My 82 240d needs an ac recharge - at first had 5 psi low side. Had already been converted to r134. So I picked up some refrigerant, started the car, jumped the compressor, and got it up to 30 PSI low side. My car has very low idle (another issue I am dealing with) and have seen that when recharging, you want the engine up to 1500 RPM. So I get in the car, and now have cold AC. Go back and check the pressures, and its showing 45 PSI. Since the car is about to die, I revved it up a little (my car does not have a Tachometer) and it went down to 36 PSI. If I rev it up a lot, it goes down to 25-ish PSI.

My 2 part question is, should the pressures change that much by revving the engine? And since I have no tachometer on my car, can I just assume when I hit 1500 rpm and be safe at 35 PSI?

Details of the system:

Ambient Temperature is 85 degrees - my chart shows that I should have a 30-40 PSI low side charge at that temperature. My car reads:

About to die 45 PSI
Revved up to what I assume is 1500 RPM is 35-36 PSI
Revving the engine to higher RPM's goes down to 20-25 PSI
AC off completly shoots up to 100-110 PSI

Thanks for the feedback.