Which engine do you have? On the 2.0 non-turbo the idle rpm isn’t adjustable, controlled by the ECM only. Make sure the engine is airtight from the air filter all the way to the exhaust pipe. Check especially the boots between the air filter and the throttle body & the exhaust between the cylinder head and cat for leaks. Make sure everything not necessary is switched off: AC, headlights, rear window defroster, seat heating, etc. If automatic, leave in P. And make sure there are no diagnostic codes. If there are codes, post them here.
Have you tried spraying starter spray into the air intake? If the engine rpms stabilize, that indicates it is currently running too lean. Then try spraying the starter spray at various places where there might be a vacuum leak allowing unmetered air into the engine. If the engine rpm speeds up at a certain spot, there’s a vacuum leak in that area.
If the starter spray has no effect the problem is probably spark. So use a spare spark plug to check for a good healthy blue-white spark. If the spark looks ok, other things that can go wrong
- engine coolant temp sensor
- throttle valve control module
- throttle valve may just be sticking, needing cleaning
- evap purge function not working correctly
- egr problem
Make sure all the powertrain fluid levels are correct of course.