1988 4900 heat problems

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.

 

No, any WS or Freightliner Dealer can get it, with the last 6 of your vin#. Might as well replace both of them, if you have a bunk. Even the manual valves my Freightliner go bad, I just replaced the cab manual cable controlled one on my 2000 Freightliner. My bunk has a electric valve, no air to it. My 99 Freightliner did have the air over electric on the bunk, same as my 92 WS. WS had them on cab also, mounted on passengers firewall. Just follow the heater hoses. Not the ones running to the waterpump, they’re returns. The other 2, probably coming off the oil cooler.

 

Well the valves somewhere between the engine and heater core. Follow the heater hoses, you’ll find it. See if it has an air line, or not. Wether it’s electrical or simply a manual cable type. Usually a slider heat control on dash is manual, dial type is electrical, with or without an air line running to it. Best to go to dealer if you can. Find out what you have, ask them to print the parts page. Gives you a good picture of where everything is, and a future parts# reference.

 

Sounds like the valves under the dash between the firewall and heater core. Take the kick panels off passengers side. You’ll probably see it in there. Usually easily accessible. Be sure to turn off both hoses on engine, before changing it. Lol. Should be 2 for cab, and 2 for bunk. 2 running from Waterpump are returns, 1 from cab, 1 from bunk. Other 2 running off block, or oil cooler are supply hoses, 1 runs to valve under dash, other to valve in bunk. If they get crossed up, coolant won’t circulate and heat won’t work.