So my heat is stuck on all the time only way to make it quit roasting me is to get out and turn the valves on the motor off had to do the same for summer so the ac would work I am going to assume that a blend door is the problem but so far I can't find out how its supposed to work so I can fix it
1988 4900 heat problems
Discussion in 'Western Star Forum' started by n4ane, Nov 3, 2020.
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Chances are it’s the heater control valve. Turns coolant to heater core on and off. Might be under dash. Might be on firewall. I think 1988 is on firewall. Air over electric design. There’s another one for the bunk, inside the Truck. Sounds like It’s wore out and not closing, allowing coolant to keep flowing to the heater core. Sometimes they go bad and put air into coolant. Pushing coolant out of the overflow. If you have a small air line going to your heater control valve on the cab or the bunk, be aware, and if you ever have excessive pressure in radiator. Symptoms mimic a blown head gasket. Turning off the manual water valves keeps the heater control valves from leaking air into coolant. They’re nice when working properly. Allowing you to turn the cool/ hot dial and set it like a thermostat. They’ll work good till desired temp is achieved, then, if bad, once closed 100 psi of air blows through them into the cooling system. Causing coolant to blow out of overflow. Also if Truck is running with the radiator cap off, and heater on, while the control valve is open, when you shut engine off, valve will close and coolant will blow 20 feet into the air. Many unnecessary head gasket or head replacements and complete overhauls have been done because of a bad $30 air over electric heater control valve. Almost happened to me on my 92 WS.
Last edited: Nov 12, 2020
n4ane Thanks this. -
That would explain why there is a 1/4 air line in that area that has been leaking I didnt know what is was for so I capped it off any idea what the part number was for that?
Rideandrepair Thanks this. -
n4ane Thanks this.
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Unfortunately there isn't anything on the firewall anymore the previous owner had taken it off
Rideandrepair Thanks this. -
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So I know that at the controls its cable that goes to a air thing then on the firewall it goes straight
Rideandrepair Thanks this. -
Last edited: Nov 22, 2020
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