Replaced factory RT1252 with RT1258 today

[Hellwig]

Hey guys I also have temperature concerns when towing. Last very hot day towing my 6500 lb trailer up a big hill i heard and felt a big loud clunk, got a wrench symbol on dash, scared the crap out of me - I slowed down and road levelled off and temp cooled back down without further concerns. My dealer said i was pushing it too hard but I suspect a faulty thermostat overheating the tranny but when researching replacing RT1252 all sites indicate the RT1258 will not fit my 2020 Max Expo. Is the Motorcraft part RT1252 now updated ? Any feedback would be appreciated.

Most likely the teeth on the inner side of the outer shell were damaged. RT1258 definitely fits your vehicle. You got the message simply because RT1258 is not recommended for Expeditions (it's default for Explorers & Mustangs).

 

[East-TN]

What are you using to measure temperatures?

Ours warns me when we get over 228F in the engine, which happens occasionally when towing ann 8,000# travel trailer uphill, especially when the oil is a couple thousand miles old.

Our transmission almost never gets to the point where the car gives me a number because it’s over.

But I can’t see temperatures otherwise when it’s in the normal range.

This is a 2019 expedition.

I think most are using Forscan to change the instrument cluster to display a digital readout of temperatures above the gauges (See pic). That is what I did so now I can monitor the engine and transmission temp more easily. You could also attach an OBD II reader and see your temps that way too.

 

[Mr Big]

My 2019 Platinum has a heavy-duty cooling system with tow package. My temps are generally low. And yes, FORScan can add temp digits above the gauges. It definitely takes the wonder what my temps are out of the equation.

 

[VBreithaupt]

Replacing the thermostat with the newer version is a must for anyone 2019 on who is towing. Solves the spikes in temp with heavier towing loads. Why your at it you might as well replace the to heater core lines that they put plastic y connectors in. They will eventually crack and the dealer does not cover them under any warranty. I recommend after market hoses that have metal y connections. They can be found on Amazon.

 

[Mr Big]

Lower temperatures are not necessarily better. A HUGE misunderstanding about thermostats is that people think it makes the engine run cooler. Not totally true. The cooling system and load on the engine determines how hot the engine gets. The thermostat, no matter how open or closed, will still rely on the coolant system's size and ability to remove heat. Factors are cooling system size, number of small fins, air flow through the radiator, fans on or off, speed and load of vehicle, and coolant flow through the radiator. If the coolant flows too quickly, it loses less heat. If it flows slower, it loses a lot more heat. So, you don't want the flow to be too high. Sometimes lower temp thermostats can cause a reduction of the cooling system's ability to cool the engine. So, one problem with a low temp thermostat is that there will be times when the vehicle will run at a temperature lower than its specifications, resulting, over time, in increased wear on the engine's parts.
If one is worried about too high running temps, additional coolers may help. I know my factory heavy duty radiator and heavy-duty auxiliary transmission oil cooler, with my stock thermostat, makes a huge difference when I tow or go on long road trips in our hot environment. I do flush my cooling system once a year.

 

[bb37]

A HUGE misunderstanding about thermostats is that people think it makes the engine run cooler. Not totally true. The cooling system and load on the engine determines how hot the engine gets. The thermostat, no matter how open or closed, will still rely on the coolant system's size and ability to remove heat. Factors are cooling system size, number of small fins, air flow through the radiator, fans on or off, speed and load of vehicle, and coolant flow through the radiator.

Thank you for pointing this out. For the reasons you state, the ability of the cooling system to keep the engine cool has more to do with the design of the radiator and other cooling system factors than a thermostat that opens more quickly.

These "cooler" thermostats open sooner as the engine temperature increases, so they make you think the engine is running cooler. These "cooler" thermostats have no ability to keep the engine cooler when the cooling system is operating at maximum load. No matter whether a thermostat opens at 180 deg F or 195 deg F, once it's open, the restriction it places on the cooling system flow should be the same.

Your comments about coolant flowing too quickly are spot-on, also. I have seen race car engines that overheated even with a big radiator, big water pump, and no thermostat. Put the thermostat back in and the engine cooled just fine. The system needed the restriction imposed by the thermostat to slow down the water flow.

