Weird Idle

[NoStickDrift]

I know there are tons of stuff on the idle of a 240sx, And I have searched. But mine seems a bit different of a problem. So here it goes. My 89 when in park idles fine about 700 - 800rpms. When in drive wth my foot on the brake its at 550 - 650rpms. And it fluctuates pretty bad sometimes. If i turn the AC on, it will idle at about a constant 950rpms. Why would it be constant with the ac on, and not constant with it off. Also it was doing this before I installed a cone intake, but its a little bit worse now. Any suggestions would be helpfull.

[NoStickDrift]

Comon guys I need some help hear, some of the techs at my shop said it could be a vac leak, any thoughts?

[Ni2s4s0aSnX]

Sounds like a sensor problem or sensor wire problem.

[yangs13]

if it starts to idle really rough and at low rpm, it could be one of your fuel injectors. when my car started to idle rough and the engine would shake violently, i took it to a local shop and had dianostics on it, it turned out to be a bad fuel injector.