Won’t reach operating temperature

[jerzia]

250 Proxs 2B360847 I saw temp was not working properly last time out. (Smartcraft) Motor would not reach 120, (psi 3-4lbs at idle) so I turned around and loaded up. Changed both thermostats (OEM) and today still only 115. Loaded and went home again. Flushed motor for a good 10 minutes, started on muffs and still only reached 115. Checked head temp with gun and got 131F starboard, 125F port. Impeller new (OEM as well) with less than 3 hrs on it. Since I had lower cowl off to flush with strainer removed, I changed poppet diaphragm.
Update: did another flush, still only reach 117F. Head temps still within a degree or two, I’m stumped????

And now another issue, went to check stats one last time, and when I went to loosen nut on starboard fuel rail, stud backed out of the head and nut never moved. Nut was torqued with a accurate high end wrench when installing. At a loss what to do with that….

[Bob G.]

I had the same issue with the stud coming out and here is what I learned.

https://www.bbcboards.net/showthread...ght=Thermostat

Was the impeller replaced before you started having issues with low temps? Are you checking the temps at the head or thermostat housings? Try the center of the thermostat housings if not.

[jerzia]

First off, HUGE thanks for that link, I thought I ran into something really bad. Looks like a pretty easy fix. Yes impeller was changed, however went out a few times with it with no issues. Motor got up to 134 and down to normal 125. I did not check temp at center if T stat cover but will tomorrow. I’d hate to have to spend weeks in the shop if it’s just something I can address.

[Bob G.]

It helps a lot maintaining these motors having a Mercury manual, Diacom software, parts from European Marine AND a lot of help on this forum!

[jerzia]

Absolutely! Now if I can figure out that stupid temp issue....

[jjbz21]

Are you sure the thermostats are OEM? I had a similar issue and found that the aftermarket thermostats I purchased were letting enough water by to keep it from getting up to operating temp. Went to a Mercury dealer and bought new Mercury ones and no issue.

[jerzia]

They are absolutely OEM. I was fortunate when I bought my boat that it came with a box of spare parts, all OEM. Even found a poppet diaphragm which I changed to no avail.

[jjbz21]

Are you sure nothing is stuck in them? Gravel, Grass etc? It doesn't take much. The first trip I took in the spring was to a lake with a gravel/dirt ramp and the water was low. After the first run (engine warmed up no problem). The second time I stated the engine it would not get to temp. I idled in and when I got home found some tiny bits of grave in the thermostats. The first run the next trip after cleaning them out, this time it was a concrete ramp. The same thing happened on the second run. I guess there was some gravel still in the cooling system. I flushed it out and no more issue since. Hopefully its something simple like that.

[apdriver]

I believe Don recommends changing all the components of the poppet valve if there’s an issue….not just the diaphragm. I had the same issue as you on my 250 and changing all the components fixed it.

[JOE54]

They are absolutely OEM. I was fortunate when I bought my boat that it came with a box of spare parts, all OEM. Even found a poppet diaphragm which I changed to no avail.

marked 130 or 120 ???? need to know

[jerzia]

They are 130’s 8M0057307.

Removed both stats today just to check if flushing brought any crud, both clean. Going to have to wait until tomorrow to do much more. Reinstalled the fuel rail bolt back in, so I want to give the red Loctite a chance to cure overnight.

[EuropeanAM]

I would recommend you carefully open and clean those thermostats even if you think they're clean. Not much more than 1 grain of sand can make it run cold.

[jerzia]

I would recommend you carefully open and clean those thermostats even if you think they're clean. Not much more than 1 grain of sand can make it run cold.

Actually I did that today, completely apart and thoroughly cleaned again. Hopefully tomorrow afternoon will tell a different story.

[jerzia]

I had the same issue with the stud coming out and here is what I learned.

https://www.bbcboards.net/showthread...ght=Thermostat

Once again my sincere thanks for pointing me in the right direction. My heart sank when it first happened, but it was as easy to fix as you described.

[EuropeanAM]

Make sure you pull the strainer and verify it's clean. Every once in a while we get one that just has lots of gritty stuff in the system and have to repeatedly clean the stats and flush the system.

[jerzia]

Make sure you pull the strainer and verify it's clean. Every once in a while we get one that just has lots of gritty stuff in the system and have to repeatedly clean the stats and flush the system.

I did remove it and it was clean as a whistle. Flushed with it removed a good 10-15 minutes.

[JOE54]

report back with results

[jerzia]

So here's where I'm at. Started motor on muffs. Idle roughly at 580rpm @3psi. Temp rose steadily until 127 then dropped to 109. Slowly rose to 120 and held there. I increased rpm to 7-800. Temp hit 126, down to 118 and back to 120. After a minute or two, rose to 126 and settled back at 120 (580rpm) It never did hit the 130-134 mark as it has in the past, nor did it hold at a constant 125 as in the past. Of course, the motor was not under load so maybe that would have some impact?? Any thoughts?

[JOE54]

id say its read to go with those temps stabilizing at or near 120 mark , other thremos were a lil lazy on opening

[EuropeanAM]

120 (or better) is the number you are watching for.