2003 Nissan Frontier Runs Hot When Idling

Our mechanic is stumped. The Frontier runs hot when idling. The clutch fan and thermostat have been replaced. The radiator has been flushed. When you are driving or you turn off the A/C, the temp goes down. The next suggestion is to replace the radiator, but I’m already hundreds into this so I hate to do anything else unless I’m pretty sure it is going to fix it. Help!

We do know that the catalytic converter needs replacing. Could this have anything to do with it?

Sounds to me like a partial blockage of coolant flow. This would be more consequential at idle since the water pump is turning slower than at high speed.

Was the radiator flow tested after flushing? That’s the only way to tell if it’s totally unplugged.

Also remove the lower radiator hose and check inside for a partial collapse. This can happen even though the hose looks OK from the outside. The inner metal support spring rusts out and the inner hose wall collapses, restricting flow. You won’t be able to see it from outside.

Another possibility is eroded vanes on the water pump, but I would check the bottom hose and radiator flow first.

I will let my mechanic know about this and see if he has tried these things. Thank you! I’ll get back and let you know.

You could need a new thermostat. It may not be opening all the way and at idle the flow is slowed and you overheat. When you are on the road the air passing thru the radiator helps cool the engine and bring the tamp guage down.

I remember my dad heating a thermostat in boiling water on the stove with a candy thermometer to check when it opens.

A clogged exhaust can indeed lead to overheating. Since you need a new cat anyway why not replace it and see if it fixes the problem.
When it overheats at an ilde, are the cooling fans coming on?

Yosemite, thermostat has been replaced. oldtimer11, I thought about going ahead and changing the cat. I don’t know if the cooling fans are coming on at idle. I assume the mechanic has checked that but I can ask.

I do appreciate all the advice. My mechanic may have checked all of that, they are very good and very fair too, but this one has stumped them.