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Pressures wandering

Theojo on Tue June 24, 2008 6:20 PM User is offline

Year: 1988
Make: BMW
Model: 325
Engine Size: 2.7l
Refrigerant Type: R12
Ambient Temp: 78
Pressure Low: 50
Pressure High: 250
Country of Origin: United States

First the questions, then the narrative.

When I got my Yellow Jacket 2 stage pump, I replaced the oil with the AC Kits bottle. It was just enough. I've now pulled down about 6 cars over the year. Is the oil still good?

Will air or moisture in the system, cause high side pressures to go down from 350 to 250 initially after recharging?

If the pressure stabalizes at 120 on both high and low side after shutting off the compressor, and I go back in the morning and it's down to 60 on both sides, does that mean there's a leak in the system?

Will adding more refrigerant ever cause pressures to go down?

I've had this vehicle for 2 years and with summer coming, thought I'd fix the AC. Last year I had good luck fixing 2 vehicles with some good advice and parts from this site. Flushing, new driers, oil etc. But it's not worth doing all of that to this car, 237K miles, so I thought I'd just try a quick fix and see if I got lucky.

It had 0 pressure on both sides. Since it's a Bay Area vehicle, I hoped that it was just minor leaking over the years, and it was never recharged; not a condensor or evap leak. I vacuumed it on a hot day and it held at 29 (no micron gauge) so I installed some leak detector oil and some r134a just to get it to cycle. The pressure appeared to be high on both sides but thought it might be because I was using the wrong type of refrigerant. Waited till dark and checked for leaks. None found up front. Didn't check at the evaporator, TXV Orings, since didn't feel like removing the center console.

Next day recovered and vacuumed again for an hour. Installed 24 oz R12, (Capacity is @36oz). At idle (900 rpm) pressures were very high at first, 60 and 350. At 2500, 40 and 380 but after a minute they went down to 25 and 260. After alternating between idle and 2500 about 5 times, I got it down to 45 and 230 at idle and 25 and 255 at 2500. Vent temp is cold at 2500 and cool at idle on a 78 degree day. There is froth in the sight glass. Rather than waste another can of R12, I thought I'd just ask throw it out there.

I'm guessing I've got a bad TXV or moisture in the system?

Theojo on Tue June 24, 2008 8:56 PM User is offline

More data. Just did a road test.

In drive way in Park at idle 900 rpm, 60/350 and climbing, (prior to the road test, I actually got 450 on the highside and the compressor shut off only to come back on a few seconds later after the pressure receded.)

Down the street at 1500 rpm, 40/200

On the highway going 60mph at 2000rpm, 30/170, nice and cool (for an 83 degree day).

Back in driveway, in Park at idle, 1100rpm. 45/260.

Based on my browsing here, sounds like a clogged condenser or weak fan, although the radiator never overheats.

Well it's a mystery, but as long as it works on the road, I'll just remember to shut it of at idle. I'm actually pretty happy with it considering it's working at 2/3 capacity of refrigerant.

Theojo on Tue June 24, 2008 9:30 PM User is offline

Mystery solved. As read about elsewhere on this forum, I thought I'd try the spray condenser with garden hose trick. At 1100 idle in Park, it went from 60/325 down to 30/110 while being sprayed! And what else did I find out? There is another fan, an electric one, in front of the condenser, and you guessed it, it was off.

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