I'm sure you'll get some push-back on this.

 

[East-TN]

Lower temperatures are not necessarily better. A HUGE misunderstanding about thermostats is that people think it makes the engine run cooler. Not totally true. The cooling system and load on the engine determines how hot the engine gets. The thermostat, no matter how open or closed, will still rely on the coolant system's size and ability to remove heat. Factors are cooling system size, number of small fins, air flow through the radiator, fans on or off, speed and load of vehicle, and coolant flow through the radiator. If the coolant flows too quickly, it loses less heat. If it flows slower, it loses a lot more heat. So, you don't want the flow to be too high. Sometimes lower temp thermostats can cause a reduction of the cooling system's ability to cool the engine. So, one problem with a low temp thermostat is that there will be times when the vehicle will run at a temperature lower than its specifications, resulting, over time, in increased wear on the engine's parts.
If one is worried about too high running temps, additional coolers may help. I know my factory heavy duty radiator and heavy-duty auxiliary transmission oil cooler, with my stock thermostat, makes a huge difference when I tow or go on long road trips in our hot environment. I do flush my cooling system once a year.

Agree to disagree to some of your logic.

 

[Dice Roll]

Ford redesigned the thermostat. The old style was a metal one, the new ones have plastic for some reason. I don’t think mine ever fully opened from new. It finally caused a few times I caught it bounce the engine temp on the gauge. I never did like that the trans ran 205 all the time, but thought that was what the engineers designed it to run at for whatever reason maybe mpg since I’d watched it be that from day 1.

It spiked to around 212-215 pulling the camper. Now with the new stat it runs 180-195 all the time. I might’ve seen 200 pulling a grade with the camper last year.

 

[Mr Big]

Ford redesigned the thermostat. The old style was a metal one, the new ones have plastic for some reason. I don’t think mine ever fully opened from new. It finally caused a few times I caught it bounce the engine temp on the gauge. I never did like that the trans ran 205 all the time, but thought that was what the engineers designed it to run at for whatever reason maybe mpg since I’d watched it be that from day 1.

It spiked to around 212-215 pulling the camper. Now with the new stat it runs 180-195 all the time. I might’ve seen 200 pulling a grade with the camper last year.

I prefer metal stats. Plastic eventually cracks and needs to be replaced, where aluminum won't. Plastic housings seal better right out of the box, but one can seal aluminum housing with an OE quality gasket or high-temp sealant. Metal housings are generally more durable than plastic in terms of resisting leaks. But plastic housings can be more durable than older aluminum ones, against coolant degradation.
Using the specs below, your tranny was almost in the middle of the optimal temp specs, making it the ideal temperature. And you were still under the high side of the specs, pulling the camper.
The optimal engine operating temperature range for a Gen 4 Expedition is generally between 195°F and 220°F.
The optimal transmission operating temperature range for a Gen 4 Expedition is generally between 175°F and 225°F.
The optimal EcoBoost operating temperature range for a Gen 4 Expedition is generally between 225°F and 250°F on the highway. Punching it hard may cause it to go as high as 236°F, dropping back down real fast.

 

[Mr Big]

Agree to disagree to some of your logic.

It's not my logic, it's Ford's logic.

 

[Dice Roll]

It's not my logic, it's Ford's logic.

Except you disagree with ford logic. They redesigned this part to correct what they fouled up while using us as beta testers for their product. Those of us using the new part are having a better go of it than those who have the old one.

 

[Mr Big]

Except you disagree with ford logic. They redesigned this part to correct what they fouled up while using us as beta testers for their product. Those of us using the new part are having a better go of it than those who have the old one.

Redesigned for new models. Don't get confused that one thermostat fits all Gen 4 Expeditions. And Ford parts don't list the RT1258 as fitting an Expedition. Ford Thermostat RT-1258. They list this one Engine Coolant Thermostat - Ford (ML3Z-8575-A).
Temperature is the most important factor. A 2018 EX may have different specs than a 2024 or 2025. Mine uses Ford (HL3Z-8575-B) .
I was speaking generally about maintaining proper temperature. But you can choose which one you want if it makes you feel safer to have a lower temperature stat.

