2002 neon revs up to 4000rpms at start and idles up an ddown 1500rpms-2000rpms after. and begins to backfire near engine .

Hi Jeff, I'm glad to help! Jeff, with the engine running tap on the throttle position sensor, If it revs-up when you tap on it then replace it. Hope this helps and have an awesome day Jeff.

check vaccum lines. inspect throttle cable and spring. if cable is warren can stop it from releasing all the way

What control idle is RPM sensor[TPS] to check it is OK.

sound like its missing
check sparkplugs wires and coil pack for fire if not that run a compression check

theres a air leak! check out your throttle body

check ur themostat take it out boil some water in a pot drop ur thermostat in the pot watch to see if it opens take it out watch it close if working properly,make sure ur pump is moving water take off ur radiator cap to insure water is moving,then check to insure ur cap is getting a good tight seal u only have ur cap,pump,hoses,radiator,thermostat,unless of course youve overheated recently???then u could have other issues

back side of the throttle body under air cleaner. but if you put a new one on. it will rase up at first the idle down after about a min. then take it down the road at 40 mph. should be OK then. don't forget to use carb cleaner on the port where you take it off from. good luck

The most common source of that problem is a sticking idle control motor. As a first step, purchase a new gasket and/or O-ring for the idle control motor. Remove the motor and clean it out using aerosol carburetor cleaner. If the idle control motor was sticking, that should cure it. It the fix only lasts temporarily, you need to replace the idle control motor. This is a common problem with vehicles with over 80,000 miles on them.
Charlie

check everything you can under the hood your air intake system all of it especially make sure all clamps are tight .. and make sure all vacuum hoses are good and not cracking or broken or just loose . you might run a little fuel cleaner through a tank of gas too cleaning the engine fuel system up some. just a few ideas.. Jerry

sounds like some kind of vacuum leak or a bad IAC motor