In hot weather when I use the a/c, the engine temp seems to climb pretty good at slow speeds or idle. Fan clutch? How do I test it?

 

How far does it creep?

 

Gauge was about 3/4 of the way to "hot", then I was able to hit the freeway so things came back down, but I'm confident it would have kept climbing had I not got out of traffic. Never used to get up that far, ever.

 

Could be the fan clutch OR the radiator...

When you're sitting at idle, and your truck is beginning to overheat just over the normal temp, give it a touch of gas. Do you hear the fan clutch kick in nice and loud?

Get or borrow one of those laser thermometer guns, and use it to check the radiator for cold spots from left to right. If there are cold spots, then the flow of coolant is restricted in those areas, which could also cause the viscous fan clutch to not engage fully. Keep in mind, the viscous fan clutch relies on hot air from the radiator to engage.

-JT