Coolant Temp Readings Dithering

[BrownTrout]

Brand new 2021 Merc 225 Pro XS 4-stroke, serial number 3B011982. Five hours on the motor. Coolant Temp on the Level 2 Merc Monitor is nowhere near steady after warmup. For example, reads 143, then 138, then 140, then 135, then 142, etc. Does this all day long. Taking the boat in tomorrow to have a water pressure sensor installed (dealer didn't tell me this was optional on this motor) and I would like to discuss the Coolant Temp issue with the mechanic while the boat is there. Any ideas or suggestions would be appreciated.

[06 SB]

I sounds like your thermostats begin opening at 143 and cycle around just below that temp. Mine is similar. It is normal IMO. My ProXS 2 strokes did the same. Assuming these are idle temps.

[BrownTrout]

I sounds like your thermostats begin opening at 143 and cycle around just below that temp. Mine is similar. It is normal IMO. My ProXS 2 strokes did the same. Assuming these are idle temps.

It does this while running, not just at idle.

[BrownTrout]

Talked to two other anglers this morning who have new 250 Pro XS 4-strokes and the Coolant Temp on their MercMonitors does the same thing (dithering) while running. So . . . . . . .

[EuropeanAM]

You are simply observing the rise and fall of temps as the thermostat is performing it's duty (opening when engine is warm, closing as engine begins to cool).

More noticeable on the 4-stroke as it's only ONE thermostat (as compared to two that were "smart averaged" on your old engine).

[BrownTrout]

Thanks Don

[howieranger]

Just to add to the conversation; Jim's dithering is not what you expect from a fluctuating thermostat. When I was driving Jim's boat @ 5000 rpm the digital temp reading would change +- 10 degrees moment to moment consistently and at times hold steady. My motor, 250 Proxs V8 does vary in cold water but not very rapidly. I would not think that the water temp could go from 145-135 and back to 143 in less than a second. during warmup the temp rose in a linear fashion as expected. I was watching the temp closely because this was the motors break-in run and the WP gauge was inop.

[JS!]

Dithering?? WTH? LOL. Fluctuating. Is this just me? No biggie just wat to be sure we are speaking the same language!

[JAZII]

Just to add to the conversation; Jim's dithering is not what you expect from a fluctuating thermostat. When I was driving Jim's boat @ 5000 rpm the digital temp reading would change +- 10 degrees moment to moment consistently and at times hold steady. My motor, 250 Proxs V8 does vary in cold water but not very rapidly. I would not think that the water temp could go from 145-135 and back to 143 in less than a second. during warmup the temp rose in a linear fashion as expected. I was watching the temp closely because this was the motors break-in run and the WP gauge was inop.

You have to take inconsideration it's not a closed cooling system, it's pulling lake water which is of drastic temperature difference from the engine.

JAZII

[BrownTrout]

You have to take inconsideration it's not a closed cooling system, it's pulling lake water which is of drastic temperature difference from the engine.

JAZII

Lake surface temp was ~41 - 45 degrees. Coldest temps I've seen on this lake in the 18 years I've fished here. Are you saying these cold lake temps could cause the fluctuating readings on the temp gauge?

[EuropeanAM]

Just to add to the conversation; Jim's dithering is not what you expect from a fluctuating thermostat. When I was driving Jim's boat @ 5000 rpm the digital temp reading would change +- 10 degrees moment to moment consistently and at times hold steady. My motor, 250 Proxs V8 does vary in cold water but not very rapidly. I would not think that the water temp could go from 145-135 and back to 143 in less than a second. during warmup the temp rose in a linear fashion as expected. I was watching the temp closely because this was the motors break-in run and the WP gauge was inop.

The other possibility here is AIR being introduced into the cooling water (such as might occur from a blown gasket in the water pump).