Were you able to solve the problem? I am having the same issue with my wife's 2006 Jeep Liberty. It only heats up when idle and with the a/c on. Any suggestions?

Check to see that the radiator fan comes on when the AC is turned on. If not check the fan and the fan relay.

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Jim

2003 Liberty Renegade
Leather Electric Seats
Self-Dimming Mirrors
Sun Roof
MOPAR Rock Rails, Skids, Rear Hitch
EMU Heavy Duty shocks and Springs 2.5" lift
Front Tow Bar (for RV)
Firestone 255 70 16 Destination AT Tires
Straps and Chains

Unsolved.

Fan blows hard when ac button pressed, but
when I come off interstate, the fan runs slow.

On hot days, it will begin to overheat.

I guess start with fan. It only happens in hot days, 90°+