99 Suburban Won't Heat Up
 

My Mom's 99 Suburban runs normally at 155* and when your hard on the throttle it will get to 170* max. This is a problem because winter is coming and the heater blows cool to very lukewarm air. I've replaced the thermostat twice with a dealership 195* and the heater control valve (controls the flow of hot water into the heater core, not sure of the technical name). The rear heat only blow cool air. Its got a 150k on it, 5.7 engine. I've flushed the heater core, seems to have great flow.... Any ideas?

Flushing the heater core was a good start, did you check the lines and hoses going from the enging to the heater core for both front and rear, i have a 99 Sub with over 200k and I have had to flush the rear line due to them being pluged from front to rear. Chevy used steal lines and rust can build up in the lines, also there is a vacuum operated water valve that open when you want hot water to be supplied to to the heater core, that may need checking to see first if you are getting a vacuum to it and that it is opening when the vacuum is supplied to it.

I've flushed the lines and replaced and checked the vacuum operated water valve.

Shouldn't the engine be running at 195 degrees? Not 155*?

Welcome to the forum.

Try checking the t-stat in some hot water and thermometer on the stove to see if it is opening and closing fully.

check the action on the climate control panel and the blend door motors.

Make sure its going in the correct way...

Update on the project... Replaced the radiator (it had a very small leak). Replaced the heater core. The heater core warms up when the engine runs for a few minutes, too hot to touch. At an idle there's no heat when the fans on, but if you rev it up to 3 or 4k it blows warm to hot.... So I'm thinking the heater core isn't getting enough flow, that is the water pumps job.

Do the easy least exspensive thing, replace the thermostat with a 195 deg. Go from there if that dosen't solve the problem.

Did that and bench test it.

Can you feel flow by pinching off the heater hose to the core?