some other little gremlin friends, too!