Temp Spike On Highway Hill

[zak99b5]

I just completed a 2500 mile road trip in my 2020 limited Max, first one in this truck I bought used last summer with 65k miles.

On the last leg of the way home, a 9-hour drive, I noticed that on a few long uphill sections of the highway (NYS Thruway, so nothing crazy), at 80 mph or so, the temp appeared above the idiot light. It was 23x*, and when I let off the throttle it dropped down and disappeared. Did not notice this on the drive out. We had 6 people plus luggage for a week, with a long Thule roof box on (19.4 mpg for the trip, btw). Outside temp was low 80s.

So I want replace the thermostat, seeing a couple threads about it on here. Apparently it was revised, and the updated part has a lower opening temp and black plastic on the end of the plunger. But I found conflicting part numbers.

Is the correct part RT1251 or RT1258?

And is there anything special I need to do to bleed the system when refilling the coolant?

TIA for any helip.

 

[TheDoug]

I am not sure about part numbers as I upgraded my thermostat to a 170 degree one from spd. (And modified my tune to accommodate it)

As far as install I bought a gallon of new antifreeze and topped it off after install.

 

[zak99b5]

Seems like the 1251 opens at 190*, and the 1258 opens at 180*.

1258 it is, I guess.

Or do I want a 170* tstat?

 

[TheDoug]

Since I have a tune I opted to get the little bit cooler thermostat. I think the 180 is fine for stock application.

 

[99WhiteC5Coupe]

Seems like the 1251 opens at 190*, and the 1258 opens at 180*.

1258 it is, I guess.

Or do I want a 170* tstat?

A thermostat that is rated lower than the OEM specification can cause lower fuel mileage due to slower full warm-up and also slower HVAC heat output in cold ambient temperatures.

If the engine is designed to operate in the 19x F range, a lower temperature thermostat (than OEM) will NOT cause the engine to run cooler. Once the thermostat opens at 170 F or 180 F - the engine will continue to get hotter (as designed).

 

[zak99b5]

Yes, I understand that the thermostat doesn’t cool the engine; rather it guarantees a minimum temperature.

I ordered a 180* t stat. So cooling starts ten degrees lower than specified.

Unless my t stat was sticking, there’s zero reason for my coolant temps to be approaching 240* under the circumstances that it was. Truck was not overloaded (it should handle 6 people and luggage with ease), ambient was not very hot (under 85*), 80-83 mph isn’t all that fast, and the hills weren’t all that extreme—I didn’t need to floor it to maintain speed. Coolant is proper Ford coolant, and when cold the level is between min and max in the degas bottle.

My thinking with the 180* t stat is cooling will start from a slightly (5%) lower point, giving me a little more headroom.

 

[zak99b5]

I forgot to add that I checked the trans temp when I saw the near-overheat happen, and the tranny was at 210*, which seems normal, and indicates to me that I wasn’t working the truck too hard.

 

[chuck s]

A larger radiator is really all that will lower the coolant temperature. I thought the Max came with HD Tow which at least used to include this -- if not it's fairly easy to fit one.

Don't be so sure "it should handle 6 people and luggage with ease." Not sure your cargo capacity but mine is a mere 1416 pounds which allows 236 pounds for each of the 6 passengers body weight and luggage. Plus that cargo thingy on the roof weighs something. A couple hundred pounds overweight shouldn't matter though.

-- Chuck

 

[BMW2FORD]

Part number for the updated 180 degree T stat is BR3Z8575G. I’m using it and couldn’t be happier. Trans temps stay around 185-195 and coolant around 190-200 cruising on the highway. Occasionally, the temp does rise past 200f on hard long pulls on the highway and a bigger radiator will help that but about 99% of the time it’s in the 190-200 range. I did not see any loss in fuel mileage and had plenty of good interior heat all winter even when the temps were below 20 degrees compared to the stock 195F. If you cross reference this part or go to Ford.Parts.Com and check applications you’ll see it’s used in all Mustangs, Police / Utility type vehicles, Continentals and several other pretty common but hard use to be expected cars.

 

[sixsix]

Same thing mine was doing. Bought the updated OEM thermostat and swapped it out and she's all good now.

Ford quietly redesigned the thermostat in 2022...replace it

If you are having cooling issues you should know Ford quietly redesigned the thermostat in 2022. Replace it now if you have the old one. The problem was the thermostat sticking closed. The other thing to know is thermostats do not last forever and should be replaced every 50,000 miles or any...

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