I got the cel P0304 about a month ago since then i have replaced all four plugs and the coil pack to cylinder 4. I drove the car around a little over a week and the same code comes back. Could any one help with suggestions before i take the car to a mechanic. The car rides fine but when I'm at a light i can hear it sputter a little bit with some vibration.

 

If it's sputtering a little bit with some vibration, it's clearly still misfiring. It could be a faulty fuel injector issue. There are 2 ways to check for a faulty injector, stethoscope and multimeter. Easiest way would be with a stethoscope.

Place one end of the stethoscope on the fuel injector. Listen for a distinct clicking noise which would be the injector firing. An injector that is functioning will make a clicking sound each time the valve opens. If the clicking sound is not heard, the injector should be replaced.

 

+1 on injector issue

 

also with it missing like that it may be the EGR valve not sure just throwing that out there as another option.

 

We don't have an EGR valve on the tC.

 

oh ok well thats good to kno...cause mine coded and somebody told me that it might b the egr valve atleast i dont have to wrry bout that now...but than for the info

 

thanks for the info guys... havent had the time to look at the fuel injector yet but ill get to it this week. i dont have a one of those scope things but i can get my hands on a multimeter. would anyone know what im looking for if i was to go that direction?

 

It looks like the fuel injector seems to be good. I checked the resistance and they all read the same (13 ohms). I also swapped the injector that was in cylinder 1 over to 4 and 4 over to 1. I rescanned the car and it pulled up the same code P0304. I've seached online and found that it could also be a bad oxygen sensor or bad catalytic converter. My motor is stock as they come. No aftermarket parts what so ever. Guess I need to find out how i can chekc the oxygen sensor and the cat.

 

Um.... it coudl be a vacuum leak, be sure and check all vacuum lines you could have knocked one off by accident. That happened to me once and I was racking my brain for two days straight until I actually noticed the vacuum line that goes on to the intake manifold was loose when I re-connected it, miracle the engine idle evened out and she was purring like a kitten again. then I cleared the code and it never happened again.

 

i doubt its vaccum cuz it would throw a cel for the vsv, which i believe is the only vaccum source in the engine bay. i also doubt it would be the maf sensor but u can try cleaning it and seeing if it clears up, but than again it would throw a maf code too...hmmm....
usually if its a cat or o2 sensor it should throw a cel for that as well but i have seen stranger things.

 

Update- I finally got around to doing a compression test to the cylinders. Cylinders 1, 2, and 3 where in the 160 range while the 4th cylinder was at about 90. Now that i have pin pointed my problem for the cel to the 4th cylinder, what could be the problem to the low compression?

 

Bad ring, I found a toasted ring on mine last year although not as bad as yours is reading.

 

sounds like a bent or a burned valve if your only getting 90psi on
cylinder 4. do a leak down test. it will tell you if your leaking at the valves or at the piston ring. If you are all stock just take it to a dealer they may cover it under your warranty.

 

Got the leak down done and came out that the air was escaping from the tail pipe. From what i read that leads to bad exhaust valves. Now my warranty is no good since i've got over 100k miles and i've had the car five years now. A local shop told me my head would need to be sent off to a machine shop to get redone. He quoted me about $1800 in total. That seems a little steep. I'm sure i can pick up a used motor with half the miles and for about $800 less and do the swap myself.