Let me start by saying I looked at a lot of threads. This is an 02 C5 coupe.
When starting out running down the highway 60-80mph, the AC blows cold and hard. It was 100 and humid, so I expect a good amount of condensation. Eventually, after an hour or so the blower starts to go slower and slower. My wife says there is a faint gurgling noise coming from the vents ( I can’t hear it). I assume this is water in the blower area. I pull in for a stop, a good amount of water drains out the tube on the firewall and runs out from under the car. We get back on the road and things are better.
I have cleaned the tube from blower to firewall and out though the rubber boot, it was all clear. It seems at speed it’s not draining. Has anyone experienced anything like this? Thanks.

 

Sounds like your evaporator coil is icing up, lack of airflow and condensate, pull over and shut it off melts, then works good until next time.

Or your blower motor is getting weak and slowing down.

May be slightly undercharged, or weak blower motor.

 

Absolutely a sign of undercharge and will cause ice to build up on your evaporator coil. Why or where it’s leaking in anyones guess at this point, but an appt with the a/c doctor is called for.

 

Good leads, Thanks

 

It blows real cold. Would it still do this if undercharged? I’ll have the charge checked. I have decent sniffer, I’ll dig into it. Blower motor is 20 years old with some rust on it, might as well change that also.
Thanks for the advice.

 

Could a bad orfice tube cause this by letting too much refrigerant flow? If the blower motor speed is reduced it isn't supposed to cause icing.

 

Undercharge is a shades of gray, so it could be low and still cool. Probably wouldn’t hurt to check and make sure the condenser is not full of road debris.

 

Moved to C5 Tech.

 

It is hard to say exactly what is going on by stating it blows "real cold." If you, or a friend have a small thermometer, or temp gun, the actual temps would give you a more accurate starting point. I believe most A/C units are considered OK if they're blowing approx 45°, give or take a couple degrees. 49° would feel real cold if it's 100° outside. I'm sure you get what I'm trying to say here. If you have a sniffer, I'd get inside the car, roll the windows up, and turn the A/C fan on high, and the temp set at its coldest. Put the sniffer in front of the interior vents, and see what happens. Make sure your hood is closed!! I once had a high side port valve leaking, and with the fan on high, it got into the air intake for the HVAC, and gave a false reading that the evaporator was leaking!! If after taking all these readings and precautions, and your sniffer tells you there's a leak, it's probably the evaporator, which is one of the biggest PITA jobs to R&R there is on a C5 interior. Hope this helps........

 

If you drive the car and this happens again, try turning a/c compressor off but keep blower on high and recirculating, in about 5 minutes see if the air pressure comes back up, if so you melted the ice.

If this is the case, your blower motor and resistor are probably ok, and may just be a tad low on refrigerant, since you say it blows strong when first turning it on, and if it the air pressure feels strong again with compressor running again.

 

It will work great until the evaporator freezes over because of the low charge, blocking airflow. Then the ice melts, and the cycle repeats. I have had this very problem in my car.

 

Always check the easy things first.

 

The same thing applies to your house AC, BTW...

 

Our C5’s have a variable displacement compressor. It regulates the suction pressure at the compressor inlet to try to maintain the lowest pressure and corresponding lowest temperature it can without freezing the evaporator. The control valves are set to different suction pressure control points based on the vehicle, the length of the suction line, and the pressure drop in the suction line. Harrison put a different color paint dot on the visible end of the control valve to identify its pressure setting. The control valve also has a bias by design. As the high side pressure gets higher (imagine a 100 degree day), the compressor discharge pressure will run higher. Due to the bias feature of the control valve, it will reduce the low side pressure it tries to control to.

So, if you are freezing your evaporator, the steps to diagnose if it is a bad control valve would be:
1. Evacuate and charge your system.
2. Connect pressure gages to the low and high sides
3. Measure the air temperature out the center duct (better yet, measure the temperature of the evaporator outlet pipe).
4. Drive down the road at a fairly steady speed and monitor the pressures and temperature. If your low side is darn close to 30psi (or lower) and your outlet air temperature is in the low 30’s and your high side is under 250psi, you may have a bad control valve.

Now if your high side pressure is at like 300psi just driving down the road, then you may have air in your system or something else that has reduced the effectiveness of your condenser. Air drives up the high side and will fool the control valve into running at a lower (which means colder) evaporator and suction pressure.

 

Due to the variable compressor, it is a good idea when trying to charge the system to do it by weight from empty and not just by the gauge pressures. Hard to diagnose the correct amount of refrigerant to add to an already kinda working system since it is not fixed orifice....been there.

 

Another method I've found, on GM cars with climate control, NOT conventional dials, is this:
If one side of the cars vents blow slightly colder than the other, you are usually (not always, but usually) low on refrigerant. My 2003 LeSabre was like that, and so is my C5. BOTH had/have climate control. FWIW, YMMV......