Question
Ok I will try to be as specific as possible. I have a 2001 Hyundai Elantra GLS with about 80,000 miles on it. Recently I've been having a problem with vibrations at idle and sometimes low speeds. I've noticed that when I go in reverse and do wide turns, the car vibrates even more (mostly going in reverse). I had a basic engine tune up.. I've had my spark plugs changed twice (first time NGK's, second time Hyundai). The check engine light will not go off.. it is giving me a cylinder 3 misfire code. The spark plugs have been changed and I've had 3 different mechanics look at the vehicle and they see nothing wrong.. It is almost like a "fake light".. but the car is still shaking so I know that there must be SOMETHING wrong.. I also had three mechanics check my oxygen sensor. Two of them (more reliable) said that it was fine, and the other (less reliable) said that there was stripping on the wire for the O2 sensor.. I am lost and do not know where to go from here. Please help!
Thanks..
AnswerP0303 doesn't necessarily indicate a problem with the spark plugs (or another secondary ignition component). While a majority of my responses for misfire codes reflect these issues because they are most frequently the problem, the code simply indicates that a misfire exists, not the nature of the cause.
First, it's important to try to ascertain whether the shaking you describe is related to the P0303 code, i.e. whether you can detect that a misfire is actually occurring. If the shaking is indeed caused by a misfire, you should be able to feel a power loss when it occurs. For the rest of my answer, we'll assume that this relationship exists.
First, since it causes most misfire problems, start with the secondary ignition system. Since you've replaced the plugs twice with no resolution, we'll assume they're okay. Take each plug wire loose from the plug and place back down on the plug without snapping it in place. Then start the engine. Now, one at a time, gradually lift the plug wire off the plug. The spark should come out the end of the wire and jump to the plug tube or the plug. If it comes out the side of the wire or anywhere else it shouldn't, the place of the arc is the location of your problem. If the arc comes out of the side of the wire, replace the wires. If it comes out between the coil tower and wire boot, you'll probably need to replace both the wires and coil. If it comes out of the coil, you'll need to replace the coil.
If all this checks okay, swap the #4 and #3 injectors. If the misfire moves to cylinder #4, the injector was the problem. If not, it's time to perform a compression check. The remaining possibilities are that there's an internal engine problem (which a compression test will usually uncover) or a problem with the injector wiring (which will usually, but not always, set a code for that injector's circuit).
If your car is indeed vibrating due to a misfire, I'm incredulous that any mechanic would tell you they see nothing wrong. If this is occurring, these mechanics are not qualified to diagnose and repair your problem and you need to find a mechanic who understands your fuel management system, your ignition system, and the theory behind internal combustion engines.
Is the P0303 your only trouble code? If so, the above will probably lead you down the correct path. If not, let me know what other trouble codes are present. They may provide information about the nature of the problem. Although there may be an issue with your oxygen sensor, it's unlikely this is the cause of the issue about which you've asked. The oxygen sensor affects all cylinders equally, so it shouldn't cause a misfire on only one cylinder.
