2001 gmc jimmy engine warm but heater blows cold air

If the temp gauge is now more stable, the problem is in your heater system, not the engine cooling system.

sounds like water pump is broken

might need to be recharged and might have a leak in the system. take it and get it checked

common for this vehicle to have clogged heater core

Try stopping on a steep incline--does the heater stay warm? If yes, what you have is an air bubble that hangs at the heater hose until pressure from the water pump blows thru it as the engine speeds up. Park on an incline, open the radiator cap, run the engine until the thermostat is open, and fill the radiator to the neck. Wait until the level stops dropping and make sure bubbles don't continue to come up. If they do, you may have a blown head gasket that will keep forming these bubbles until you change the gasket. Please let me know the outcome, as we are getting a lot of these cases and are trying to improve the answers.

UNHOOK BOTH HEATER HOSES AND FLUSH OUT HEATER CORE
DO IT FOWARD AND BACK FLUSH / COVER DIST. CAP OR YOU WILL
FILL IT WITH WATER / IF IT CLEARS OUT HOOK HOSES BACK UP AND
BLEED AIR OUT OF SYSTEM / IF IT DOES'T CLEAR OUT HEATER CORE
WILL NEED TO BE REPLACED (DASH COMES OUT TO DO HEATER CORE)

check for a thermostat that is stuck open and will not let the block warm up when running,if it can't warm up to operational temperatures,it will tell the computer to keep the fuel/air mixture rich and this will will make it run rough, like having the choke on partway in an engine with a carburetor controlled engine

make sure heater core is hooked up some times when the heater core goes some people just plumb it back ti itsself

heater control valve

Let the vehicle warm up to normal operating temprature, touch the heater hoses going into the firewall to the heater core, they should both be hot, if only one is hot the heater core is stopped up. If this is the case you will want to try flushing it before you replace it. If you have to replace it be prepaired to spend $400.00 or more depending on shop rates. NOTE: BE CAREFULL WHEN CHECKING HEATER HOSES YOU COULD GET BURNED! Thanks and good luck with your repair.