Heat Only Works When Moving

Mitragorz · May 13, 2019

This has been an issue for a few months, but never gave it any attention, since it’s also been a few months since I’ve really driven the truck. I can’t justify 12mpg when we also have a Ford Fusion and Escape.

Anyway, it’s a 2005 Silverado, and the heat only works when I’m driving. I first noticed it last fall but, like I said, the truck has been pretty much parked since then. We recently gave our fusion back to ford (it was a lease), so my wife is now driving the fusion and I’m back in my pickup.

I noticed when I’d use the remote start to letvthe car warm up, even for 10 mi or so, I’d get in and the air blowing out would be cold. Once I started moving it’d warm right up.

I forgot about that problem until today. The battery in my fob is going bad, so I went out and started the truck with the key, like a savage. Let it warm up while I got one of my little guys out of bed and dressed for school.

Went outside to leave, and cold air. Coolant gauge was right in the middle. Started driving, got hot air.

Then I stopped at a red light. After a minute, the air was cooling off again. Started moving, warm air.

Thoughts?

Maybe related: the radiator fan doesn’t shut off. It doesn’t come right on with the key, but once it kicks on it doesn’t shut off. And it’s loud. It hasn’t been hot here yet (hence the heat), only 50s-60s. High 40s when I waltz out the door at 7:30am

Hillwood · May 13, 2019

From the way you describe it I would guess that your heater core is partially plugged.. Take both hoses off the engine and flush the core with a garden hose both ways..

kickass audio · May 13, 2019

I think it's as hillwood mentioned with a partially clogged heater core. If you can start your truck up and let it run for a bit to warm up and grab both your heater core hoses. One should be really hot and the other will be warm. If one hose is burning hot and the other is really cold then you have a clogged heater core and need to try flushing it out to see if you can get it flowing properly again.

Another idea is that when your truck is warm and sitting, rev up your engine and see if you get more warmth out of the vents. It will spin your water pump faster to make more coolant flow through the system and would also possibly indicate a clog.

edit: If your fan starts out on a low speed and gets very loud after awhile of idling then you probably have a failed fan clutch. The clutch on your truck is thermal and not electric so if it fails it can make it engage at incorrect temperature limits. They are not that bad to replace though. Does your truck take a long time to get up to operating temp?

Mitragorz · May 13, 2019

I'm out of town for a few days, but I'll give those things a shot. The truck has 207,xxx miles on it, so it could very well be clogged.

As for the fan... Not sure why, but I thought it was electric. Maybe I thought it sounded like an electric fan. The truck seems to warm up in the normal amount of time. 5 min or so. I'll start it, head back inside to finish up some things, then come back out and the temp is right where it should be. It's just blowing cool air.

But in any case, changing the clutch is no problem. That's the first thing I do when I get my hands on an old plow truck... Swap the fan clutch out for a heavy-duty.