1999 Kenworth (CAT C12) High Pressure in Coolant Resevoir

Uhm,,,bad head gasket ..

 

Pull a sample of the oil if you havent changed it yet and get a test done for coolant in oil, if you take off plug to add oil ck. for white oil on top, on top of dip/stick also?

 

One thing I just thought of. Does the truck have an air controlled water valve for the heater? Sometimes when those go bad they can pressure up the cooling system with your truck's air pressure. That could explain why the cooling system holds high pressure overnight and why it pressurizes so quickly after start up.

 

I haven't looked under the hood of an 800 in a while but I would suggest finding your heater hoses and following them back to the cab. They are generally 5/8" or 3/4" hoses and probably pass into the cab on the passenger side firewall. It could be located there or inside the dash by the heater core. If you can't find the valve easily I would suggest calling your KW dealer with your VIN and find out where the water valve is located and if it is air or cable operated before spending too much time on it.

 

Those taps are shutoff valves and yes they can be shut off in the summer if needed. The valve I'm talking about should be located inline with those taps and is used to regulate how much coolant flows through the heater core and is controlled from inside the cab. If that valve happens to be controlled via an air signal it is possible that it could leak air from your reservoirs into your cooling system.

 

Yes may be another air valve on heater hiding under bunk of sleeper may also be powered by air just look for small air lines at both heater cores on rubber lines, the rad. cap should let off a over supply of air, so ck. rad cap spring to be sure its not rusted closed?