ac problem
 

I have a very intermittent problem. If I have the AC on for more than 30 minutes or so, cold air stops blowing in the front vents. Back vents blow fine though. Last time this happened, I turned the system off for 10-15 minutes and turned it back on. AC worked again although not as well, but well enough. Also, if I have the AC on pretty strong, I don't have this problem. Any ideas?

an idea
 

There is only one expansion valve and evaporator - the cold air gets ducted to the back from the front, so I think you can assume the main loop of the AC system is working fine. I think you are left to consider '

1) hot air is being introduced into the front (heater valve, etc) - I don't know why this would take so long - unless your 20-minute delay is how long it takes your car to warm up (but that seems too long)

2) the flaps and actuators directing cold air are somehow failing/getting mixed up, etc., after some period of time

3) the fan in the front is failing or slowing down (weird failure mode of the fan resistor?), allowing more hot air/outside air to leak into the front vents. The back does have its own blower fan. Do you have the same problem when you set the fan speed manually and set the return air (recirculate or fresh air) selector manually?

Thanks for the quick response. : )
Same problem no matter where or how the air is blowing. It doesn't blow hot air, it just doesn't blow anything. I've also noticed when I turn up the fan, the vents way in the dash work fine, but there's no cold air blowing from the front vents.
How much do you think the repair would run me?

Do you know what vent temp the air is blowing when it is working? Could be that the evaporator is icing up after it been running for a while. The ice will block air from passing through the evaporator. So instead of passing through the evaporator core, the air pass around the core and doesn't give up its heat. This can happen if the refrigerant is low since the low side will be low and the lower the low side, the more colder it will get. Get a reading of the low side and high side and the ambient temp.

Also do you recirculate the air or are you drawing in fresh hot air?

I don't know what temp the cold air is blowing and I just realized when I have the ac on at night, I don't have this problem. The dash is black and I live in the desert so it gets very hot during the day. Do you think that might have something to do with it?

I would say check the (1) heater control valve..most common problem.
https://www.youtube.com/watch?v=tP-Vp2L36Ew

Sometimes you can take apart the solenoids and clean them and the valve buts it's better to change it. The solenoid or the valve itself can stick and there are two of them.

jeck, when your cold air stops blowing from the front vents, do you mean...no airflow? or do you actually mean warm air?

Our X5 just started doing this. about 30 minutes in (not every trip mind you), suddenly very little airflow from the front vents. The air that comes out is still ice cold, but even with the fan on max, no too much is coming out. It's not our FSR because i can hear the fan on, and it gets faster incrementally as i turn up the speed. since the vents aren't always broken, i assume its not the damper adjustment itself.

My only current conclusion is that it's the actual control panel. thoughts?

Replace the blower resister

if you dissasemble your ihka (control panel), there is a sensor that sucks ambient air in so it can see what the cabin temp is. my guess is yours is covered in fuzz and its causing the system to think your car is already to the correct temp. very easy to clean (I recommend MAF cleaner.)

I thought if the fan was working fine on all speeds, and it turned off with the ignition being off, then the FSR was fine? it's usually this way with the other chassis' why would the E53 be any different? not saying it's not possible, but i'm curious as to why the FSR in my case? remember, the fan works fine. it really seems like the dampers close halfway or don't adjust when i ask the HVAC to move to windshield, center vents, footwell.

i'll take a look at this too.