Poppet valve/Thermostat

[apdriver]

250 ProXS
Model 1251P73KD
Serial 1B962763

Merc Monitor installed so I watch my temps before getting on plane to make sure I come up to temp and the thermostats are working properly and open. In the past, my temps would always show a steady March right up to 130-140 and the thermostats would open. Now the temps will stall, slowwwwly increase a couple degrees at a time, stall, I have to idle up to get things progressing. In my mind, I was thinking trash in my thermostats or a piece of wood/debris and I have had this before. Yesterday, I pulled my thermostats, inspected them, tested them per the service manual instructions. Not a piece of debris, opened well, both seem OK.
So my first question is could I have a poppet valve issue. Possibly a torn diaphragm, weak or improperly operating spring, bad valve or something else amiss?
Second question. I see in the manual, the valve is in maybe the mid section of the engine? To access it, do the lower cowlings have to be removed and is it on the port or starboard side? Thank you guys for your knowledge, time, and expertise in keeping our motors in tip top shape. Your help extremely valuable!!!!

[Savage]

See if water is coming from the thermostat dump hoses when the motor is cold, when first started.

[EuropeanAM]

+1

Also, what is your IDLE water pressure?

[apdriver]

Water pressure at idle is 3.7. Pretty good stream of water coming from both dump hoses before thermostats should have opened. Is that normal? Oh, I ran it in a barrel not on muffs.

[Savage]

Water pressure at idle is 3.7. Pretty good stream of water coming from both dump hoses before thermostats should have opened. Is that normal? Oh, I ran it in a barrel not on muffs.

Nope.....thermostats need checked again, and/or replaced.

[apdriver]

Nope.....thermostats need checked again, and/or replaced.

Thanks Savage. I’ll just replace them both.

[apdriver]

Just replaced both thermostats along with the TPS, lever and cover for an intermittent high idle. Before putting it all back together, I decided to test the thermostats while the dump hoses were still disconnected at bottom. Water still streaming out with the new thermostats on cold engine. Is there a possibility some water leaks around the thermostats at idle? Even though the thermostats have a rubber seal around the edge, it’s not gasket tight in the housing.

[EuropeanAM]

Some water (small amount) may bypass the thermostat even when cold.

[Savage]

Did you replace with Genuine Mercury Marine Parts?

[apdriver]

Some water (small amount) may bypass the thermostat even when cold.

yes, sir. I believe it does. It’s not drips but it’s not full dump hose either. It’s a steady stream though. Maybe 5%-10% of dump hose or so. Do you think I’m just getting impatient waiting for the temps to come up? Could there be another issue? I will time my time to thermostat opening next time at the lake but it seems slower than in the past.

[apdriver]

Did you replace with Genuine Mercury Marine Parts?

yes sir, I did.

[EuropeanAM]

yes, sir. I believe it does. It’s not drips but it’s not full dump hose either. It’s a steady stream though. Maybe 5%-10% of dump hose or so. Do you think I’m just getting impatient waiting for the temps to come up? Could there be another issue? I will time my time to thermostat opening next time at the lake but it seems slower than in the past.

That would be a LOT of water bypassing (too much).

It is quite possible that you have debris in the system that is now lodged in the NEW thermostats. If so- make sure you remove the strainer and clean it, as well as backflush with the coastal flush attachment (while strainer is out).

[apdriver]

Ok. I’ll do all this. Try to make a movie but I’m technically challenged. Just for discussion purposes, there’s no trash in the thermostats, I just cleaned the strainer with little run time and I mostly fish in gin clear lakes, and I still get a steady stream out of dump tubes.........where do I go from there? I ask this question because I feel this will be the outcome. Just a gut feeling because I feel like I have four good thermostats right now. Again, I appreciate all your help!!!

[EuropeanAM]

Let's get the information AND the temp you are able to achieve AT IDLE SPEED (in neutral). You can worry yourself to death on some of this stuff... make sure they are clean, get the temps, and let's go from there.

[Savage]

I have cleaned BAD thermostats and found no debris, yet when tested again now seal up.....Moral of the story, you would be shocked to know how little it takes to allow a thermostat to pass water too soon. Are you disassembling the thermostats when checking for debris?

[apdriver]

I did not disassemble the ones I replaced. I tested per the service manual instructions and they tested good. When they opened in the boiling water per the service manual instructions, I brushed the sealing surface with a tooth brush. I never saw any debris.
I thought maybe the springs were weak and they were opening prematurely. I’m sure the OEM are fine but still bypass water. As you said, maybe clean but still a bad thermostat. That’s why I replaced them.

[apdriver]

Took my boat out today and off the trailer at idle, water temp was 72, went from cold engine to thermostats opening at 134 degrees in 2min.30 sec. I think I’m just getting impatient. Does this seem about normal?

[Savage]

If you had an issue, you wouldn't have made it to 134* at all.

[EuropeanAM]

+1

Time to go fishing.

[apdriver]

Thanks again for your time and expertise. Headed to the lake!