I have a '91 gmc half ton. The a.c. works perfectly most of the time, but if i drive roughly twenty miles(non stop), the blower's pitch/sound becomes muffled. The air output with max a.c. and with the blower on high will just barely blow out of the vents. It almost sounds like a plastic bag got sucked up but I checked and did not find one.
Here's the kicker. If I keep on driving, the lack of air flow persists. If I cut the a.c. off and leave the blower on high, the airflow will gradually go back to normal after a couple of minutes. With the airflow back to normal, I can turn the a.c. back on, and it will last another 20 miles. Is it possible that the coils are freezing up and blocking airflow?
I don't know if this is related or not, but a month ago, I noticed my carpet on the passenger side was wet. I thought it was my heater core, but after pulling the carpet, it was pure water and not antifreeze. I figured the drain got stopped up, but it was fine. The rubber elbow on the drain nipple was missing so I put another one on and it has been dry ever since. But I never really found a problem.
Thanks in advance Hutch