Because of the colder temperatures, I decided to replace my 180 degree thermostat with the OEM temp. The only one I could find locally was a 205 degree thermostat. I bought that and installed it, and now the truck is running too hot.


When I first installed it, the needle on the gauge seemed to rest a touch to the right of center, which was about where I expected it to be, given that the thermostat is a little hotter than stock.


On the drive home today, however, it went all the way to the last hash mark before "H", which caused a great deal of panic on the drive home. It never did get all the way into "H" or to the right of it, but it's never been this high before.


I topped the coolant off when I replaced the thermostat, so I'm scratching my head as to why it's running so hot. When I stopped the truck after reaching my destination, the top coolant hose was hot to the touch, however the center of the radiator, and the lower coolant hose was cool to the touch. The very edge of the radiator closest to the passenger side of the truck was warm, but for all intents and purposes, the radiator was cool as well.


What could be at fault here? Thermostat, radiator, water pump? When idling in the driveway, the gauge sits at about 3/4 of the way but doesn't go past that. It's only when I'm driving at highway speeds (RPM at 2500 or higher) that I notice it get far into the hot side of the temp gauge.


Temp gauge has always worked reasonably well, so I have no reason to suspect it being at fault. Before changing the thermostat, the truck coolant system was bulletproof, would never go above about 1/4 on the gauge under any circumstance. Is it possible to get a bad thermostat?


Is there something I missed in changing it that I should have done? Bleed the system somehow..?


For what it's worth, with the truck running, I could open the radiator cap and peer down into it. I saw coolant in there, but nothing worth noting, no movement or anything. When I shut the truck off, I forgot to put the radiator cap on, and within 60 seconds or so, all of my coolant started pouring out of the top of the radiator, and it was steaming hot.


I'm at a loss here and need help quick, as the truck is my daily driver for awhile. Any help / suggestions / input, etc. would be GREATLY appreciated!

 

The classic thing to do is put the old part back and see if the problem goes away or not.

Relying on factory gauges isn't really ideal.

It is possible the new thermostat is sticking, it is possible there's an air bubble in the system, it is possible that..........

 

I'll bet you have a bad t-stat that's not opening all the way.

 

Bingo. Wanted to believe it wasn't the thermostat, but all signs point to it being the culprit. Stuck the new part in a pot of boiling water and it didn't budge. Put old part in same pot; it opened quickly. Hard to believe a new part can be such garbage, especially one that has the potential to ruin a $1,000 engine. *smh*

 

Order the OEM Motorcraft RT1139 thermostat and be done with it. They work, aftermarket parts store thermostats are JUNK! been there and done that. I had a Motorad 180* act like a 160* thermostat. Never looked back.

 

Sometimes old Ford parts are better than the new stuff....

 

Whats your altitude in Colorado?
It well could be that water boils at or below 205*.

On the other side, I see people here on the forum all the time that put a 160* thermostat in an EFI truck and then complain about stumbling and poor mileage.
(because its got to help, right? )

WTF do you think is going to happen if the computer is reading a cold engine all the time?

Care to share what brand thermostat, so we can all avoid them?

 

If his radiator cap is not faulty, his boiling point will be pretty high. Heating water under pressure raises the boiling point. There is probably a chart somewhere that will tell you under what pressure water boils. Of course you would have to subtract what the chart says for a high altitude, but the boiling point would still be way higher than 212.

To the original poster; It worked out for you, but you can waste a lot of money relying on the factory temp gauge. If you get a aftermarket gauge with numbers on it, you can actually see the needle go up to 180 or 205, shoot past a little bit, and then drop below as the thermostat opens. Don't trust your oil pressure gauge either.

 

He said he tested in a pot of boiling water...
There is no pressure there.

I DO agree that under 13 psi there should be no pronlem

 

XR3Z-8575-BA (replaced D9BZ-8575-A & F3TZ-8575-A) .. 192 degree Thermostat (Motorcraft RT-1119) / Available from Ford.