has anybody else had problems with temperature swings with the air coming from the vents inside the cab? I had my defrost set at 80 the other day on a cool morning and noticed that it would blow out hot air most of the time, but at times the air coming out was cool. It seems to do it more when I'm stopped, say at a redlight, but then starts blowing out warmer air when I get moving again. It has blown out cooler air when driving though. When it blows the cooler air, it only lasts for a minute or two, then back to warmer. I don't have the "Auto" climate control on, so the truck isn't trying to maintain a certain temperature in the cab.

sounds like a coolant issue. when your coolant gets too low it wont fill up the heater core enough to allow it to produce heat. when you come to a stop, your water pump isnt turning fast enough to supply enough coolant to the heater core and when u take off, the pump will pick back up due to increase in rpms and then it will fill the heater core back up to produce more heat. have you checked ur coolant level??

or your thermostat?

sounds like a coolant issue. when your coolant gets too low it wont fill up the heater core enough to allow it to produce heat. when you come to a stop, your water pump isnt turning fast enough to supply enough coolant to the heater core and when u take off, the pump will pick back up due to increase in rpms and then it will fill the heater core back up to produce more heat. have you checked ur coolant level??
I had to add a little coolant about a month ago, but I'll check it again.When I added coolant last time, it was about 20oz low (I believe the low coolant light came on, but temp never rose above normal).

or your thermostat?
I'm not much of a mechanic, so I have no idea to tell if the thermostat is good or needs replacing. What would let me know if it's time to replace? The truck never runs hot whatsoever, but that's not to say that the thermostat isn't the culprit

I'd be looking into why the coolant went low. Any stains below the water pump on the front skid plate?

Check for coolant leaks around water pump and on the floor. If you continue to lose coolant have it checked for EGR Cooler leak into the engine.

has anybody else had problems with temperature swings with the air coming from the vents inside the cab? I had my defrost set at 80 the other day on a cool morning and noticed that it would blow out hot air most of the time, but at times the air coming out was cool. It seems to do it more when I'm stopped, say at a redlight, but then starts blowing out warmer air when I get moving again. It has blown out cooler air when driving though. When it blows the cooler air, it only lasts for a minute or two, then back to warmer. I don't have the "Auto" climate control on, so the truck isn't trying to maintain a certain temperature in the cab.

Defrost also uses the A/C system to dehydrate the air, I have noticed the same thing in other vehicles I've owned that actually idle up when the A/C compressor kicks on. I don't notice the A/C compressor kicking on at idle in these trucks, but it still kicks on and off while defrost is on.
Check and see if it does it with just the heater on and no defrost.

Mine does the same thing.
Seems like I notice it more when I have the temp hotter like 80.
Temp gauge stays the same, so thermostat is good, and coolant is full.
I just thaught it had something to do with cab temp sensor taking a while
to read temp. Drives me nuts that it can't find a even temp.

Check for coolant leaks around water pump and on the floor. If you continue to lose coolant have it checked for EGR Cooler leak into the engine.
I checked the coolant level and under the truck- no leaks and level is where I filled it to last. I'll be sure to keep a check on it.

I had a similar problem just search my posts. Mine was needing a recalibration of the thermostat that controls the inside temp.