Engine overheating when heater in running

Need help !!!...my engine is overheating randomly when the heater is running. When I turn the AC unit off it comes back to normal. It is very odd, and it is hard to duplicate the issue when I try to troubleshoot it.

Normally it happens after I drive it for a while and come to idle for about ten minutes.

I see no leaks, coolant is fine, and the only thing I could think of was to replace the thermostat, but it did not solve the issue.

I took it to a garage, and they say I have no flow. However, I do feel both heater hoses hot (but do not have a temp sensor). They say it could be that the impellers of the water pump are eroded away, and that they need to take it out for inspection.

I know this is possible as it happened to me once on an older XJ, but it happened because my dad would put tap water instead of coolant in the system, and the fluid was in very bad shape and rusting everthig off.

Could this also happen with standard engine coolant ?. My XJ has 130k miles on it. Any other thoughts ?

Thanks !

IF you have never done the water pump after 130K the chances are that it is toast. Might be a good idea to replace. Aslo may have a pulg in the radiator. While doing the water pump pull the radiator and have it roded out by a shop. The thing that is odd to me is if your running the heater your operating temp should be going down. You are going to see a increase in temp if your running the AC but the heater should cause a drop in temp.

Totally agree !!!! you are on the money … hence my frustration. The symptoms are not logical.

I do not think it is a plug. I flush the system every 30K…. well, I think I have my work cut out for the w-end. I will pull out the pump and see…

After some thought the heater control valve might be stuck. This is the unit that your heater hoses are pluged into before they go into the firewall.

Onkover said:

After some thought the heater control valve might be stuck. This is the unit that your heater hoses are pluged into before they go into the firewall.

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If it only happens when the heater is on, this is the only thing that sounds like it should cause the symptoms.

I wasn't aware of this valve. I looked at the OEM repair manual, and there is no mention of it. Also, I don't see it. All I see are the hoses going into the firewall.

Mine is 2001. Could it be that this was there on older models ?

Help ! ! help ! pleaseeee.

I removed the pump yesterday, and it was in perfect condition. Pretty much as expected due to the regular maintenance I do on my jeep… I guess I was hoping this was the cause of my problem.

Any other ideas ? Thanks so much !

Are you sure it's really overheating?

Could be a voltage drop caused by a bad ground or bad blower motor throwing the gage reading off.

Normally it happens after I drive it for a while and come to idle for about ten minutes.

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Another indication of a voltage problem.

You know… I thought about that since the beginning. That it could possibly be just a bad reading i.e. like a bad temp sensor, but when I thought about it, since this is electronic, I didn’t think it could “partially” fail, so I ruled it out.

However, it seems that it is possible.

I guess my next test is to get it to “overheat” and probe the temperature using an infrared gun to find out if it is really overheating.

I only need the gun… oh well, I think I have a good excuse to get me one

If you un-plug the temp sensor it will go to full hot on the gauge.

It actually does the opposite. I goes to 0, and the engine light comes on.

???