1999 Suburban A/C Problem

I have a 1999 Suburban (old body style) with the 5.7L engine and 106,000 miles. During the past summer, when using the front A/C, the cold air coming from the instrument panel vents would gradually diminish in volume until no air (cold, warm, or hot) was coming out. In fact no air was coming from the floor or windshield vents either. However, I could still hear the fan motor humming merrily along. The rear A/C was normal.

I tried moving the fan selector to high, but there was still no air of any type being pushed out of any vents. I found that by switching off the A/C, but leaving the the air delivery selector knob on the insturment panel vent setting, the flow of air would gradually increase in volume to where it had originally started. I could then turn the A/C back on and everything would be fine - a good volume of nice, cold air - until the problem surfaced again after driving several more minutes. Sometimes the problem would resurface after only a few miles, and sometimes I could drive for an hour or so before it cropped up again. Bottom line - it was manageable so I decided not to have it looked at until it got worse.

Now, however, I find as it's gotten colder, when I use the defog setting on the air delivery selector knob (sending conditioned air to the windshield vents and floor vents), a similar situation occurs. This time, the air coming from the floor vents gradually decreases in volume until there is nothing. If I move the selector knob to heater only, the flow of air out the floor vents gradually increase to full volume again. This process also repeats itself as I continue driving.

Because this occurs when I am trying to use conditioned air, and does not seem to occur on any of the settings that do not call for conditioned air (vent and heat), I am assuming it has someting to do with the A/C. Has anybody had a similar problem they were able to fix? Or does anyone have a diagnosis? Thanks.

Possibly the air duct for the fan is restricted or the fan is slowing down when the fan motor gets hot.

Could it be the evaporator core freezing up??? Your description was excellent. The evap core is in the heater plenum and could be freezing up which could be cutting off the airflow. If this is the case, I have absolutley no idea why or what to do to fix it. I hope you'll talk with some a/c experts to see if this is a possibility. One thing you might try is to crank the heat full hot and run the a/c or defrost and see what happens with that. Good luck to you and welcome to the forum.............Steve

Another thing just entered my feeble mind. Could it be the evap drain hose plugged up?? This hose allows the moisture to drain on the ground from the evap core. Not sure were that is but on my '98 Sub there's always a water puddle close to the right front tire after running the a/c in the summer. So many things to think about.