We have a 2004 explorer with the 4.0 flex engine it is idling below 500 rpm's stalls out sometimes and the ac compressor only works while driving shuts off when idling.Just put new plugs in checked the MAF sensor,cleared and reset the onboard computer as per specs and still the same problem.

with the ac controls on?.. there is a circuit that bumps up engine idle so extra horsepower is created to run the ac compressor for defrost and such.. this is normal usually..what kind of vehicle..repost

TRY REPLACING YOUR PCV VALVE

run the fault codes and check for problem idle control sensor
500 rpms are too slow for idle should be around 850 for automatic
also have the voltage regulator /alternator checked as the problem is not enough voltage ti run the car and the ac because the available voltage isn't high enough

IDLE AIR CONTROL VALVE IDLE SPEED TOO LOW. IDLE SPEED SHOULD BE 750 - 850 RPM. IDLE SPEED NEED TO BE CORRECT OR IT WILL CAUSE ENGINE RUN ROUGH OR STALL OUT.AC COMPRESSOR WILL TAKE SOME POWER FROM ENGINE WHEN AC IS OPERATING.

if this happens while the defrost of ac is turned on, it could be a seized ac compressor which will cause the serpentine belt to squeal and drag making the engine sometimes stall. if it happens with the ac off and the selector on say the floor, or dash vents, the issue could be a bad alternator and battery. this is confirmed when the blower motor engages and the lights dim and then the truck goes dead.

you may have a faulyt IAC sensor not idling up enging when a/c is on

Idle control sensor on thr trottlebody or if eequipped; airflow meter ...off the top of my head, hope it helps.

Check the RPM if it drops when you have the AC on while the car is not moving. If it does drop you may want to check on the idle compensator. Its job is to bring increase RPM to make up for the added load. You may also want to check on the idle if it is below the normal speed. The added load may decrease the speed and send you to stall. If they all check out OK, you may want to check out the compressor as well if it gives the engine too much load. This can be caused by a faulty compressor or too much freon, or the bearing may be bad (with the bearing, you will get to hear a screeching or whinning sound).

this is caused by slow engine idle speed, do the procedure below.
Here is the common cause of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

I took mine in to change the plugs and they said there are four O2 sensors in the fuel mixture system and two were bad. I didn't replace them since they were a little pricey ($500 to replace both) and I have 103,000 on the car. Has very similar roughness and occasional oscillations, but no stalling. Hope this helps.