Ambient temperature is approximately 85F. Before starting the car and AC, the low side pressure is 100psi, and the high side pressure is 100psi. After starting the car and putting the AC on Max with recirc on, the low side pressure is 40psi, and the high side pressure is 200psi. When I leave the car and AC running for 5 minutes, the low and high pressures have dropped to 27 and 175 psi, respectively.

I recently replaced the receiver/drier and vacuumed the system. I was able to get to approximately -25 inHg, which is ok because I live at 5500ft in elevation and there should be a correction of +5 for vacuum at this elevation.

The condensor fan runs, and the compressor clutch is engaged. The air out of the vents is cool, but not cold.

According to the AC pressure graph in the service manual, normal low side pressure for 85F should be around 28-48psi, and the high side pressure should be 280-360psi.

My low side pressure looks to be in an acceptable range, but the high side pressure is quite low. Can any AC experts offer some suggestions as to what to look at next?

One last note, this morning, with ambient temperature at around 75F and the AC running, the pressures were 25 and 200 after a few minutes.

Edit: I forgot to mention, it's a 94 Civic DX

 

Run the A/C for 10 minutes as you did and then measure the pressures with the engine idling at 1500 rpm. Post the ambient temp and the temp of air output from the center vent.

Did you leak test the system when it was evacuated? What exact weight of R134a did you add after evacuation?

 

Ambient is 80F. I left the AC running for 10 minutes. Low side pressure is 32, high side is 180. I get 65F at the center vent.

To test for leaks, I pulled a vacuum and let it sit overnight. It was still in vacuum in the morning.

I added 2 12oz cans. The yellow line connecting to the AC gauges was purged for 10 seconds for each can. I realize the system is supposed to take 21oz, but since my high side pressure never went into the range I was expecting, I just added the whole second can.

 

Based on the chart, your low-side pressure should be around 35 psi and the high-side pressure should be around 300 psi.

Therefore, the low-side pressure seems fine but the high-side (discharge) pressure is low:

 

Hmm, I don't have the related symptoms. It takes a while for the pressures to equalize. Also, the low pressure gauge isn't at vacuum. What else could cause this?

 

Was the test done with the engine at 1500 rpm?

 

Unfortunately the car doesn't have a tach, so this was at idle.

 

Your pressures can only be applied to the chart if the engine rpms are at 1500 or a little higher.

You can hook up an external tach or a scan tool that reads live data.

 

OK, I had a friend help me measure it at 1500 with a tool he has. When it's at 1500rpm, the low side drops slightly to 20psi and the high side goes up to 250. Ambient is at 80F. Referring to the chart again, it seems like I might be slightly low on r134a. What do you think?

 

Given that there were no leaks and if anything you added too much refrigerant, it's most prudent to assume the refrigerant level is NOT low.

Your high and low side pressures are low. Here's what the manual says: