Erratic Cooling

[Swacer]

Just noticed this in the last couple days. Its about 30-40F outside.

Truck will heat up normally, but when I start coasting anywhere, the temp gauge will start going back toward 'cold'. On typical coasting, it will come back enough to uncover the temp symbol. On a big hill, it will come back about 1/3 of the gauge.

As soon as I start giving the truck gas, the temp gauge will go pretty quickly back to normal temp.

My first thought was a stuck open thermostat, but I did notice, if I sit at a light, the truck will heat back up to normal temperature and HOLD there. I had a thermostat stick open on the truck once, and the truck would cool down while sitting at a light. Thus, this thermostat is only about 25k miles old.

Heater Core Replaced at about 40k miles. Coolant was flushed at 50k miles or about 9k miles ago.

Is it possible that I have another thermostat kicking the shit? Or is there something else I should be looking at?

 

[tltoronto]

Honestly, it sounds like you have a bubble in the system. Fill your reservoir, and go find the steepest offroad hill (easier to find) and let it idle on it for a few min with the heat on. That's what I did to fix my Nissan frontier when I experienced the exact same thing.

 

[Swacer]

Honestly, it sounds like you have a bubble in the system. Fill your reservoir, and go find the steepest offroad hill (easier to find) and let it idle on it for a few min with the heat on. That's what I did to fix my Nissan frontier when I experienced the exact same thing.

Its interesting that you mention that, I had found a few other F150 forums mentioning the same thing. I will see what I can find.

 

[Swacer]

Honestly, it sounds like you have a bubble in the system. Fill your reservoir, and go find the steepest offroad hill (easier to find) and let it idle on it for a few min with the heat on. That's what I did to fix my Nissan frontier when I experienced the exact same thing.

I gave that a try. Used an extremely steep driveway, sat there with the heat on for 10 minutes....no avail. Still doing the same thing.

 

[MTF]

I don't think there's a problem, mine behaves the same way for 9 1/2 years in the winter months.
1 1/2 years NA and 8 years Supercharged.
I run a 170 thermostat. It does what yours does just cooler.

 

[onthebrake]

Just change your thermostat. It's a cheap fix. I just had to changed mine on my '17 with only 20k miles, my truck was running way too hot in the winter. 35$ later, problem solved.

 

[Swacer]

Just change your thermostat. It's a cheap fix. I just had to changed mine on my '17 with only 20k miles, my truck was running way too hot in the winter. 35$ later, problem solved.

I think thats what I'm going to do. Did you just remove the hose downstream and just lost some coolant onto the the floor? You didn't drain the entire radiator, right?

 

[onthebrake]

I think thats what I'm going to do. Did you just remove the hose downstream and just lost some coolant onto the the floor? You didn't drain the entire radiator, right?

I went with the SPD 170F thermostat. I don't hear my fans coming on anymore and the truck still warms right up, no problem. It wasn't that big of a deal. I did stress about it a little bit before hand. I get it. The struggle is real with all of us. I just sorted out all the right tools, and took my time being methodical. Don't rush it and don't strip the threads on the water pump housing and you'll be fine ! You just need to remove the ''Y'' tube that connects to your airbox. Then three bolts that connect the thermostat housing to the motor. You do not need to remove the hose from the housing. You'll need 2 - 3 drain pans underneath the truck. I lost about 1 1/2 gallons with my truck parked uphill slightly. Best upgrade I've done to my truck to date, and I've done a few.

 

[MTF]

There is a plastic drain plug on one of the side tanks at the bottom facing the engine
but I just used a 1/4 hose and siphoned a gallon or so into empty clean jugs a have laying around.
Didn't want to waste time removing the skid plate.

 

[tltoronto]

Man that sucks! Im pointing to the thermostat, though I wonder why it shit the bed so soon. There aren't many other parts associated to the cooling system other than the water pump. Does anyone know wether the impeller is plastic and could be worn? Maybe tolerances are worn and its not moving the same amount of coolant?