Hello:

I have a 2004 1500 Silverado. The heat works for awhile, then blows cold, then hot, then cold for varying amounts of time. I thought maybe it would only blow hot when I was driving faster, but that doesn't seem to be the case. Also, the temp gauge seems to move around both sides of 210 degrees when it seems like it would stay more constant in the past (just below 210). Maybe connected to the heat problem in some way?

Anybody have any ideas?

 

Maybe a sticky thermostat? Any ideas?

 

Check the coolant fluid level in the radiator and overflow tank. If you're a little low the temp sending unit will often hit a "bubble" and change temps until hit with fluid again.

I'm more of a Ford man (don't hate me) but many trucks use a vacuum controlled blend valve to regulate the mix of cold and warm air. If GMC does it like this (Ford does) then this valve may be faulty or have a vacuum leak in the control line.

Just a few things to check. Good luck.

CMOS

 

It looks like its a lack of coolant. It's way below the 'Cool' line after driving home from work. Where could the coolant be going? There isn't a puddle on the floor or ground where it's parked. Please don't tell me it's going in the engine or something.... :-o

 

I have a 04 Silverado as well. I'm having a similiar issue with my heat. I can't run the defroster when i'm in auto mode so I have to switch it to manual mode. When in manual mode the valve is spose to direct the flow of air over the heater core depending on the temperature that you have set. When I have mine set at the lowest (which is 60 degrees F)it will blow hot air continuously and then suddenly change to cold. It will switch back and forth without me making any adjustments. Now I don't have any issues with my engine cooling system. I have read some system descriptions and they say that there are a couple of outflow temperature sensors. I have reset the ambient temperature sensor. The question that I have is if the various temperature sensors feed into the heater core valve when in manual mode or not. I was thinking that when in manual mode that the valve is set at what position the temperature on the climate control commands. Can you give me any insite into how this sytem works and if I should be looking at the sensors or the climate control system for faults.

 

Open the hood and give the Upper and Lower Radiator Hosed a good squeeze. Keep an eye on the Hose from the Filler Neck to the Expansion Tank. If you don't squeeze some Coolant into the Overflow Tank ... there's a very good chance that you have an "Air-Bound Cooling System"

Make a gallon of 50/50 Mix.
Take of the Radiator Cap.
Start the Engine and keep the level of coolant right to the upper lip of the Filler Neck.
Have the Heater ON.
As air escapes from the system you top-up the Radiator. Keep topping up the Radiator until the Thermostat OPENS and the Coolant Flows out of the Radiator.
At this point ... keep the Coolant Level topped right up so no air is circulated.
Let air escape and keep the level full.
When the Temperature of the Coolant rises and opens the Thermostat ... add coolant until all the air is out of the system and then Replace the Radiator Cap and fill the Overflow Tank.
Undo the overflow tank and work-out any air bubbles in the line and reservoir.

That ought to do it!