[alexwhittemore]

So my AC was underperforming and, despite my best attempts to pre-inform myself before going to town with a recharge kit, it seems I may have cocked it all up.

I measured the AC outlet temperature on Max today (windows closed) at 60F at idle, 55F with the engine reved to 2k RPM. That was markedly higher than the 45F I got the car at (per CPO checklist), so I thought, yep, time for some work.

I used a recharge kit from pep boys, as you do, but it seems I may have overcooked it. The low side right now, AC off, is a bit under 100psi. On, it's down around 40-45. But here's the odd thing: at an idle, the system blows 65 degrees or so, and the low-side pressure stays even at 40psi. When I rev up to 2k, instead of the AC chilling harder, the low-side pressure fluctuates between 45 and much higher than that, maybe 110 or 120psi. It seems like the high side is venting when the compressor kicks on, then it falls back to nominal.

So the question is, what do I do about this? There's the manifold pressure gauge from HFT, but even if I get that, where's the high-side outlet and what's the procedure to get the charge back to normal?

EDIT: additional info: with "Max" off, the AC does get quite cold, even 45 degrees. But on "Max," quite a bit warmer as mentioned. In all cases, with the AC on, the fan rails all the way up, which seems unusual to me.

[Squark]

The A/C performance depends on the ambient temperature. If it's a 90 degree day the air won't get as cold as it would on a 70 degree day. Hitting the Max button recirculates the air and turns the fan speed all the way up. The higher the fan speed, the warmer the air is going to be at the vents. It's possible the CPO check was at low fan speed on a cooler day. Also, if the car has been closed up all day and is 120 degrees inside, the air might get colder when it's bringing in fresh air from outside. All this just to say there are a lot of variables at play.

If you're concerned about over-charging, I would suggest having the system evacuated and recharged by a shop. They can meter the exact amount of refrigerant called for by BMW to ensure the system works optimally.

[alexwhittemore]

For anyone coming back for reference, I took care of this myself last night and this morning. I did, in fact, overcharge the system and the high side pressure was too high (something like 300psi or so when I tested at 75F ambient.

I borrowed a manifold gauge set and vacuum pump from a friend. I vented the high side a bit until I got pressures in line with another thread (http://www.e90post.com/forums/showth...charged&page=2) at 35psi low/225 high (I just vented until I got 225 on the high side.

That seemed to fix the problem I'd created by overcharging, but I'd added the stuff with leak sealer and whatnot to the system and wanted to do it properly, so I went back to the store and got two 340gram cans of clean R134a. I vented the system, pulled vacuum for 30 minutes, then let it sit without the pump for 30 minutes to see if the vacuum held. It did, although at this point I realized one of the two R134 cans had a manufacturing defect and somehow had the wrong thread. So I left the vacuum until the morning when I could go back for a new can. It held, no leaks

With the system at vacuum, I weighed both cans on a digital kitchen scale and determined how much of the second can I needed to add to get the 590 +-10 grams. Dumping the first can was easy, turn the car on, crank the AC, and shake it/heat it in water until it's empty. The second can was obviously trickier. I set it up on the scale with the hose attached and, very carefully to not disturb the hose, let it out into the system being careful to see the mass drop I calculated. It takes a while to do without actively shaking the can, since the R134 boils off more slowly.

In the end, I lost 604 grams from the pair of cans, so with a little lost here and there as I screwed and unscrewed the cans and plumbing, and with the couple grams of liquid in the high side line, I think I'm probably in-spec.

And now that it's all done:

(Max fan)
Temps before:
60F idle, 55F cruise

Temps after:
55F idle, 50F cruise, 45F fan low idle, 40F fan low cruise.

So I'm pleased . Definitely get the right tools!

EDIT: forgot to mention pressures: I forgot to take a picture of the gauges because I'm dumb, but I think I finished at 35psi/210psi (75F ambient)