Engine Idle and RPM a problem?

[DKVL]

I have several questions about rpm and engine idling.

1. When my GT is idling at neutral, it revs at around 600rpm and this is constant.

When my AC is on while idling, it revs between 600-900rpm and this do not remain constant. Over the course one to two minutes it will switch between the two reving regions. Meaning, it will rev at 600 rpm for about 20-30 sec and switch back to 900rpm. The durations in how long each change in rpm is not constant and at times seems random. In all instances, the range is betwen 600-900rpm. Is this normal?


2. When my AC is on while driving and with my gear at neutral (for example: coasting in neutral until I come to a complete stop for an intersection), I observed that my engine do not idle at the normal 600rpm. Instead it revs at exactly or near 1000rpm for maybe 10 seconds before dropping down to the normal reving at 600rpm. If my AC is off and when I switch to neutral, my engine almost immediately drops the rpm to about 600rpm. Is this normal?


3. I noticed that during idling my rpm will drop from 600 to 400 if I step on my brake pedal. Once it drops to about 400rpm, it will readjust itself and return to 600rpm. Is this normal?

Also, HAPPY NEW YEAR TO ALL!!!!!

 

[squeak93]

Sounds like u have an electrical gremlin if it drops that bad when u either have the a/c on or hit the brake pedals. have u had ur alternator tested or battery tested? (most local parts stores can do this while they are still in/on the car.)

 

[4tun8]

I'm new to working on mustangs but all of that sounds normal to me. With other cars the ECU has different settings for IDLE when the AC is on and when the AC is off. The switching back and forth will be normal as well.

 

[DKVL]

The last time i brought my GT in for service (1 month ago), they took a look at the battery and alternator and both are normal. Not sure if it is electrical.


I hope this is normal.

Any other inputs?

By the way, thanks for responding.