Heater Control Valve 4.8is Cabin Temperature

[X5M-ISH]

As the Northern Hemi has rolled into winter, and as such, A/C usage wanes to heater usage, I have found that my 4.8is has the oddest climate control issue. While I have searched here and other forums, most all of those issues were solved by simply shelling out the dollaridoos for a replacement heater control valve unit and swapping that in for the old one. However, mine is somewhat new as well as the hoses. Service records from the PO reveal that this unit was replaced about 20k miles ago. Appearance wise, the unit and hoses even looks new.


The issue is this: when the truck is turned on and creeps toward operating temperature (and at operating temperature), the front & rear cabin temperature is nice and warm and cozy. But, after about 15 to 20 minutes of driving the front cabin temperature drops off to what feels slightly cooler than room temperature while the rear climate control vents (back side of center console/arm rest) is still billowing out pretty toasty air. The dash center vent dial is also set at warm. However, setting this dial to cold makes the air unpleasantly cold. Real cold.


To the touch, the hoses at the heater control valve are warm/hot between the rad and valve, but from the firewall to heater control valve those two respective hoses are warm, but not scalding. Turning the key to POS 2 and touching both solenoids on the heater control valve appear to be working but I'm not exactly sure how obvious this is supposed to be. Conducting a continuity test shows that the installed unit is showing continuity between pins 1+ & 3- as well as 1+ & 2- (three pin connector). This test is consistent with a new heater control valve unit I have.


The current heater control valve can possibly be fried, yet looking new. That wouldn't be uncommon. However, the heat throughout the car for the first 15 minutes or so then falling off once I get on the freeway is consistent and perplexing to me. Are there any other tests I can perform before replacing this heater control valve or worse - getting into the dash for something else?