I have a 2006 Chevy Uplander,,I flushed the coolant system. Now it's not overheating but running hot as soon as I turn the heater on the gauge goes back down to normal operating temp

SOURCE: 1994 Century Buick Heating problem

It sound like you have a block or plug up heater.If you just used a garden hose that is not a flush .A reel flush is called a power flush it used about 150lb. of air&water mix. Find a Radiator shop to do this. as far as your temp. going up and down you may have an air trap in eng,an radiator run car for 20min without the cap on. I have been doing Radiator work over 34 years

SOURCE: 2001 mitsubishi eclipe heater does

It could be very well your water pump from the sounds of it. When was the last time you had that check or replaced? That would be my next guess to the problem...what's your mileage on the car?

SOURCE: 2002 KIA Sedona Overheating

most likely, replace thermostat .

SOURCE: my 1994 acura legend keeps overheating

when bleeding the system to get the air out, you may want to turn of the heater to help get all the air out of the system.

SOURCE: overheating fto

make sure you have 50/50 antifreeze and coolant.if its been awhile replace radiator pressure cap and thermostat.