EmpHi everyone. I'm a new member and the proud owner of a 2002 sonata 2.4 automatic that is just about to hit the 200k mark . Its given me minimal trouble so far except for the crank pos sensor going out last year. I replaced the timing belt when I replaced the cps and it ran fine fine...... until now. Upon first start of the day in the morn its a bit cold blooded and you have to hold the idle at about 2k for a bit until it will hold its idle. Its done this the whole time I've owned it its annoying but no biggie. After this it will hold idle and run fine for about 20-30 min of driveing . After that time has passed its just like someone flipped a switch what used to be smooth idle and powerful acceleration instantly turns into rough car shakeing idle and choppy bucking very weak take off. In park it smooths out only a little when in gear it almost stalls if your not on the gas . Ran a check for codes thrown. And nothing no engine light on no fault codes no visible problems under the hood. And it only does this after the car has reached operating temp any ideas on wht it could be

 

Ok I narrowed it down to the #4 fuel injector . When it was cold and running good I went and unplugged each injector 1 at a time. Each one I unplugged made it idle rough and had a noticeable difference not unlike what it sounds like when it gets hot and starts running rough. After that I waited till it warmed up and started running bad then went through and unplugged each injector again all but the #4 made it run so bad it almost died. The #4 however made no change at al used my noid light and all injectors are pulseing did the screwdriver test and I could hear each injector fireing even #4 so what would cause this

 

could be a coil issue . Check your wire connections for corrosion, check ground connection is clean . If there is paint under the ground connection, sand it off.