As the title states, at an idle the rig will start to climb up on the temp gauge but when you increase the RPM you hit the accelerator the temp will come down.

This started as we were sitting at a light, and I was like - you feel the cold air coming from the vents? As I noticed this, at the same time I was watching the temp starting to climb until I hit the gas - which at the same time hot air started to and did come back to the vents.

Happened multiple times while we were at the light, of course stuck in traffic and continued till I got it back to the house to which I stuffed 'er back into the garage and am planning on how to get started with the repair plan.

My thought is that the water pump snipped the shaft but is catching enough to get the water though the system when the RPM is off idle. If its the case, I think that might be a good time to swap out the T-Stat as well.

Anybody else have some thoughts on this one?

 

2nd on the water pump. The impeller is probably shot. That would explain why your heat only comes when you rev the engine. Would also explain the overheating at idle.