Malfunctioning air conditioner mystery - recharge makes car run worse

Question

The air conditioning in my 2002 Subaru Impreza has sort of worked the last couple years. This year it got bad enough that I began trying to fix it, but even though it's totally useless on a 30 minute drive on an 80-degree day, it still puts out 40-degree air for almost a minute at the beginning of the trip (so the compressor runs, obviously).

So I decided to try to recharge it. I did this, and the pressure both before and after the recharge was basically 0. It was an 18 oz can. I did this with the car running, and after I was done the air that was blowing was hot, so I figured it didn't work.

Then, on my first drive, I got cold air for probably 3 minutes - longer than normal. It kind of came and went though - unsure if it was tied to throttle/speed. Anyway at the end of the three minutes it didn't just blow hot air, the car started "coughing" every 4 seconds. Like it was trying to start the compressor (guessing) and some safety mechanism was saying "uh, nope, abort that" and there was a noticeable hiccup in my engine/speed. I turned the air off and this went away immediately.

Two theories.

1) I have some problem other than low refrigerant and now that I've charged it I have an overcharged system that still has the original unknown problem.

2) I need even more refrigerant, because it's working better now and air conditioners with low refrigerant tend to freeze up during use and maybe that's what's causing the hiccuping. It's anyone's guess if I have a super-slow leak that took 12 years to get to where it is or if I have a really fast one that makes recharging a waste of money.

I guess the immediately question is whether I should try more refrigerant or if I should go immediately to the catch-all answer of "take it to a mechanic to test for leaks and other problems and between 100 and 900 dollars later come home with working AC." Or, is there any other troubleshooting I can do at home?

Answer 1

tl;dr: It sounds like an O-ring failed.

Funnily enough, I just solved an almost identical set of problems on my 2004 WRX. Here's how things played out for me:

1. Checked AC on the first hot day in a while - sadly, a whole lot of nothing happened. No cool air, no compressor-caused dip on the voltmeter guage.

2. Purchased two seriously overpriced cans of R134A. Note: NAPA autoparts and I are no longer friends.

3. Recharged the system on the low-pressure side and observed that new pressures were well within the target range. Confirmed cold air from AC. Expressed joy.

4. Parked car in garage, returning after about an hour. Noticed a funny smell and heard a dispiriting hissing. Noticed that the fitting where the high pressure line leaves the compressor was hissing and bubbling green lubricant. Expressed frustration.

The next day, the AC was again non-functional. I didn't see the coughs as the compressor tried to kick in as the system could clearly tell that there was no longer any refrigerant in the system to compress (confirmed with guage). Perhaps your system is triggering slightly later, giving the compressor a moment to try to turn on and then immediately switch off.

Since I could see the location where the leak was occuring, I decided that there was a good chance that I'd be able to fix the problem. Based on research, I purchased this set of O-rings which contains a variety of diameters and thicknesses. I replaced the O-rings on the low and high pressure sides of the compressor. This time, I added just enough refrigerant for the compressor to catch and then did an immediate eye-ear-sniff inspection trying to find any remaining leaks. All seemed well, so I finished off the can and let the car run the compressor for a few minutes.

Net result: O-rings in the right place fixed the leak. The trick was to find the right place in the first place.