Why does my 2004 avalanche not heat up when i am

Check the coolant level. in the winter time It will not over heat very easily but vapor locks can occur and low coolant level will do what your describing ...When you drive the water pump is spinning faster causing coolant to intermittently flow through the heater core so it gets warm then cold at an idle.. also a thermostat that is stuck open will also cause this its also in the cooling system, if the cooling system is full I would replace the thermostat..good luck hope this helps

There are a few problems that could be occurring. check for coolant leaks, external or internal (check coolant level), check water pump seal for leaks, you should also check to see while in park hold engine rpm at 2000 when warm and check the heat from the vents if it stays or not. Also poor water pump flow or restricted heater core (get it flushed or replace) could be possible. Also a thermostat that was stuck open would cause poor heat at idle and really long vehicle warm up. Also if you have had any service to the cooling system previously air trapped in the cooling system would cause poor heating inside the passenger compartment and poor cooling of the engine (called "air locked"). hope this will help you

This sounds a bit like 2 problems, make sure your coolant is topped up then try replacing the thermostat, and see if that solves the problem entirely... If that doesnt help then you wanna get your water pump checked. If this is to blame then save yourself a headache later and have the mechanic also replace the timing belt... The labour is the same making it a total no brainer...