I have a 2000 GMC with the 5.3 - About 100,000 miles. The temp gauge reads right around 200 but the heat output is not really that good. I just noticed it when it took longer to clear the windshield of frost.
I turned the heat on high on my wifes trailblazer and turned the vents on and I could barely hold my hand in front of it, On the truck it is warm but not anywhere near as hot. Any ideas on what it could be?
I thought of the thermostat but the guage reads right at 200 so I can't see where that would make a difference.
Thought maybe someone ran into something before.

Thx.

LK

Couple of thoughts:
If it uses DexCool, has it ever had the 'green stuff' mixed with it? Mixing the 2 types of antifreeze cause a sludgy buildup that could be clogging your heater core. No flow=no heat! One test is to see how hot the lines to & from the heater core feel, if they're both 'cool' after the engine is warmed up and the heater turned on, you've got low flow.

Another thing is there could be debris buildup on your heater core restricting airflow through it. Again, no airflow across the fins, no heat transfer.

Hardware working properly? If the air diverter doors are not opening properly, you might not get the airflow either.

Blend door is not doing its thing, or you have a plugged up heater core. When I say plugged up, I'm talking about the inside of the core.

It seems as if I turn the knob very slightly to the "cooler" side it really changes it. Is there any adjustment on this thing? Seems like it's all vacuum. Never any green stuff mixed in. I'm going to look at it this weekend, Just looking for some ideas.

Thx.

LK

check your coolant level its its low that would affect the heater

Try performancetrucks.net, seems to be a fairly active board, those guys know those trucks inside and out.

It seems as if I turn the knob very slightly to the "cooler" side it really changes it. Is there any adjustment on this thing? Seems like it's all vacuum. Never any green stuff mixed in. I'm going to look at it this weekend, Just looking for some ideas.

Thx.

LK

Heater core is plugged up. Disconnect the heater hoses from the core at the fire wall, and GENTLY blow compressed air into one of the core inlets with a blow nozzle. DONT GIVE IT THE FULL 180LBS. hook up your lines back up and, add coolant that you lost from disconnecting the hoses. Get the motor warmed up, and see what you get. If the heat is warmer, your core was plugged from the inside, which is how I have almost always seen them. At this point dump in some radiator flush, and flush your cooling system. Follow the rad. flush directions. This should clean up the rest of the gunk.

Also, our trucks have a HEPA filter in the blower housing. These get plugged and your airflow will be way down, and it might just feel like it isn't getting too warm. I do know that the S-10's had alot of trouble with plugged heater cores, so that might be it as well. A good shortcut to use so you don't lose much coolant, is to take a couple of long nose vice grip pliers and clamp off the heater hoses before you disconnect them.