AC makes car idle rough (2002 Protege5)

[LunarStray]

I have an '02 Protege5 with a K&N cold air intake in a manual, the engine runs great in all but one circumstance - when the AC is on. Turning the fan on without the AC switch engaged doesn't affect anything but when the AC switch is on, the car will idle around 200 RPM, very rough, and sometimes stall the car, but it continues to run fine so long as I'm not at idle. Once or twice, during heavy acceleration with the AC on, I had the charging system light come on and the AC shut off for a few minutes, then the light went away and it came back on. I don't hear any belt squeak, I was curious if maybe it was the tensioner? Any ideas?

 

[LunarStray]

Update: I let the car idle with the AC on at a stoplight for a moment and got a check engine, oil warning and charging system light. I pulled over and turned the car off, plugged in my scanner and found codes P0506 and P0090, for "idle air control system RPM lower than expected" which makes sense for the car idling so low, and for "Fuel Pressure Regulator 1 Control Circuit Open." I didn't see anything physically wrong with the fuel rail, and I'd assume there'd be other symptoms than running rough exclusively when the AC is on, and the car is at idle if that were the problem?

 

[Saudade]

When you turn on the AC, you engage the compressor, which increases the load on the engine. The PCM then commands the IAC to increase the rpms to compensate. This, in turn, maintains engine idle speed, keeps the the charging system going, keeps oil pressure up, etc.

For the P0506 - Idle Speed Control System RPM Lower Than Expected, I 'd try cleaning the throttle body, the IAC, the electrical connectors. Check for any intake leaks. Then reset the codes and see if it comes back.

The P0090 - Fuel Pressure Regulator 1 Control Circuit Open, means there's an open circuit somewhere in the wiring. Again, check the harnesses for any signs of damage, clean the connectors. Reset the codes. test drive and see of they come back.

If they do, then you dig into the next level of diagnostics. I prefer diagnosis before replacing parts.