History-I replaced the complete thermostat and housing due to a leak in the plastic housing about a month ago. After this, I noticed the temperature gauge was running a little cooler than before (before was pointing at the center, now maybe 3/8 of the way). Took it on a 80 mile trip Sat. night and the gauge was too low in my opinion (1/4 or less), but the outdoor temp was in the 30’s, so I made a mental note to look into it. Then the car decided to help me remember by throwing a P0128 error code (low coolant temp…).
I am going to assume the new t’stat is stuck open and replace it. My question is what is the ideal recommended operating temperature of a new thermostat? The Mac stores show everything from a 170 to a 192. I neglected to look at either the old one or new one (don’t know if it’s still stamped into their bodies). I can’t find anything in the owners manual. I live in central IL. If I had to lean one way, I’d rather have a cool(er) engine in the summer than an overly hot heater in the winter.
That is actually an oversimplification of the actual reason for the vehicle manufacturer’s specification regarding the temperature at which the thermostat opens.
Because engines burn less gasoline when they are fully-warmed up, and because they emit fewer pollutants when operating at their “design temperature”, it is a good idea to use the same specification thermostat as the vehicle manufacturer.
Additionally, when engines run “cooler” they build-up more sludge in the crankcase, and that sludge will eventually impede the passage of oil through the tiny oil galleries within the engine.
“Hotter” thermostats help to enable longer oil change intervals, and anyone who uses a “cooler” thermostat would be well-advised to do oil changes at much shorter intervals.
I suggest that you get the thermostat from the Ford dealer’s parts counter, or–if you want to save a few bucks–find out the temperature specification that Ford uses and then buy one at the local parts store that matches that temperature specification.
I should have phrased my question differently. We don’t have a Ford dealer in my neck of the woods. I was hoping someone could tell me the temperature specification that Ford recommends for this engine. I did some pecking on the computer earlier but couldn’t find any specifics.
Most of the OE replacement thermostats on rockauto are 190F or 192F, so that’s what I’d use. If it runs hot with one, that means you have a problem elsewhere in the cooling system, like a partially plugged radiator.
Thermostat specifications;
Thermostat Opening Temperature 4.0L SOHC
Starts to open 90 - 94 degrees C (194 - 201 degrees F)
Fully open 106 degrees C (223 degrees F)
Stant states that the OE thermostat for your vehicle is rated at 198 degrees.
https://www.rockauto.com/en/moreinfo.php?pk=7056676&cc=1428229&jsn=357
Tester
I saw that, but the several other ‘OE’ ones were rated at 190-192. I guess that’s opening temp, Nevada’s specs fall right in between.
Gentleman, many thanks!!! (I’d insert a happy face icon here if I knew how). @VDCdriver; thanks for that tutorial. I know remember dad saying something along those lines 20 years ago. That man could remember everything - I’m having trouble trying to recall what my name is half the time.
Our local Autozone has the 192 degree in stock, so I can tackle that one evening this week before it turns cold for good.
You are very welcome!
Even though I retired from the classroom many years ago, I think that I still have the ability to impart some good information…at least some of the time…
Beware: many of those aftermarket thermostats are junk.
So what you have in there might be the right rating but it’s just not doing the job.
I suggest you get or order an OE (Motorcraft?) 'stat.
If your coolant level isn’t correct, you can also get P0128
Seen that happen a few times 
It’s a good idea OEM or aftermarket replacement stat to test it before installing. Easy enough to do with some hot water in a pot on the stove & and thermometer. Make sure it opens at the correct temperature and reaches the spec’d fully open dimension.
I’m trying to schedule a long lunch hour later this week - with the time change, it’s too dark to try this in the evening. I get to drive my weekend cruiser for a while.
The coolant level is full, made sure of that first thing.
Once I get to the t’stat, I’ll know if the Dorman housing came with a junk one, or if I have other issues, and I’ll be sure and post an update.
Is the reservoir AND the radiator full . . . ?
Yes, both. I’ve been checking it regularly in the last three weeks since I changed the housing/t’stat assy making sure I didn’t have any air pockets.
I promised an update - changed the thermostat yesterday with a 192 degree. The temp gauge came right up to halfway; where it was before all my headaches started.
I tested the new one on the stove before installing.
I tested the old one after removal-it opened right at the correct temp, so I repeated several times. I finally caught it getting slightly cocked to one side and leaving a 1/64" gap around its circumference, so I am hoping the old (three week old) one was indeed bad.
Let’s hope this is a closed case!! Thanks again for all the good advice and help.
The Motorcraft RT1167 (OEM) is a 190 degree thermostat.
Good for you OP for getting it resolved. I’ve also had replacement thermostats fail right out of the box like that. You know the stove trick now, so all’s good.