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DIY AC System Flush

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17 November 2007
Raleigh, NC
My AC system was leaking on my 91 at the evaporator connections (9 and 10 in the labeled diagram below), so while I had the system evacuated I decided to preventatively replace the compressor (Kaz has a great post on which one to use - I used the Denso 471-1194 and moved the old coil \ clutch assembly over) and several other o-rings. I wasn't sure how much oil to add to the system, and as Kaz informed me, there's no way to know for sure, so it's best to flush the system and start from scratch. I'm posting my experience with the manual AC flush since I didn't see any other posts on the subject.

The only reason I didn't follow his advice and have a professional flush the system with a machine, which would have been WAY easier and also flushed the evaporator, is because I already had the car apart. If you've got the option, I'd definitely look into going this route. Flushing the system was a PITA and exposed me to some pretty heavy-duty solvents.

Parts \ Materials:

There's no method defined in the workshop manual, so the following procedure was the best I could come up with. For reference, steps 2-15 took about 6 hours. I labeled the system o-rings in the diagram below and refer to them by number.

  1. Removed the compressor
  2. Drained \ removed the radiator. This made working on the lines up front way easier, and I was preventatively replacing it anyways.
  3. Removed front left (FL) fender liner, unbolted (but did not remove) FL condenser
  4. Unscrewed fittings for 4, 5, and 6 (4->5 is the cross pipe that connects the condensers)
  5. Moved FL condenser away enough to free the cross pipe (4->5)
  6. Flushed from the compressor HP connection to 4 (FR condenser outlet)
    1. I used a 90° ¾" PVC fitting to direct the solvent down. Note that the solvent will come out with some force, so make sure it's constrained! I learned this the hard way.
  7. Reinstalled the cross pipe with new o-rings
  8. Flushed from compressor HP connection to 6 (FL condenser outlet)
    1. Same note with the 90° PVC fitting. Do not let it fly off.
  9. Removed dryer
  10. Removed the FL condenser -> dryer line (6->7) and cleaned \ flushed outside of the car since it was easier to do so, then reinstalled
  11. Unbolted lines from evaporator since I needed to replace both o-rings
    1. I cleaned around this area first since it was nasty and I didn't want to contaminate the system. Because there was so much dirt accumulation I'm guessing this is where my leak was
  12. Unbolted dryer -> evaporator line (8->9) from chassis and flushed
    1. Flushed from the dryer connection towards the evaporator. I caught the solvent with a plastic bag I shimmied over the evaporator-side end. As long as I didn't go overboard with the solvent volume this worked ok
  13. Unbolted the center tunnel cover under the car and the brackets holding the AC lines to the chassis. I also loosened the front bracket holding the lines together to give myself some more wiggle room with the LP hard line (10->11)
  14. Flushed from LP compressor connection (12) to the evaporator connection (10). Similar to before, I caught the solvent with a plastic bag over 10
  15. Replaced Schrader valves and re-installed everything except for the new dryer, which will be installed last

I ended up replacing the following o-rings. I didn't replace all of them because, as I've learned in the past, it's better to not fix things that aren't broken.

1NoNot leaking
2NoNot leaking, buried up under coolant lines
3NoNot leaking
9YesWas leaking
10YesWas leaking
11NoNot leaking

And for those replacing a compressor on a 91-92, here's a link dump for the threads I found helpful:

This post on my blog

The amount of total system oil for R134a (ND-OIL8) is 160ml (Min) - 180ml (Max)

How to tell if AC clutch is bad