I've been chasing this problem for about a year now, it seems like the heater core doesn't have coolant flowing through it. I've checked everything from the heater doors inside the truck, changed thermostat, the heater core isn't plugged or leaking, I took the dash apart to make sure all vacuum hoses for the climate control aren't leaking or broke, I have a cold front over the entire grill. The heater used to work great, when I got it back on the road after I wrecked it in Dec 05 it hasn't worked very well. I get luke warm air after the truck has warmed to operating temp. When I first start the truck in the morning I can hear coolant girggle in the heater core. Has anyone heard of an air lock in the heater core? Or is their anyway that the heater core could be blocked from getting coolant to it? Any ideas would be great.

 

Im not sure this is your problem, but I had similar problems on my 1500, all I had to do was flush out the heater core seperatley via the 2 smaller lines under the hood, saw all types of junk/sludge come out, was nice and hot after that. I blew all the old fluid out with air first, then the acid cleaner stuff, then flushed with a hose, air again, then filled with coolant. PS I hear my heater core bubble every morning and it heats just fine.

 

I hear coolant flowing in my heater core every morning, and mine works fine. If your hearing the coolant flowing, then it's getting to the heater core. After the truck is up to operating temp, lift up the hood and check the rubber hoses to the heater core at the firewall, both should be warm. If so, you have coolant flowing thru it.

Next thing I would do is check the blend door. It can sometimes get stuck or the servo that operates it can malfunction. Any easy check (at least on my 97) is to drop the blower motor, and reach up there to find the blend door. It is basicall above the blower motor or behind the glove box.

See if it's stuck. It may also be the servo and those can be a bugger to get to. I think there is 4 of them doing various functions of the heater/ac system.

The hoses at the back of your heater control switch are color coded and each color represents a certian function. A schematic is floating around this site somewhere with the color code key. That might help you out as well.

The servo is basically a vacuum diaphram and the diaphram can develop a leak. I would first check the temps of the hoses at the firewall, then the blend door next, as those seem to be the easiest things to check. KD

 

I'll try dropping the blower motor and checking the blend door, and flushing the heater core this weekend. The hoses in and out are warm from the heater core so coolant must be passing through it. Maybe it's just something simple like the door being stuck. Thanks for the advice I'll let you know if I find anything.