Cold Air -- Dodge Ram (2004) quad cab 4x4 hemi

[aanderud]

Driving back from the St Cloud area last night, had a sudden drop in temperature in my cab. It started about 10 miles NW of Maple Grove, and by the time I got to Carver (sw metro), I could see my breath.

First thoughts were the usual suspects -- thermostat, low coolant level, heater core blockage. My copilot did some research while I drove, it sounded more and more like a blend door issue -- especially since it was so sudden (most heater core blockages tend to be more gradual, from what I have been reading). Apparently Dodge is "infamous" for this.

Engine temp seems to be just under 1/2 way from 'cold' to 'hot' on my gauge. It goes slightly lower than that at times. But never extreme in any direction. Makes me think it's probably not a thermostat issue (stuck open would take forever to heat up, and stuck closed would overheat, no?).

Both coolant lines going to the heater core seem to be warm/hot. The top one is slightly hotter than the bottom one. I think this means it's doing its job.

Apparently dodge used motors on all of their HVAC doors (rather than vacuum systems), and they used plastic parts on the door flap hinges that connect to the motor -- and those tend to break. Since the issue was so sudden, I figured maybe this was the culprit. But then when I went out today to do some diagnostics, the dang heat was working great. After a 10 minute warmup or so in the driveway idling, it was blowing 140 degree air out the vent in normal mode. Took it for a drive to see if it would get any colder or hotter. Seemed to work just fine.

Today, all the actuations to each of the zones worked perfectly, and all hot to cold actuations worked flawlessly. Last night it seemed like some of the zone (defrost/feet/front-vent) actuations were failing, and ALL of the hot/cold actuations were failing to do anything. Today When I set it to recirculate, it cranks up to about 170 degree air temp at the vents. Unreal. Wish I had that last night. That's the first time I've ever tried recirculate on this heater, I just did it out of curiosity really -- it's almost like a sauna now..

Any ideas what this might be? I'm reluctant to do the 4+ hours of digging in on these 'blend doors', because to me it seems like if they're broken plastic stops on the hinge they should always fail. This intermittent issue seems like something else.

Thanks in advance!