Heater on my 1999 GMC Sierra 1500 is not working, i.e. fan runs, but no heat is produced, just luke-warm. Coolant is OK, but possible thermostat/water control valve malfunction? I can't see a water control valve, does this model have one?

change your oil, possibly, and check the specific gravity of your coolant.

do you smell coolant ?

check your coolant level first low coolant can cause bad heat and check heater core to see if its cloged. i ussaly push water threw each side with garden hose.

Check coolant level. Check for heat at the heater hose going from water pump to heater core at firewall. Check to see that cable is moving coolant control valve from the closed to open position. Low mileage so it shouldn't be plugged heater core.

heater core is leaking needs to be replaced

no,, the system works from a vacuum and has to be worked on in a service garage. sorry about the bad news...

Your heating system is run by your cooling system. If your cooliant is low then you are not going to get adiquiate heat disapation to your heater core. So check the entire cooling systerm. Your water pump (low flow), hoses (leakage), you already checked the thermostat & heater core. So other than that I don't know.

After replacing the thermostat I pulled the heater hoses off and blew air through the heater core at firewall. Can only blow in left side as directional valves will prevent opposite. Chunks of stuff came out with coolant. I then put a piece of hose on the left side and shot water through until it came out clear.
Put it all back together and heat works fine.

It is possible that you may have an air lock in the system. You need to drain the rad again and remove the upper most heater hose from the heater core and raise the hose up high . Fill the rad until coolant comes out of the heater core pipe. Carefully reattach the heater hose. Fill the rad slowly allowing trapped air to escape.This may take a while. Once the rad is full put a catch basin on the floor under the rad filler neck.Start the car and let it run to operating temperature.Allow the expanding coolant fall into the basin.Once you see circulation in the rad you can slowly top the rad off.once you do this replace the rad cap and fill the reservoir bottle 3/4 ways to the top. Shut the car off and let it cool off filling the bottle as necessary.Hopefully this will cure your problem. Good luck :-)

How hot this gets is often regulated by the engines ability to properly heat up. This is controlled by the engine thermostat. Alot of times, people who have an overheating problem simply remove the thermostate without replacing it. This does not allow the engine to heat up properly (it actually is running too cold) thus the water coolant does not heat us. result, barely warm air through cabin heater.