Air pump causing poor idle?
 

As of lately my 94 FD started having difficulty at an idle when warm. The idle would dip down pretty low and sound like it was about to die. This is with no electronics on, and if I put my foot on the brake it would get worse. Also something to note is that with the hood open I can hear and see the air punp turning on and off rapidly, and everyone it cycles is when the idle gets worse. I checked the TPS voltages and they are all perfectly within range. Does this sound like a grounding issue? All the time the idle is low the dash lights all start lighting up like the alternator is not charging. And at an idle for voltage I have mid to high 12's. I checked with my meter to see if there was a voltage difference between the battery negative and the intake manifold, alternative housing and there was no indicated difference. Anyone have a similar problem in their experiences?

How low does the idle dip down? Compression numbers? Mods done to car? Brake boosters pull vacuum from the motor so there is no surprise it gets worse when you press on your brake. If you feel like your air pump is the culprit, I would suggest unplugging it and seeing if it changes or not.

I tried unplugging the air pump, and it didn't help that much. But one thing I did do was hook up a battery charger to it while it idled and that made it much smoother. The car is all stock other than a down pipe, and cat back straight pipe. I do not know the compression numbers it pulls about 16-18 in hg of vac(indicated) at an idle so I don't think the compression is low, it runs flawlessly other than this new idle issue. Do you know what the voltage should be at an idle? I would have to imagine the alternator does charge at an idle

The idle dips to just under 500 then back up to about 750

I think I just figured out the problem, with the car off battery voltage is 12.18, now when it idles the voltage is 11.8. So I'm pretty sure it's the alternator

Looking at the FSM, it says that your battery should actually be reading a minimum of 12.4v.

It also says that the alternator at the ring terminal should read 14.1 to 14.7 volts at idle.
The top terminal should read 14.1 to 14.7 volts at idle
Bottom terminal should read 12.9 to 13.5 volts at idle

I hope this helps out a little bit. The electrical systems on these cars always feel sub par to me imo.