whenever i put on the a/c, the rpms tend to drop low or sometimes the itself car would shut off. i wondering if it could be something to do with the timing or adjusting the idle.
Have your freon checked and make sure it is not low. If it is low the compressor has to work more and the ecm does not have time to compensate for idle changes. If it is good then replace your IAC and reset your idle like this:
Begin with a cold vehicle. The idea here is to get the car to a firm cold idle with enough air bleed capacity left in the idle circuit for IAC adjustment.
The idle stop should be set first. Back out the idle stop screw, away from the bell crank arm, until about 1/2 turn past the point where it no longer makes contact (blade fully closed). Using a 0.010" feeler gauge, tighten until gauge just drags between screw and bell crank arm. Remove feeler gauge. Tighten screw exactly 1 1/2 turns. If the screw is very loose, put a drop of loc-tite or silicone on it, so it doesn't work out of adjustment.
Now remove the connector to the Idle Air Controller (IAC) just on the other side of the throttle body. Start the car and allow vehicle to warm for 2 minutes. Give a small "blip" to let it settle. If it is having a hard time staying running you may have to get an assistant until you can get to the front of the car. Now open or close the air bleed screw (CCW opens) next to the IAC until the car idles at 575 to 600 rpm. For guys with aftermarket cams and an EEC tuner, you might want to idle a bit more briskly, say 650 to 675.
Obviously, this rpm range is by what the car and driver wants...IE, no set idle speed, whatever works for YOU.
Turn off the car. Now count the number of turns clockwise to close on the idle air bleed screw. If it falls between 1/2 and 2, it's okay, now reverse it out the same number of turns. Log the number somewhere in case you need it for the future. Reconnect the IAC. You are done.
If the air bleed screw is above 2 turns, it's a good idea to tighten the idle stop screw another 1/2 turn, and then repeat the idle setting. If it is below 1/2 turn, then loosen the idle stop screw by 1/4 to 1/2 a turn, and repeat the idle setting. Be sure to put another drop of silicone RTV on the stop screw if it was disturbed. Reconnect the IAC.
Now remember we adjusted the set screw on the throttle body. That means that the voltage reading from the TPS sensor has changed. It should read between .96 and .99 volts. Anything outside of that range will cause all sorts of issues including misfires and rough idle.
you will have to back probe the TPS connector. With the connector attached to the TPS stick a paper clip into the rubber boot on the connector where the top and middle wires go into the connector. The rubber is very flexible the paper clip will slide in between the rubber and the wire.
Set your multimeter to volts. Turn ignition to ON. Then place your multimeter leads on the metal pins. If it comes up as a negative that is okay as long as you are -.96 to -.99. If you get this reading then great you are done and hopefully your issues are gone. If not proceed to step 9
This is where and extra pair of hands come in handy. Using a large screw driver you will need to loosen the bolts that hold the TPS. DO NOT USE A SCREWDRIVER THAT IS TOO SMALL BECAUSE YOU WILL STRIP THE BOLTS. They probably will be really tight so you have to really lean on the screwdriver and use some finesse.
Once the bolts are loose you will need to move the TPS up or down and continue to read the voltage. Once you get it to the desired setting you can retighten the bolts. What I do is I find .97 and then as carefully as possible I tighten the TPS down. what will happen is the voltage will change usually to .96 or .98 but that it okay. Once you are done with this manually open the throttle body a few times and close it then reread the voltage and make sure it is still within our desired range.
Depending upon how loose your set screw was you might want to cover it in RTV to hold your setting. At this point you have CORRECTLY reset you idle.