Could be an air lock in the hot water system.
Normal procedure is to slacken a hose clip at the highest point and bleed the air out.
Don't try this when the engine is hot or someone could get scalded. Surprised the dealer didn't bleed the system as they would have a special radiator cap that allows the system to be put under pressure
SOURCE: 1997grand voyager is blowing cold air new
I would look for a vacum leak on the control valve for the heater. There are 2 lines from the radiator leading back to the firewall. One has a control valve on it that should be vacum controlled. When you back flushed to the heater core you may have forced this valve open temporarily. The control may have low vacum or insufficient vacum to open a stuck valve. The good news is you can actually manually open this valve and tie it in the open position which will at least temporarily solve the problem. Check your vacum lines for leaks and check the valve to make sure it isn't sticking.
SOURCE: replaced water pump engine overheats
You still have air in the system. remove the radiator cap, if you have one, if not remove the resevoir cap. start the car and let it warm up to operating temperture. keep check on the level and fill as necessary. you may have to do ths a couple of times.
SOURCE: heater blows cold air
Air
Temperature Control Blend Door Failure No Heat In
all modern cars there is a tiny DC electric motor driven gear drive
that moves a plastic door that blends heated and cooled air from the
AC and the heater core, this is called (interestedly enough) the air
temperature control blend door actuator. The motor that moves this
door fails because the door starts sticking from warping, most common
complaint is no heat but it can be no AC as well. To replace these
parts (blend door and blend door actuator) you must remove the entire
dash from the car. Always have this problem confirmed by a dealer or
qualified repair shop as it is a very expensive repair.
1 run truck to operating temp 2 feel both upper and lower heater hoses near firewall both should be same temp if not clogged heater core 3 if both same temp mode door not changing from a/c to heat
Air
Temperature Control Blend Door Failure No Heat
In all modern cars there is a tiny DC electric motor driven gear drive that moves a plastic door that blends heated and cooled air from the AC and the heater core, this is called (interestedly enough) the air temperature control blend door actuator. The motor that moves this door fails because the door starts sticking from warping, most common complaint is no heat but it can be no AC as well. To replace these parts (blend door and blend door actuator) you must remove the entire dash from the car. Always have this problem confirmed by a dealer or qualified repair shop as it is a very expensive repair.
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