 

[Dice Roll]

The 1252 is what they use for the old ones now when you ask the dealer for a new stat. As the op said, that is the 195 and is what results in lower temps than the assembly line one did before they updated it. That’s what mine has, but you don’t agree they should’ve put that one out for some reason.

 

[Mr Big]

The 1252 is what they use for the old ones now when you ask the dealer for a new stat. As the op said, that is the 195 and is what results in lower temps than the assembly line one did before they updated it. That’s what mine has, but you don’t agree they should’ve put that one out for some reason.

I think you mean the op said "Replaced factory RT1252 with RT1258 today", so the new lower temp stat for his 2024 would be RT1258.
I don't disagree, maybe confused; however, I'm just looking at the specs of the 1258 and it shows it does not fit the Gen4 Expedition. I decided to verify with my dealer and they are using this: Engine Coolant Thermostat - Ford (ML3Z-8575-A), which is designed to open at a temperature of approximately 194.1 degrees and is spec'd for 22 to 24 Expeditions. But maybe some dealers use whatever fits.

 

[Dice Roll]

Nope. I meant what he said is what he said. Not sure why you are struggling with this topic. The guys at ford redid the stat for a reason and it’s at odds with what you want. It results in a cooler engine and trans, and it has some sort of space age thermoplastic, all of which doesn’t agree with you. Plus you parse posts to knit pick. I don’t care what you do, burn your rig on up.

From the OP

“The engine coolant working temp used to be from 190F to 195F with RT1252 (90.6 C/195F),”

 

[Mr Big]

Nope. I meant what he said is what he said. Not sure why you are struggling with this topic. The guys at ford redid the stat for a reason and it’s at odds with what you want. It results in a cooler engine and trans, and it has some sort of space age thermoplastic, all of which doesn’t agree with you. Plus you parse posts to knit pick. I don’t care what you do, burn your rig on up.

From the OP

“The engine coolant working temp used to be from 190F to 195F with RT1252 (90.6 C/195F),”

I'm not struggling, I read verbatim. You meant what he said but you typed it wrong.
Read your post and the OP.
YOU: "The 1252 is what they use for the old ones now when you ask the dealer for a new stat. As the op said, that is the 195 and is what results in lower temps than the assembly line one did before they updated it. That’s what mine has, but you don’t agree they should’ve put that one out for some reason.
OP: "Replaced factory RT1252 with RT1258 today
The engine coolant working temp used to be from 190F to 195F with RT1252 (90.6 C/195F), and now it fluctuates from 176F to 181.4F with the new RT1258 (82C/180F)."
The 1252 is the OLD stat and the 1258 is the NEW stat. You just switched the numbers.
We can agree to disagree, but there's no need to be insulting and derogatory, when someone posts FMC mechanic facts.

 

[Dice Roll]

You’ve been combative this entire thread. As it stands, you’ve stated you will go with the ford logic, yet don’t agree with it. At all.

I believe my stat was replaced with the superseded part for 1252, and you have said the 1258 doesn’t fit the expy. In fact, you linked a 1250 stat if you look at the other names section of your link.

 

[Mr Big]

You’ve been combative this entire thread. As it stands, you’ve stated you will go with the ford logic, yet don’t agree with it. At all.

you linked a 1250 stat if you look at the other names section of your link.

It's not being combative; it's all opinion based, and I assumed you understand opinions. I accept yours is different.
The only difference is you chose to be insulting and condescending, rather than be professional and just disagree with my opinion.
Regarding thermostats, one needs to check the Genuine Ford Parts description to make sure the thermostat one uses is the correct part for the specific Ford year and model.
In closing, we all have opinions and that's all they are. In the end, we can decide what's best for us.

 

[East-TN]

Yikes. This went south pretty quick. I was agreeing to disagree with some of the minor word nuances used in previous posts. I've been using the RT1258 for 3 months and am happy with the 180 temp.

 

[Fastcar]

You guy's ought stop measuring your *****! Ya beat this subject to death imho.