This one has me stumped. Vehicle is an '05 Grand Cherokee, 3.7L with the manual HVAC. The head gaskets and timing components were replaced along with the heads resurfaced about 1 year ago due one HG leaking after overheating.

When the outside air temp is 25*, and engine temp is a steady 203*, the vents will blow only 75* when the engine is at idle yet when the engine rpm is raised to 1500 rpm the outlet temp increased to 135-140*. The engine warms up quickly, temp on the scanner stays a constant 203-204* once warmed up and the system pressurizes quickly. The temp difference between the heater hoses is only 10* at idle with the heater on high.

First thought was the heater core. It was only 4 years old, but figured that was the most likely culprit given the symptoms. Replaced that along with the soft heater hoses & thermostat for good measure. I also cleaned the hard lines from the firewall to the core by snaking a piece of wire through them with a small piece of rag attached to the end. That resulted in no change. None whatsoever.

After that I redid the HVAC relearn/calibrate procedure. It did it's thing, with no codes, and still no change in heater performance.

Thinking it was a flow issue at this point, I replaced the water pump as a last ditch effort. Wouldn't you know, no change. In fact, the temps from the vents lowered a little bit, now I'm lucky to get 130* out of the vents going down the road. I wasn't a big fan of the new pump's impeller design, lots of sharp edges; where the OEM unit appears more aerodynamic if you will.

One thing I noticed while burping the system after the pump change is that the temp out of the vents was 125* at idle. This is while the coolant level was an inch or so low in the radiator and the cap removed. It was about 40* that afternoon. After it cooled down and I ran some errands, the vent temp was till low at idle.

Anyone have any idea on what might be going on or ideas to help diagnose this?