I have a 2002 Liberty Sport 3.7L engine. It too began to overheat at highway speeds up to 3/4 on the temp gauge. No coolant loss. Became progressively worse. I had interior heat at idle no heat upon acceleration. I first thought blown head gasket but pressure tests and hydrocarbon tests proved this to not be the cause. The water pumps on these 3.7's are very easy to access and remove which i did to find that the plastic impeller had cracked where it joins the shaft, upon inspection it engaged with slight slippage with little effort, explaining the vehicle having heat inside and not over heating at idle. it also show that with further effort, the impeller actually slid away partially from the **** and did not spin at all, explaining the no heat inside and overheating upon driving. a simple water pump change solved all issues. I have a 2002 Liberty Sport 3.7L engine. It too began to overheat at highway speeds up to 3/4 on the temp gauge. No coolant loss. Became progressively worse. I had interior heat at idle no heat upon acceleration. I first thought blown head gasket but pressure tests and hydrocarbon tests proved this to not be the cause. The water pumps on these 3.7's are very easy to access and remove which i did to find that the plastic impeller had cracked where it joins the shaft, upon inspection it engaged with slight slippage with little effort, explaining the vehicle having heat inside and not over heating at idle. it also show that with further effort, the impeller actually slid away partially from the **** and did not spin at all, explaining the no heat inside and overheating upon driving. a simple water pump change solved all issues.