Engine rougher with AC on

[Spetz]

I have recently noticed that my car runs smoother with the AC off.
With the AC on it is considerably harsher, especially at the lower revs.

What could this be?

[JamesDean]

Does your AC compressor make noise?
This might be unlikely, but your compressor could be failing and becoming harder to run, thus drawing the engine down lower...

just an idea...

[Spetz]

There is no noise, and the air coming out is cold.
It causes harshness at the same rpm that the engine already exhibits some harshness, however it is intensified about 3 times or so

[david s poole]

how new are the motor mounts?

[Spetz]

Less than 6 months.

I am suspecting there is a hole in the exhaust, and that the additional load from the AC is just making more noise and hence vibration.
May go to an exhaust shop next week

[Ferdman]

Spetz, the motor mounts were replaced. What about the transmission mount? That affects engine vibrations too.

[Spetz]

Yes, both engine and transmission mounts have been changed less than 6 months ago.

When the car is idling and I put my head under it, there is some weird noise coming from under the car towards the engine, like the exhaust has a tiny hole in it. Also, the noise from the rear muffler kind of sounds weird, its not drony, or noisy though. Could the muffler/resonator/cat hollowed out or something?

[david s poole]

generally speaking if you add load to an idling engine and it becomes noticeably rougher, it's because engine isn't making enough torque at idle.i would be checking for small vac leak by checking mixture via o2 sensor reading and also taking o2 sensor out and running engine to test for exhaust restriction.