I have a 90 XJ. When I drive it for longer than 20 minutes or so, then there is an obscene amount of heat radiating from the dash. I have redone the cooling system in the jeep. I installed the open cooling system with newer radiator, overflow bottle, and 97 heater hoses. I would like to install a ball valve in the upper hose to close off flow to the heater core. My question is - do I need to plumb in a bypass loop, or just isolation valves? Does fluid have to flow from the thermostat housing to the pump?

 

Just put a valve in line on one of the heater hoses. The problem is inside your dash. The blend door foam is either gone, or it's partially open. Either way, it's probably more work than you want to do. One valve in the heater hose. It's all you'll need.

 

Do you have carpets in the cab?

 

The heater controls are vacuum operated, check the solenoids. there is one in passenger footwell, the hose can get kicked off the solenoid and when that happened on my truck the heat was stuck on.

 

There's only one door that would cause this. It's the one attached to the cable that's operated by the slider control. You can see it under the dash below the glove box. Has an adjustable clip. All the doors except for that one are vacuum actuated. No solenoids or such in a 1990.

 

The jeep has the stock rubber carpet. It's the most basic of basic models. I'll check the cable on the blend door, thanks cruiser54.

I ordered a ball valve with hose barbs on each side to put in the upper hose.

 

I thought something like that would be the case. If you put in the OEM style carpet with backing, you will find your floor temps go down a lot. I suspect much of the temp issues you are having is from the metal in the firewall and floor getting hot. Headliners are a good thing for that too.