Temperature of engine still not right

[Champ2005]

I've cleaned the t-stats (taken apart and cleaned well with toothbrush), changed the water pump (at idle it pumps 3.5 psi), cleaned the strainer, ran it on muffs, flushed it using the port on the back of the engine and my engine still won't heat up after my initial run on the lake. Well, it does at first. Three times now I've put the boat in the water and it warms up, then I run it to a fishing spot and when I go to leave the temp won't go above 85-90. I have tried two different sets of t-stats. All Mercury no aftermarket. When it was on the muffs my temp came up just fine as well, but I didn't do it more than once. I just ran the motor for 5 minutes.

Anything else I can try? The t-stats opened fine in the hot water after I cleaned them and closed. Both sets did. There has not been anything visibly stuck in the t-stats when I pull them. There was some sand in the strainer hole. I could feel it when I put my finger in there. Also, when I took the lower off to change the water pump there was some sand there as well. Not a huge amount. I do fish quite a few shallow sandy bottom Michigan lakes as well as Lake Michigan which is sandy.

Thank you for any help, because this has become an exercise in frustration. I literally want to sell the engine...lol. It only has 110 hours approximately.

[Savage]

Pinch off the thermostat dump hoses COMPLETELY, if the temperature comes up then you STILL have a thermostat problem. Sand can kill thermostats.

[Champ2005]

By "dump hoses" you mean the hoses that connect to the t-stat housing? Do I just use a couple hemostats to pinch them? Should I do this using the muffs? Or, take it out on the lake? Thank you.

[Bob G.]

I would pinch the dump hoses while running on the muffs. Easier to stand behind the motor in your driveway than at a dock while on the lake.

[Savage]

Pinch off the thermostat dump hoses COMPLETELY, if the temperature comes up then you STILL have a thermostat problem. Sand can kill thermostats.

I use clamps, or clamping pliers designed specifically for pinching closed hoses. You can do it in the lake, or on muffs.

[Champ2005]

Just to be clear, the dump hoses are those connected to the t-stat housing? Thank you.

I use clamps, or clamping pliers designed specifically for pinching closed hoses. You can do it in the lake, or on muffs.

[Savage]

Yes, but the hose has to be COMPLETELY pinched closed.

[JOE54]

cork stoppers and tie straps

[5keepers2]

Do you know gauge is working correctly?

[Champ2005]

I had a merc monitor that I purchased from Don. It was installed about 2 years ago. I guess to answer your question, no I don't know if it's working correctly. This problem started last September and I've been chasing it ever since (minus winter..lol). The t-stats open just fine, but I guess they aren't closing. But, I can't even get one trip fishing without this happening (actually one run with the boat and they quit).

Do you know gauge is working correctly?

[Champ2005]

This may be a stupid question...but when I pinch them completely closed they are not connected to the thermostat housing, or am I pinching them closed while they are connected to the housing?

Yes, but the hose has to be COMPLETELY pinched closed.

[Champ2005]

How do I test the Merc Monitor to make sure that is accurate?

[mikesjet]

Test your thermostats.
Open thermostat, place string (i used fishing line) in opening and let close onto string (line)
fill a pot with room temp water
suspend in room temp water
use a digital thermometer to check water temp
place water on burner of stove or heat source
heat and watch temp that thermostat falls off. That is the actual opening temp.

I had one thermostat that was opening at 112 and wouldn't let my 250 EFI get to operating temp at idle. Correct temp in my situation is 130deg. New thermostats fixed my issue.

[Champ2005]

Yes, I did that because my manual suggested it. I used a thermapen digital thermometer, which is really good and the t-stats work. I still take them apart and clean them as well. I did purchase new Mercury t-stats, so I have the old ones and new ones. Same thing happens to both. Thank you.

Test your thermostats.
Open thermostat, place string (i used fishing line) in opening and let close onto string (line)
fill a pot with room temp water
suspend in room temp water
use a digital thermometer to check water temp
place water on burner of stove or heat source
heat and watch temp that thermostat falls off. That is the actual opening temp.

I had one thermostat that was opening at 112 and wouldn't let my 250 EFI get to operating temp at idle. Correct temp in my situation is 130deg. New thermostats fixed my issue.

[lpugh]

Your description is not real clear:
At idle or sleightly above temps should come up to stats rated temp in less than 5 minutes, as you increase rpm and water pressures go up the poppit valve will open allowing the water to bypass the stats and temps will fall off depending load, rpm and lake water temps to how much it will drop. When you reduce speed and rpm water pressure returns to less than around 5 psi, the poppit valve should close allowing the stats to again control temps at their rated temps, if the poppit is sticking and does not close it will act the same as a stat that is stuck open. Also when shutting down the motor after a run and it hot soaks for a while the temps may be up in the 160 degree range until the water flow purges the hot water from block, this should happen very quickly, less than a minute
Hope this helps in understanding how they work
You may be fighting a normally operating system

[Savage]

This may be a stupid question...but when I pinch them completely closed they are not connected to the thermostat housing, or am I pinching them closed while they are connected to the housing?

Leave everything connected.
Pinch hoses completely closed.

Start the motor and run at IDLE SPEED, on muffs, or in the lake.

Watch temperatures.
IF temperature hits 130* then release the clamps, and inspect the thermostats again.

[Savage]

How do I test the Merc Monitor to make sure that is accurate?

You don't......It IS accurate.

[Champ2005]

I'm sorry I wasn't clear, let me add to my description. I put the boat in the water. Idle until the temp comes up to 125 and then make my run. I watch the merc monitor and can tell the poppet opens, because temps begin to fall. Then I begin to fish, and when I decide to run again, the temps will not get above 90. So, I idle to the ramp and go home. It's not a far drive, because I live on a lake..lol. I have never inspected the poppet valve, but I think I remember reading that Don said it should be like "butter?" No resistance, so I'll check that out and probably just replace that as well. Then I know it's new. Thank you for your detailed response. Everyone is always so helpful on this site.

Your description is not real clear:
At idle or sleightly above temps should come up to stats rated temp in less than 5 minutes, as you increase rpm and water pressures go up the poppit valve will open allowing the water to bypass the stats and temps will fall off depending load, rpm and lake water temps to how much it will drop. When you reduce speed and rpm water pressure returns to less than around 5 psi, the poppit valve should close allowing the stats to again control temps at their rated temps, if the poppit is sticking and does not close it will act the same as a stat that is stuck open. Also when shutting down the motor after a run and it hot soaks for a while the temps may be up in the 160 degree range until the water flow purges the hot water from block, this should happen very quickly, less than a minute
Hope this helps in understanding how they work
You may be fighting a normally operating system

[lpugh]

Thanks that clarifies it for me, I don't like to make assumptions
I doubt it is a thermostat as I think you have ruled that out, forty years of running Mercs and I have never had a bad on any of them, Can happen but the quality of there stats are really good. For sure inspect and or replace the poppit valve
I would stress genuine Mercury parts only and avoid the internet as there are lot counterfeit parts out there, verify suppliers are legit, Cant go wrong with European Marine and Don

[Savage]

Perform the pinch test when the problem occurs on the water. It's not hard to do and the results will point to the answer to your problem.

It is possible that the poppet valve is sticking, but most technicians would do the simple, no parts involved, pinch test before disassembling the poppet valve.