Heater problems

My 99 Jeep Cherokee has recently started having heat issues. The heater works, lots of hot air when the car is moving. But if the car is idle there is no heat. If im driving up to a light the heat will be fine, but as soon as i stop at the like the heat begins to cool, by the time the light changes its just cold air. The Jeep also seems to over heat when sitting still, the temp will get to about 225F and then the fan turns on and the temp goes back to 210 and the fan turns off, until it heats up again. If im sitting still and give it some gas, the heater will come back. So its definitely RPM based. Ive been doing some reading online and was reading that the heater control valve is vacuum controlled and so is the thermostat. Before I start replacing parts i want to make sure Im going in the right direction, so any advice?

Are the coolant fans running?

From your description, it sounds like a flow related problem that is related to engine speed. This indicates to me that your water pump may be the culprit.....looking back, there was another cooling problem recently where the water pump vanes were almost corroded away.


How many miles and have you ever replaced the water pump?

Also, do you have the mechanical fan or only electrical?

152,000 miles, no idea if the pump was ever replaced. Electrical fans.

152,000 miles, no idea if the pump was ever replaced. Electrical fans.

If both electrical fans are running when you turn on the AC, then I'd still go with the flow issue.

As far as the heater, I'm not the underdash guy, but it sounds like a vacuum issue with the hoses that control the heat.

The only indication, I have had over the years would be a leaking water pump losing coolant, thus couldn't pump coolant through the system.

Just to keep things simple for a second, a low coolant level or air in the system could cause this.

As far as blend door operation goes, on my 98 it is cable operated, on the passenger side above your left foot close to the center console. I do have to manually give mine a extra nudge to close off to heat or a/c.