AC not very cold at idle, great while driving

[Jonas]

Other than the unusual static pressures already called out, IMHO the running pressures are just about what they should be at an 80-degree ambient, except that the low pressures at idle are slightly elevated. But the idle speed should automatically bump up a couple hundred RPM whenever the A/C is turned on, lowering those pressures (and vent temps) a bit and helping with cooling when stopped at, say, a traffic light. Do you notice an increase in idle speed when turning the A/C system on? If not, that could be something else to investigate.

The low pressures of 22-25 PSI at 1,800 RPM translate to an evaporator temperature of about 25 to 29 degrees which, by the time the evaporator air reaches the dash, should result in vent temps of around 38 to 40 degrees. Are you sure there's no heated air mixing with the cooled air somehow?

I just arbitrarily listed the static pressure. It was taken shortly after the car was shut off.

Yes, the RPMs increase when the AC is on-I'd say by about 150 RPMs.

I'm curious about the heated air as well. I'm not sure how to tell. The vents seem a bit warm with fresh air, no AC on. Is that normal? They certainly aren't hot, (my heater could melt metal) but I'll check the temps in that scenario tomorrow.
Also, my passenger window-side vent has almost no air coming out, so I just keep it closed.

I should add that later today on the interstate, it was 85 and sunny with a steady center vent temperature of 38 degrees (recirculate/medium high fan.)

[Jonas]

Will do. Should that be done with recirculate on or not?

[tbirdtbird]

We have found heater control valves, blend doors, etc to be faulty, and allow heat to dilute the cold air.
Center vent of 38° is awesome and more than most ppl ever get! I think your charge is OK

[Jonas]

"Surefire way to test for reheating: let the engine block cool overnight then turn AC on immediately as you turn the engine on and immediately measure vent temps. That way, coolant would still be cold enough."

I did this test this morning at one of the center vents, recirculate, low fan, other vents closed. Ambient 80 degrees. Vent temp got down to 44-45 which took about four minutes. By then, the coolant was warmed up.

I did it again when I got home (88 ambient) and the temp dropped to 46 in about a minute. It was too hot to wait any longer.

[Jonas]

We have found heater control valves, blend doors, etc to be faulty, and allow heat to dilute the cold air.
Center vent of 38° is awesome and more than most ppl ever get! I think your charge is OK

I agree. We are narrowing it down, thanks.

[Jonas]

I also noticed my RPMs don't bump up very much with the AC on. They're steady 670 at idle, but they fluctuate between 700 and 800 with the AC on-usually below 750. It feels like the compressor is cutting out or the belt is slipping or something.

[JohnHere]

The idle speed will bump up and down according to whether the compressor is on or off at any given time. A fluctuation of 150 RPM or thereabouts is normal.

[Jonas]

The idle speed will bump up and down according to whether the compressor is on or off at any given time. A fluctuation of 150 RPM or thereabouts is normal.

I see. Good to know.

[Jonas]

Couldn't reply earlier due to shift. Talked about the superheat (i should have said "refrigerant flow", actually, but evaporator superheat is still indicative of a certain amount of refrigerant flowing through the evaporator) stuff only to explain how a system with a seemingly feeble at idle comp (a tiny clutchless, scroll or vane) still manages to provide satisfactory cooling at idle and with the car stationary. In a properly working system, that is. Once there's a leak, such a system, especially due to equally tiny nominal ref charges, basically becomes useless junk. A seriously starved evaporator won't provide any relevant cooling exactly like a flooded one. On the other hand, a deliberately slightly starved evaporator will provide maximum cooling while things are still warm.

Surefire way to test for reheating: let the engine block cool overnight then turn AC on immediately as you turn the engine on and immediately measure vent temps. That way, coolant would still be cold enough.

No change in temperature. Same at cold start as after 2 hours of driving.

[Dougflas]

Do not use superheat or subcooling in MVAC. The compressor speed changes and superheat will also change. Throw those readings out the door. These readings are used in stationary refrigeration because the compressor speed is constant. Get a new belt on there and check the idler and pulleys. Also, disable the heater core by using hose lamps or even vice grips with a protective material on the jaws. Make sure your cooling fans are working. If cooling is better with clamping the heater, you may have a blend door problem. Get away from the refrigeration system for now.

Going down the highway you have good cooling telling you the refrigeration system is working. Your problem is at lower speeds. Look there first.