While riding my rpm dropped between 2000-1000rpm constantly..

All the above is normal , except 2000rpm is a little high at idle . can be adjusted down to 1500rpm. the computer adjusts the engine speed . in drive the transmission loads up the engine idle drops to 1000rpm . in park engine unloaded rpm increase all normal.

could be anything from loss of signal from the throttle body, to emissions to the fuel system..check all 3 and look for any issues.. replace whats bad..

The ac compressor takes around 5 hp to drive so when the clutch engages the idle speed will drop around 250rpm amd when the clutch dis-engages the rpm will rise 250 rpm If you have a efi computer controlled vehicle the computer should automatically adjust the rev difference to maintain a constant rpm. If you have a carby vehicle there is a electric over vacuum control that you can fit to the accelerator lever that when the clutch activates the power to the switch from the clutch opens a valve that allows vacuum to pull the throttle open and maintain a constant rpm

idle air control valve is bad replace it. at start up choke control runs till warm up while driving throttle opening controls engine at idle ,the idle air control valve.

code is manufacturer specific so post the question again with make , model, year , no of cylinders to get an answer

Hi, please have the car scanned at your local parts chain and send us the trouble codes. If I had to guess, I would say MAP sensor, but it will be much easier to help you with the trouble codes in hand.

Please see my tip at http://www.fixya.com/cars/r5932051-check_engine_light_check. These are generic instructions. If you get stuck or need specific instructions for your car, please get back to me with model, year and engine info.

Check power steering fluid and transmission oil level

Check throttle cable at carb and make sure you have full travel. Sounds like rpm is little to low. Throttle cables are usually held in place with 1 screw and a washer. Put cable at full throttle loosen screwand pull back until throttle at carb in at full then tighten screw.

idle it down carb settings.possibily throttle cabe needs adjusting.

i'am no expert on this car, but i would chk. the T.P.S. (throutle positioning sensor) there also may be carbon build up on throutle butterfly, not letting it close completely when you take your foot off the pedel.