I have a 2018 F150 with the 5.0L gas in it. It has around 75,000 miles. I am only getting heat at high rpms.
Things I have done.
1. Replaced heater core.
2. Replaced water pump.
3. Refilled the system using a vacuum tool to ensure no air in the system.
4. Checked the blend door actuator.
I would appreciate any tips I can get on diagnosing this problem. I would especially like some info on the aux water pump and the heater core isolation valve. Using forscan I can see that neither of those are being activated.

 

No doubt, the answer here is....HAMMER DOWN!

Thermostat is good?

 

I have a new thermostat here but haven't installed it because nothing points to that. The upper radiator hose and the heater core inlet hose both get hot but the heater core return is barely warm. I seems obvious that water isn't circulating thru the core but I'm not sure why not. Maybe I'm missing something obvious.

 

Did you buy it new? If you bought it used, check to see if someone put a ball valve on the inlet hose to the heater core. It's commonly done, more so in the past though, to keep the heater core cool so that the AC will blow colder.

 

No it was bought wrecked in the back just last year so I have no history with the truck. It's actually my brother in law's truck. I'm just trying to help him out. I'll check for a valve on it. I think I would have seen it as many times as I've climbed over the thing.
I am wondering if anyone knows what controls the heater control valve solenoid and if it needs to activate in order for the truck to have heat at idle.
I appreciate the help!

 

Just to update you guys and maybe help someone else out in the small chance that you have the same issue.. It turns out it was the Hvac heater control solenoid. The solenoid was "working" when I tested, (that is it cycled when I applied 12 volts) it but it appears that the seal that the plunger seals off against had swollen and wasn't allowing coolant to flow thru freely even in the open position. Possibly the result of the previous owner using cheap antifreeze? Thanks for the info you guys gave!

 

Good find. The proper spec antifreeze is important on these trucks, as is using an OEM thermostat. If you didn't do so, now is a good time to change that coolant out.

 

Check for a restriction in the radiator.

 

Yes I flushed the coolant system and am going back in with the yellow motorcraft since that's what my dealer tells me they switched to on these trucks. Going to do my best in checking out the whole system before it goes back together. Thanks for all the help!

 

Does the temp gauge do anything unusual?

 

The temp gauge operates like it should.

 

Just wanted to say thank you, I read your post and mine was doing the exact same thing. I ordered the valve off of Amazon for 68.00 just had to change mounting bracket. But fixed the problem. Sorry you had to go down the path you did to fix it. I definitely appreciated your update! Have a good one 👍

 

Do you have a link for the hvac module? I have a 2019 f150 5.0 that recently started doing the same thing. Only gets luke warm at best until heavy acceleration. I have tried topping of with coolant and burping the system but so far no luck.

 

Heres the part i ordered from Amazon, you can also find videos on replacing it online its located in the passenger side wheelwell. You'll have to change the mounting bracket. But other than that it's identical.
Good luck!

 

Thanks I'm gonna start with the heater control valve but I was curious as to why this would be the reason the return hose from the heater core would only be warm as the inlet hose to the heater core is alot hotter? Apreciate alll your help.