I’m getting ready to replace the thermostat on my 94 F-150 4.9 300 and there are two Motorcraft parts that seem nearly identical, but one is $7 (RT1139) and the other is $36 (RT1203). The former opens at 92° and the latter opens at 90°, but also sometimes at 92°. Does that really matter? Wondering if anyone has some insight

 

RT1203 is not listed on RockAuto for that application. I would be using the $7 thermostat.

 

Yes, use the cheaper one
The temps are 190 and 192 F
I buy from Rock Auto too but do not use them for part numbers or application data

 

I see something else that is different. Apparently the RT1203 has a rubber gasket on the circumference of the circular metal flange, the RT1139 does not. It appears O'Reilly's website does not have the RT1139, various Ford dealership sites I have tried says it's discontinued, the website of Summit Racing says it is unavailable.

 

Heres the housing, the thermostat fits in the groove and the gasket is on the housing, not the thermostat

here is procedure

according to ford this is the thermostat, idk if its been superseded oranything (192)

 

So it appears that RockAuto does have the RT1203 if you search under mid to late 1990s Crown Victoria, but not the RT1139.

 

Its unclear if the RT1203 will properly work in a I6 because of the vertical setup, the perimeter o ring on the stat may mean it wont lock in per the procedure above properly (obviously would not use it)

assuming this is an accurate RT1203

this would seem to show the offset in the proper (this one) is also possibly in the RT1203, which just runs "2" degrees cooler. If the outer diameter is right is should work with the t stat housing as the service data shows I think the only reason for the o ring is for use on v8 engines with a housing that supports that, not sure what or when or which but yeah