Differing Thermostat Temperatures: Port vs Starboard

[Aruss1984]

2008 225 Mercury Optimax Serial#1B544934

While idling on my DDT I noticed the port thermostat temp around 87° and the starboard at 100°. I assume they should be similar in temperature? I removed both thermostats once cooled and checked for debris that could keep the port side open but didn't find anything. Also flushed the engine and no debris came out.

I then removed the poppet valve and found the spring to be covered in carbon. The spring seemed to move without any significant restriction but I did order a new poppet and will install. Could the poppet have been related to the differing thermostat temperatures?

PXL_20241112_215508573.jpg

[Savage]

Tell us the sensor readings on a stone cold motor before starting it.

[Aruss1984]

Will do. How long do you want it to sit for? I can get readings tonight. Engine was last running 6hrs ago

Tell us the sensor readings on a stone cold motor before starting it.

[Aruss1984]

What would be considered "excessive carbon buildup" on the poppet spring? Reading that the carbon could indicate an exhaust leak in the power head?

[Savage]

Will do. How long do you want it to sit for? I can get readings tonight. Engine was last running 6hrs ago

I would check tomorrow, this test is to ensure that both sensors have a matching read-out.

[Aruss1984]

What could be the cause if the sensors check out to be functioning properly?

[Savage]

What could be the cause if the sensors check out to be functioning properly?

Thermostat seat leakage

[Aruss1984]

Meaning...replace the stats?

Thermostat seat leakage

[Aruss1984]

Update: both thermostats are cold and reading 46°

Both thermostats seem to be functioning equally now while idling

[Savage]

Meaning...replace the stats?

If you can't get them to reseal, then yes. I have seen a tiny 1/8" piece of a blade of grass keep a seat from sealing. The piece of grass dried out on the sealing surface, and seemed glued to the seat. I had to remove it with a razor blade. I wouldn't have believed it if I didn't see it with my own eyes.

[1ranger487vs]

When I replaced my opti stats I pulled the dump hoses to see if new ones would leak. Yes they did just not as bad as the old ones. More than what I thought. Controlled temperature just fine

[EuropeanAM]

Significant chunky carbon on the poppet spring is often a sign of powerhead or adapter plate gasket leakage.

With those temps, thermostats are almost certainly the problem. Very unlikely you have multiple failed temp sensors (and it's quite rare for them to fail "in range", though not impossible.

[Aruss1984]

Update: port side thermostat ran cold again. No debris or anything obvious after I boiled it to open it up. Ordered a new one

It never seems to end

[JOE54]

if you can scratch carbon off spring on poppet its too much , replace both gaskets with HP MERC only gaskets clean tuner idle relief holes , use only OEM MERCURY quicksilver stats , report back with results of repair

[EuropeanAM]

Update: port side thermostat ran cold again. No debris or anything obvious after I boiled it to open it up. Ordered a new one

It never seems to end

Problem isn't OPENING, it's CLOSING. Place in 140 degree water until open (suspend on string) then move to cool/cold water.

Must close COMPLETELY in 10 seconds or less.

[Aruss1984]

Update: replaced the port thermostat. Initially port climbed much higher in temp compared to the starboard side. After a few mins they came down. Still seeing a difference in the port/starboard side but higher temps than before. It usually sits around 118-120 on Vessel View. Is that an avg temp of both stats?PXL_20241120_181129527.jpg

if you can scratch carbon off spring on poppet its too much , replace both gaskets with HP MERC only gaskets clean tuner idle relief holes , use only OEM MERCURY quicksilver stats , report back with results of repair

[DewRonInc]

Replace other stat also, problem will resolve

[Aruss1984]

Update: after changing the 2nd thermostat (and poppet)my temps improved. While idling out (the average via smart craft) increased gradually to 170 and then dropped to 120 when on plane.

[EuropeanAM]

Update: after changing the 2nd thermostat (and poppet)my temps improved. While idling out (the average via smart craft) increased gradually to 170 and then dropped to 120 when on plane. I think my temp issues are now resolved.

Should never reach 170 degrees while operating... and should be sounding the warning horn by that temp in most cases.

[Aruss1984]

My initial issue was running cold, and now its hot. What could cause it to rise that high? Both stats are new. Is it a sensor issue?

Should never reach 170 degrees while operating... and should be sounding the warning horn by that temp in most cases.