1997 Suburban 6.5td (is Jim still policing the use of TD?) with front and rear AC. Front AC quits cooling after a couple of hours, rear AC stays cool. Front AC cools again after an hour with the engine off. Front AC has an orifice tube, rear AC has an expansion valve. No coolant flow through front heater core, front AC vent temps on the best day are such that I don't believe there's ice on the evaporator, no evaporator case drain problems, no airflow or distribution problems

I studied the relevant shop manual which shows the refrigerant circuit for front AC only (trucks) and words to describe how the Suburban with rear AC is different. Neither diagram nor words describe why, if I can trust my eyes, the rear evaporator discharge flows directly into the compressor while the front evaporator discharge goes through an accumulator. Rear AC uses an expansion valve while front AC uses an orifice tube. Does this have something to do with it?

I have to remember the next time it stops cooling to check if the front evaporator discharge line is cold. I've seen it sweaty but never iced - how can it ice so close to the turbo? I suppose I can check the front evaporator line downstream of the orifice tube but it's tucked under the turbo and I don't have an infrared thermometer.

Possible scenarios:

1) Debris accumulates in the orifice tube after several hours of AC use. The debris slowly settles back in the condenser when the compressor isn't engaged.

2) The compressor low side port, rear AC evaporator discharge or accumulator is at a higher pressure than the front evaporator discharge confounding flow through the front evaporator.

Are these scenarios plausible? What other possibilities are there?

Sixto
MB-less