I took the mounty for a road trip, it's 90 out and working well 50 at vent with fan on low. Earlier at gas station it was 70 out, popped the hood and found just a little ice on dryer.
That's perfect!
40-50 out of the vent on low during a 90 degree day is the exact spec ford cites for proper operation.
Now, let's address the warming up on the highway. Are the vent temps still colder at idle?
It's possible that the clutch is being rapidly cycled at higher rpm due to the low pressure side going too low. This can be caused by restrictions, like dessicant or 'black death' clogging the orifice, as mentioned by
I've even seen receiver \ dryer desiccant internals burst and spread dust throughout the system. The O tube screen catches it.
He could totally be right.
Or simply from the system being low on charge. Paradoxically, overcharge can do the same thing, as the high pressure cutoff engages and disengages.
Overcharge can hurt performance exactly like low charge. This is why those cans from Autozone with only a low side gauge are dangerous. You need to see the high side, too. Otherwise one might think they're still low until they're compressor grenades.
This is where the pressure reading at 1500 rpm for 5-10 minutes becomes important. It simulates pressures and cycling behavior like you're cruising along at 60 or whatever.
I don't know how to trick the throttle by wire system, so you'll have to find an assistant.
Repeat your test, in the shade, recirc max ac, fan on low, have your assistant keep the engine around 1500. It will take 5 minutes at least to stabilize the behavior.
After that start watching the gauges and try to see what's happening. Is it cycling because the low side is getting pulled down into low 20s? Is the low side actually going into vacuum? (This will happen from the orifice tube screen being partially clogged as mentioned by the other poster.)
Or is the high side skyrocketing?
This will help you determine possible over/under charge and/or loose desiccant or contamination.
These systems, especially the systems with aux (rear) ac, are super sensitive to charge. Once you get a leak, you lose track of how much charge is in the system. Especially since most cans for DIY use have a few ounces of oil, dye, or stopleak in them, which takes up volume in the system.
It's best to the fix any leaks, vacuum the system, and fill by weight.
But I think the gauges will give us a pretty good idea.
I slowly titrated my system up by 2oz at a time until I found the mix of good idle and highway cooling performance + proper low and high pressures. (Be careful to purge your gauges of air before doing something like that as the volume of air in the hoses can start to add up.)