92 chevy 1500 4x4 with 5.7 litre. my heater doesnt work very good unless the engine is at an idle . as soon as you drive down the road the air gets cooler unless you put the vent control on recirculation. ive replaced the thermostat,heatercore,radiator,waterpump,rerouted the heater return line into the top of the waterpump and nothing seems to be helping it works good when it is just at an idle

Chevy vehicles do not heater control valves in the water lines any more...they always rust out and malfunction. What you are probably experiencing is a bad vacuum ball or a vacuum tubing leak in the heater box area under the dash. The control lead that you use to adjust the heater (temp and direction), is vacuum controlled from engine vacuum. Engine vacuum varies under load and at idle, so the manufacturer put vacuum balls under the dash to store vacuum for the controls. When you adjust the heat or direction, vacuum from the balls is diverted to vacuum motors that position the blend doors to pass air thru the heater core or A/C evaporator. Other motors direct their door(s) to position the air to the floor, mid-vents, or to the defroster. A leak in the tubing, ball, or motor can cause the door(s) to retract to default if vacuum drops down as it does at idle. Have the heater box and vacuum system to it checked out.

1. Verify the exhaust system is not plugged. Check for less than 1.5 PSI backpressure at 2,000 RPM in park, or open the exhaust before the converter.

2. Check the Manifold Absolute Pressure Sensor (MAP) for 4.8-5.0 volts Key On Engine Off (KOEO) and about 1 volts at idle.

3. Check actual engine vacuum at idle at 2000 RPM.

4. Unplug the Mass Air Flow (MAF) sensor, reset fuel trim to 0%, and road test. ALSO CHECK FOR FAULT CODES IN THE OBD SYSTEM.

I’m happy to assist further over the phone at https://www.6ya.com/expert/uttamjeet_2f3adc18600f8ede

Clean your throttle body and map/maf sensor. If that doesn't work you may have a bad iac sensor (idle air control sensor)

Usually a problem with a dirty throttle body or a faulty idle air control valve. The computer controls idle speed by regulating how much air enters the intake thru the valve.

Assuming the check engine light is not on, I would look for a dirty throttle body or a problem with the idle air control valve.

It sounds to me like you are possibly describing a low coolant condition caused by a leaking heater core. If you can smell the antifreeze, there is a leak. You just have not checked good enough.

Idle air control valve is usually the problem when you have no drivability issues and no codes other than an idle issue., Clean the throttle body. this may help but if it doesnt you will need to swap the IAC out.

One of your pressure sending units has failed. You will have to get this professionally repaired unless you are a REALLY GOOD mechanic. It involves tearing down the top part of the motor. That sound your hearing is a sticky valve and this occurs because of top end oil deprivation. Get it fixed soon and you will be ok. If not it will burn up valves and you don't want to go there$$$.

change your thermostat, yours is sticking open not letting the coolant in the engine to warm up

if engine temp. is good. i would rec. good flush of the heater core disconnect both heater hoses and flush core in both directions. (pressure washer works good)