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High Pressure Not High Enough

AC Rookie on Wed August 13, 2014 12:45 PM User is offlineView users profile

Year: 1996
Make: Chevy
Model: Lumina
Engine Size: 3.1
Refrigerant Type: 134A
Ambient Temp: 80
Pressure Low: 50
Pressure High: 160
Country of Origin: United States

1996 Chevy Lumina
3.1L V6
80 Ambient Temp
Low Side - 50-55 psi
High Side - 150-160psi

The previous owner had the AC system worked on and a new compressor installed. It worked for them for several months and then quit blowing cool. When I got the car it had lost most of its Freon charge.

I replaced the high and low port valves, added 1/4oz of leak dye through the low port, added 1/2oz of PAG46 through the low port, vacuum pump for 1 hour, charged it up by weighing 2 lbs of Freon. Fired up the engine, AC on Max, blower on high speed, high pressure 150 to 160, low 50-55. The best vent temp I saw was 62F. The data I have says the low should be 42psi and the high 340psi, which seems to high to me.

What do you guys think? Compressor issue or orifice tube issue? Where is the orifice tube on this vehicle?

As an aside. I added the PAG46 because the info I have found says that PAG150 is the GM OEM spec but the new compressors I have inquired about recommend PAG46. Since this vehicle supposedly had a new compressor installed I went with a supplement of the PAG46.

Thanks in advance for your help.

Thanks for the help!

Cussboy on Wed August 13, 2014 2:29 PM User is offline

Two questions:

1. Did you find and repair the leak? If not, you still have a leak.

2. Does this have a fan clutch/belt driven fan, or electric fan(s). These also have to work well, but a malfunctioning one won't cause your high side to be lower than what it should be. Of course, the fan shroud must be intact too.

OK, third question: did you add that refrigerant using cans or by modern machine?

AC Rookie on Wed August 13, 2014 6:26 PM User is offlineView users profile

1. No leak found as of this writing
2. Electric fans. One was cycling
3. From a 30 lb bottle upside down (liquid) on a scale. 32 oz

Thanks for the help!

mk378 on Wed August 13, 2014 6:38 PM User is offline

The main problem is that your low side is too high. Does it drop when you rev the engine? If not, the compressor (which is variable displacement) is not working properly. If you give the V5 enough rpm and keep the high side down, it is supposed to internally regulate the low side to about 25, which corresponds to the optimal evaporation temperature of about 35 (good cold air but the condensed water doesn't freeze).

I don't know where you got the figure of 340 from. High sides should stay well below 300 except in triple-digit weather.

Edited: Wed August 13, 2014 at 6:40 PM by mk378

AC Rookie on Wed August 13, 2014 11:15 PM User is offlineView users profile

When I rev the engine the low drops some (not near a normal level though) and the high goes up and then they pretty much come back to where they were.

I must confess the 340 on the high pressure came from a Haynes manual in vehicle AC systems.

Thanks for the help!

TRB on Thu August 14, 2014 12:39 PM User is offlineView users profile

Test Post.


When considering your next auto A/C purchase, please consider the site that supports you:

bohica2xo on Fri August 15, 2014 1:31 PM User is offline

Test again.

Doors open. Cabin fan on highest speed. Engine rpm ABOVE 1500 rpm. Hold the high rpm for 3 minutes to allow the system to stabilize. Record test pressures while the engine speed is above 1500 rpm.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

AC Rookie on Fri August 15, 2014 1:45 PM User is offlineView users profile

The inside vent temp I indicated, 62F at best, included a 5 mile drive at 40 to 55 MPH. Of course I did not have the gauges on at that time but I am sure the RPM were above 1500 for the majority of the drive.

Thanks for the help!

AC Rookie on Wed August 20, 2014 12:47 AM User is offlineView users profile

Well today I broke open the orifice tube fitting and the orifice tube was a mess. All cruddy and covered with black residue. Also it was in backwards. Put in a new orifice tube the correct direction and 32oz of 134A by weight. Set on Max, windows down, high speed blower and the best vent temp I got was again was 62 degrees F. This time though the high pressure ran between 225 and 250 which I believe is in the correct range. Also the compressor sounded different than it did before, more like it was working harder. But the low pressure was still running around 50 to 55?!?!?!?! Gas temp on the gauge says 50-60F. Ran the RPM up for a while and could see the high pressure cycle between 200 to 250 but the low pressure stayed the same. Do I need a smaller orifice tube for this compressor?

Thanks for the help!

Dougflas on Wed August 20, 2014 6:52 AM User is offline

he correct size tube is in the system, look at the control valve as a problem.

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