Now that the weather is officially cold here in Ohio, I've noticed that the heat in my '00 XJR is stronger and hotter at idle than when underway on the highway. T-stat I replaced not too long ago, and coolant temp is normal. I'd think that if my heater valve or aux water pump (is there one on this car?) were bad, my heat would be poor at idle and better on the highway when the engine driven water pump is moving coolant through the system.

Any ideas?

 

A true telltale sign of a stuck open thermostat, could have gotten a bad one.

 

How could that be? Water temp gets to normal and holds there on the dial. I replaced the stat because the old one was sticking open, as you suggest, which was also apparent by my water gauge needle moving below the midway point at anything cooler than about 80 ambient outside. Now, the needle climbs to midway and holds there in 20 degree temps so the problem seems more like a heater flap issue to me but ???

 

The car sitting at idle or city driving, the coolant is circulating at a slower speed, so the coolant has time to heat up, which produce warm/hot air.

At highway speeds, cool air is flowing though the radiator and coolant is circulating at a much faster speed. Which is why the air starts getting cooler.

If you pull over and let it idle then the air starts getting warmer.

With a thermostat working as it should, it would close when the coolant is below its heat rating, that slows the flow. This slowing down process, heats up the coolant, which provides hot coolant to the heater core, that the blower motor blows into the cabin and on cold days you go Ahhhhhh!.

When the coolant reaches the thermostat heat rating, it opens and start the process all over again.

With a stuck open thermostat, coolant don't have time to heat up, its just circulating, so on cold days you go Brrrrrrrrr!

There was one time with my Acura Vigor, I swapped the thermostat out four times from two different stores, all four didn't work. Then the fifth one did.

Make sure you installed it in the right direction also.

These are the 5 items that produce heat in the cabin.

1. Hot Coolant
2. Thermostat
3. Heater Valve
4. Heater Core
5. Blower Motor

Remember the coolant system is a closed system, the outside temperture has no influence on it. It could be 120 degrees outside and if you turn on your heater, guess what it will produce heat.

Read the many many post on the subject of the engine temp gauge, DON'T TRUST IT!!

 

Thanks for the reply. We discovered a couple of the heater hoses under the supercharger were leaking, so the system was bleeding off pressure, and I wonder if that also was a contributor. Anyways, supercharger and intakes are off the car and hoses are being replaced regardless!

Cheers!