GEN 1 Engine coolant won’t warm up

[thegrays]

In cold temps, truck takes a very very long time to warm up, once warmed up the top coolant temp is 158-168 F. The truck will stay there no matter the run time and hasn’t gone up. Code PO128 was registered need help.

 

[FordTechOne]

That’s an indication that your thermostat is stuck open.

 

[rschap1]

My 1st guess too.

 

[Ruger]

When changing the thermostat on your GEN1 6.2L engine be advised that the thermostat is positioned below that of the coolant reservoir. So if you just pull the gooseneck to get to the thermostat, you'll spill at least a quart or more of coolant all over your engine. You need to drain off a good gallon of coolant before addressing the thermostat. And, of course, do this work when the engine is cold.

 

[Vash]

Once you remove your thermostat, you can test it in a pot of boiling water. The OEM thermostat should be fully open around 195 degrees F. If it appears to open consistently at that temp, you could have other cooling system issues such as clogs/restrictions elsewhere in the system such as junk in the heater core. Granted, if your coolant has been changed at the suggested intervals then it’s probably just a bad thermostat that’s easily swapped using the advise stated by Ruger as far as draining a bit a coolant to empty out the top radiator hose that goes into the thermostat housing. Also a good idea to replace the thermostat gasket which is just an o-ring that’s flat on one side. You can technically reuse the current seal if it’s in good condition, but it’s cheap so you might as well replace it while you’re in there. Also, make sure you use a torque wrench on the bolts holding the thermostat elbow/housing on to prevent stripping or under tightening which can both cause coolant leaks.

 

[thegrays]

Update: the truck appears to have a cooling issue, rather heating issue as in extreme cold temps; such as 30 degrees or lower the truck cannot sufficiently bring the coolant temp above 160F. This cannot still be a thermostat can it?

 

[MTF]

Your going to have to get that thermostat changed!!!

Most owners change the thermostat to the 160 degree when they get a Tune!!!!!!

The stock thermostat is 195 degrees.

When I was up north I changed my thermostat twice a year for winter and summer months.
I wound up just leaving a 170 degree in, it was just enough to produce heat in coolest NYC months.
Now, if you're further north then a 180 or 195 will work better in the winter months.

 

[dsiggi]

Update: the truck appears to have a cooling issue, rather heating issue as in extreme cold temps; such as 30 degrees or lower the truck cannot sufficiently bring the coolant temp above 160F. This cannot still be a thermostat can it?

Yes, thermostat

 

[Ruger]

What's the problem? It's not a complex fix in the slightest.

 

[dsg2003mach1]

Update: the truck appears to have a cooling issue, rather heating issue as in extreme cold temps; such as 30 degrees or lower the truck cannot sufficiently bring the coolant temp above 160F. This cannot still be a thermostat can it?

absolutely. If it's stuck open then coolant is always going through the radiator. The AC compressor is cycling even when using heat, typically the cooling fan runs anytime the compressor is on. The coolant isn't spending enough time to get it up to temp.

I tried going from a 180 to a 160 stat in my mustang and two things happened - when hot out it still eventually got to the same peak operating temp as with the 180 and when it was cold outside I'd be cruising down the road in the 160s which is too cold.

edit: that's the whole "fail safe" design thing - it's better to fail open and run cold, allowing you to drive without killing the motor.