No power in 5th gear

SOURCE: 2001 xg300 engine runs rough - skips

I had the same car and the same problem.
My case I changed Air Intake
sensor(or call MAS), located right after air filter unit.
Good luck!

SOURCE: My 2002 Hyundai Elantra Gt (2.0L)engine runs rough

The Hyundai online shop manual and my newly arrived Hyundai shop manual kept pointing to the coil as a possible problem. While preparing to check coil continuity I discovered the pigtail connected to the bottom of one of the coils was loose and the contacts were burned. This is the coil that supplies spark to plugs 1 & 4, the same ones that kept showing up each time the check engine light illuminated. After cleaning the contacts and making sure the pigtail was securely attached to the base of the coil I have had no further problems.

SOURCE: engine revs but car doesn't accelerate

Since you did not say what type of transmission you have i will suspect the clutch is slipping if it's a manual.
If it's an automatic, it could be several things. Low fluid level...clogged filter, fluid needs to be changed..transmission serviced...weak pump, bands/clutch packs worn...etc
Good luck..i hope this helps, please rate me a fixya.

SOURCE: my 98 sputters during acceleration and check

Thanks for asking me to answer this question as I'm going to say something a little different than ther others that are trying to help you and they arn't nessarily wrong as their answers are a possible solution but not the most common. Low fuel puressure will do it but it won't nessarily turn the check engine light on. If the check engine light flashes then the computer is complaining about something that's going to damgeage the engine or the emmision system perminaly and needs to be repaired right away. Like another of the answerers reconmended, the computer needs to be scanned to see where your starting from. All check engine light events set a stored ( code) in the computer (ECM, Engine Control Module) This code can be found but connecting a scannerr ti the data linl connector that's located under the dash board on the drivers side. ( most autro parts stores will connect and read the codes for free as they hope to sell you something to fix your car. These codes just give you a place to look as the problem they point out could be caused by something else that will not have a code and also codes can come up that have nothing to do with the percieved problem you think you have with the car. ( Don't take codes as this is exactly what's wrong ( It's just a place to look). Once the code is investagated, your car can still hesatate. The spark plugs are a good idea as stated by another answerer of this question and they said that you may even have cracked or damaged the new plug as you installed it. Plug wires also fail and they can be found( sometimes) by obserbing the car running with the hood open in the dark. There shouldn't be any snapping noises and no noticable sparking under the hood anywher if things are good. Bad repairs can also cause this problem. the plug wires may have been installed on the wrong spark plugs ( firing order). If your brave ( I'm a chicken when it comes to high voltage ( like the plug wires and the coil can produce)) You can remove one wire at a time ( and reinstall it ) with the engine running and see 1) is it sparking, 2) does it change the way the motor runs 3) is the spark strong or does it jump to something else real easy and not to the cap position it came from. These cars lay the plug wires right on the valve cover and as they get old ( but not all that old) they'll start leaking high voltage to the valve cover and make the engine miss under load. Ignition CoilPart Number: E563C Manufacturer: ACDELCO PROFESSIONAL Grand Blanc, MI 800-223-3526 This is the coil pack and the plug wires go from it to the spark plugs. These don't fail that often. but the one coil fires the two end cylinders and the other coil fires the two cylinders in the center of the engine. the wires Can leak the high voltage to other things other than the sparkplugs and the miss and the mixture codes in the computer will be caused. Misfire codes are P0300 ( random Missfire( all cyclinders miss some))) P0301 ( cylinder number one is missing) P0302 ( cylinder number 2 is missing). And the other two cylinders. If it's just a 300 your looking in many places , but if it's just a certain cylinder then you know wher to look. your engine cylinders are numbered starting at the fan belt end on the engine and in line away from the belts.1234. Say the code is P0304. Take the nuber 4 spark plug out put it back into the enf of the plug wire , lay the plug with the wire connecdted on the valve cover and run the engine and see the plug spark. If it does'nt or doent fire staedly you have found the problem the computer reported. Put a differnt plug in the end of the wire and if it sparks good the plug you removed is bad , if it acts the same the wire or maybe the coil is bad. Use a different wire connected to the same position on the coil and if it too misses the coil is bad.

SOURCE: check engine light is on...have

Some check engine lights need to be reset by the OBD-II code reader. It's also possible that you've solved 98% of your problem but there's still 1 problem left (possible clogged fuel injector?) that's pulling a different error code. (possibly P0301-P0312 misfire or P0262-P0295 injector)

Best bet is to go to 1 of the auto parts chain stores that provide free diagnostic tests. If it's just a lingering old error code it will be reset or if it's something new, you'll know exactly what the issue is.