Hello I was hoping I could get some help with this. I have a 1989 Jeep Cherokee Limited with about 230K on it. The heater control valve wore out and I decided just to bypass it. But the overheating continued and the needle goes completely to the max. It does have a new radiator and I had put in a new thermostat just a month ago. If I fill it up completely with water it does stay at around 220 but as soon as I turn it off, the water and coolant as expected just boils over and vents out.

 

When was the last time that your cooling system serviced other then just replacing the t-stat?

 

U might also need a new bottle and cap, a new fan clutch and water pump. The correct amount of coolant/water ratio and a good seal at the cap/bottle should give u boil-over protection to +/-260.

 

Methinks you have a head gasket issue if your overheating that bad, alongside a pump issue possibly.