Under normal driving (even stop and go traffic) our Quest run in the middle of the temperature gauge as it should.

If it idles for 3+ minutes the temperature starts to rise and within 7 minutes it is near the top of the temperature scale.

This happens whether the AC is on or off. Could too much tension on the A/C belt be causing too much load? The check engine light is not illuminated. Does it make sense to have our local auto parts store check for error codes?

Should I start with the thermostat assembly? Or, is it more likely the water pump?

Thanks,

Chris

could be the thermostat, could be an air bubble in the system or blockage in the radiator. When was the last time you had the system flushed?

In December a broken heater core supply line was replaced after we ran over a log on the interstate - it was drained and filled then. The van has 135K so I was considering it may be the radiator itself but was thinking the thermostat might be easier to replace.
It's been a while since the system was actually flushed though.
Thanks.

A shop should be able to do a pressure test of the coolant system to see if there are issues.