I have the same problem, have swapped coils, injectors, plugs and replaced plugs w/ new ac delco iridium plugs. I have flushed out the fuel rail 3 times and tried adding seafoam, iso heet and berryman's B-12 chemtool to the tank over the last 15- 20 tanks of gas. I have also wasted money and cleaned the mass air flow sensor. NOW I FEAR THAT THE CATALYTIC CONVERTER IS GOING BAD DUE TO THE MISFIRES! (smells like sulfer/rotten eggs at times.)There is a service bulletin for contaminates and water in the fuel. It states that the fuel rail is a non-return type. This means that any water or contaminates in the fuel will enter the rail and stay there inturn clogging the injectors. The bulletin states that this causes the misfire on cylinders 1 and 4 during right and lefthand turns. So why is it only P0304 and never P0301? The dealer will charge you 300.00 to flush the injector rail. The problem is compounded by the fact that there is no fuel filter on the HHR except the one in the tank. Evidently that is not good enough to keep contaminates from reaching the fuel rail. I am going to drop my fuel tank and drain all the fuel from the system and refill w/ top tier fuel from a top tier station. Search for top tier fuel on the web. If it comes back again, I am going to get a ford focus. I can't keep worrying about this nonsense. Too bad too because I really like my HHR. Reliability is more important to me now, after all this time and money.
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Swap the #4 coil with #3, swap #4 spark plug with #2 and swap #4 injector with #1. Clear codes. Drive the car until it has a misfire code, again. If you get P0303, it's a bad coil. If you get P0302, it's a bad plug. If you get P0301, it's a bad injector. If you get P0304 again, it's a mechanical engine problem, like a burned valve, bad intake manifold gasket, worn cam, etc. or a wiring harness issue to the coil or injector, maybe even a bad computer.
P0304 is engine misfire #4 cylinder Several things can cause a misfire #1 electrical which you have covered #2 Mechanical Test engine compression is should be around 150 cranking and 120 running . internal valve train components can cause bad compression and the valves then self. #3 fuel injector can be the cause suggest swapping with a know good cylinder and see if misfire follows injector.
I hope our chat was informative. I am assuming, since you said your Jeep is equipped with a V-6 that it is a 4.7L. You can't swap plug wires because there are none, but you can swap coil packs from one cylinder to another to eliminate the coil pack as the cause of your misfire. As for the "solution" above that says to replace your catalytic converter, all I have to say is if you are not getting the "dreaded" code P0420 "Catalytic Converter Efficiency Below Threshold" - don't worry about the cat. - you MAY get this code once the misfire is fixed, but I wouldn't run out and lay down hundreds of dollars on a converter until the computer tells me it is time....and simply "resetting" codes doesn't fix ANYTHING!!! Just resetting codes and continuing to operate the vehicle with a misfire problem is a sure-fire way to guarantee that the catalyst will need to be replaced. Misfires destroy catalytic converters. Let's hope you get it fixed BEFORE this happens!
P0304 Cylinder 4 Misfire Detected,A code P0304 may mean that one or more of the following has happened:
Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs).. I would suggest spark plugs, sparkplugwires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced.
it is always a good idea to stay with o.e.m parts and yes there is updated parts in the newer coils. and yes the M.A.F will also cause misfire faults but will not make check engine light flash or make car shake and shudder. also check you wire harness going to your coils make sure your ground wires are not cracked. That could be the main culprate. good luck hope this helps
P0304 is actually pointing to a misfire in cylinder 4, not a camshaft sensor problem.
There could well actually be two misfires rather than just one because Ford use a wasted spark ignition system wherein two cylinders are fired at once, one of them on its compression stroke and the other on its exhaust stroke, hence the wasted spark on the exhaust stroke.
You say it is "popping" which would lead me to believe that there is a spark issue rather than a fuel issue, because the popping sound would be the fuel igniting either in the exhaust or the intake rather than the cylinder, and this would be confirmed if you were to run the engine and then switch it off and pull the no.4 spark plug and it were to be covered in fuel.
If you were to find two plugs covered in fuel and the rest ok, then the coil pack feeding those two plugs would probably be the problem.
If only one plug were covered in fuel then either the plug or lead feeding that cylinder owuld probably be the issue.
If none of the plugs are covered in fuel, indicating a fuel problem, and you were to swap the no.4 injector with one of the other injectors, and then the fault code were to change to the cylinder into which you swapped the no.4 injector, then the injector would be the problem.