Hey all - 2003 F150 SuperCrew XLT with 5.4 engine. About 2 weeks ago started noticing that the heater would start blowing cold air periodically while out running errands. Outdoor temps were probably 30-45˚ F during this time. It was not consistent enough to cause concern. Today I started the truck and let it run about 6-8 minutes before driving it (about 25˚F outside). It was decent when I got in but in less than 10 minutes it was blowing cool air while driving city speeds. Never really warmed up. Went out again at lunch and it was the same thing. Started warming up ok but after driving it got cool again while driving city speeds. I jumped on the interstate for about 5 minutes at one point, and it was blowing so hot I almost turned it down. As soon as I got back to city streets, it cooled down and stayed that way. Below is a pic of the temp gauge. This is about the highest it got during the entire time.

I checked the coolant level the other day, and it was just under full in the overflow tank. I didn't have any antifreeze on hand to top it off but got some today. I'll try that and see, but didn't seem low enough to cause issues. Maybe?

In the meantime, I was wondering if you all had any other thoughts of things to try? Shop can't get it in for another week.

 

As a follow up, I was much lower on antifreeze when I checked it today. Topped it off and it's heating like normal again. Now to figure out where it's going!

 

either it's going into your oil, or the valley under the intake, or leaking out your heater core

 

Or leaking out the radiator....