My car has sat for 2.5 years (Long Island, NY) Started right up but after 2 min in idle the rpm surged back and forth from 1200 - 2000. Shifting into Drive with the brake on it was stable at 2000 rpm. Driving performance seemed normal. I did get a "Check Engine" code P1129 (MAP sensor circuit high voltage) I changed it along with new plugs, wires, dist cap and rotor and air filter. No change. Used CRC Throttle Cleaner to clean out the sticking throttle butterfly. No change. Had the Alternator checked. Putting out only 11.5 volts vs 14 volts required.
Had no "BAT" light after starting the car, but noticed that the rpm went to Zero (siting in Drive with brakes on) and stayed there even when I increased power. Shortly thereafter the engine slowed and quit. A start attempt got only "clicking"(as in a dead battery). A recharge brought everything back including the surging.
Could a weak/bad alternator cause this engine surging in Idle?
One other possible problem is the (IAC) Idle Air Control Valve. However it seems almost impossible to get to remove and replace. At least for me.
Engine is 2.3 L
The surging is most likely be due to a stuck IAC. Remove the air hose and spray carb cleaner into the ports of the IAC to let it soak. Turn ignition switch on and tap the IAC with a screw driver for a few minutes and hopefully get the stucl valve to work. Start engine and if idle is still too high, continue to tap the IAC until the idle drops. If it faisl, you would need to remove the throttle body to get at the IAC. You can try cleaning after detaching the IAC from the throttle body.
As to the charging, the alternator is definitely not working. The carbons could be stuck after sitting for too long. Tap the alternator lightkly with a hammer and test the charging again after starting. Ensure the pulley is turning while engine is running and drive belt tension is correct.