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Discussion Starter #1
We have a 2009 Cobalt and the CEL is always coming on with the PO304 (misfire #4). We have had the spark plugs replaced, the coil packs, the fuel injectors, did a compression test (was fine) and cleaned the system out, replaced the intake manifold gasket (was leaking) and it still constantly comes up with this code. The car runs fine, but idles bad. Especially with the A/C on. We have tried everything! I'm tired of dumping money into this car. Has anyone else had this problem or have any insight as to what it may be and how to fix it??? I'm at my wits end and my wife is ready to kill me if I don't fix this for her.
 

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This might be an obvious solution but have you had the light turned off? I have the same issue when AC is on, but no light. Maybe you fixed but never turned the light out.
 

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Discussion Starter #3
Yes, I have reset it each time after doing something to it. But thanks, sometimes it is something simple. Also, the service traction off light constantly comes on with the CEL. Very frustrating!
 

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I would think that you might have an ECM that is going bad or at minimum needs a reflash. You can turn off the traction control by pulling the ebrake up one notch from what I've heard.
 

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Unless you have a tech 2 you would have to go to your dealership to have this done. A primative version would be to unplug your battery for 15 minutes, but it's not the exact same. Your local dealership should be able to reflash the PCM easily, but I'm not sure as to the cost associated.

Here is some info I found useful:

For example, on certain GM vehicles the Check Engine light comes on and sets a code P1406 that indicates a fault in the position of the exhaust gas recirculation (EGR) valve. Cleaning or replacing the EGR valve and clearing the code does not fix the vehicle because the code usually returns. The real problem is the OBD II programming in the PCM. When the PCM commands the EGR valve to open to check its operation, it isn't allowing enough time for the valve to respond. A brand new valve takes only about 50 milliseconds to open but an older valve may take up to 350 milliseconds or longer - which is not long enough to cause a real NOx emissions failure but is long enough to trip a fault code. The fix in this instance is to reflash the PCM with new instructions that allow more time for the EGR valve to respond.

Another example are rich codes that may appear on some late-model GM vehicles. The problem here is that the original OBD II self-diagnostic programming does not allow enough leeway for changes in intake vacuum that occur as the engine ages. After 60,000 miles, intake vacuum isn't as high as in a new engine, which can create a rich fuel condition. The cure is to flash reprogram the PCM to compensate for the drop in vacuum.
 

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Discussion Starter #7
Thanks for the info! That's what I thought you meant by it and I did unhook the battery for about 30 min to hopefully reset the computer. That didn't work either, but like you said, it's a primative way and not exactly like the dealer would do. The car has 69,000 miles on it.
 

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Without knowing the car and being able to check it out I wouldn't be much assistance. We have a memeber that knows the electrical systems better than most. Hopefully he will have some insight for you, but that is about the only thing I can think of if you have already fixed the assumed problem and cleared the codes.
 

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Does your antifreeze go down? could possibly be the head gasket leaking on cyl 1. Yes they will leak into the cyl and still show good compression
 

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I think you have a lot of borderline problems causing this DTC to set. Idle speed too low, weak or older battery, dirty throttle body,throttle up signal from "BCM" and possibly your ECM control voltage for cylinder 4. Check battery voltage with engine off, a good battery will read between 12.3 and 12.8, then measure voltage at idle speed after warm-up it should read 14.3 approx.
If changing the plugs and coils did nothing then it must be voltage related and the weakess link is first to set a code ( in this case cylinder 4 )

DTC P0304
DTC P0304: Cylinder 4 Misfire Detected


Circuit/System Description
The engine control module (ECM) uses information from the crankshaft position (CKP) sensor in order to determine when an engine misfire is occurring and uses information from the camshaft position (CMP) sensor in order to determine which cylinder is misfiring. By monitoring variations in the crankshaft rotation speed for each cylinder, the ECM is able to detect individual misfire events. If the ECM detects a misfire rate sufficient to cause emission levels to exceed mandated standards, DTC P0300 sets. Under certain driving conditions, a misfire rate can be high enough to cause the 3-way catalytic converter (TWC) to overheat, possibly damaging the converter. The malfunction indicator lamp (MIL) will flash ON and OFF when converter overheating, damaging conditions are present and DTC P0300 is set. DTCs P0304 correspond to cylinders 4. If the ECM is able to determine that a specific cylinder is misfiring, the DTC for that cylinder will set.

Conditions for Running the DTC
• DTCs P0121, P0122, P0123, P0221, P0222, P0223, P0335, P0336, or P0338 are not set.

• The engine speed is between 420-7,000 RPM.

• The engine coolant temperature (ECT) is greater than -30°C (-22°F).

• The fuel level is more than 11 percent.

• The ECM is not in fuel shut-off or decel fuel cut-off mode.

• The ECT is less than 47°C (117°F).

• DTCs, P0304 run continuously when the above conditions are met.
 

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Anything ???
 

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Discussion Starter #12
Thanks for the info, to all of you. I haven't had a chance to do anything to it yet but will hopefully work on it this weekend. Coby7, you might be on to something. My wife told me that a few times she didn't think the car was going to start, turn over so it may be voltage (battery related) Will def look into that. Thanks again and will post my findings!
 

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If battery reaches under 11.8 Volts while running it gives all kinds of ghost codes and problems that aren't really there.
 

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Discussion Starter #14
You say it will give ghost codes but will it also make it not run correctly? It runs great until you are idling or in park and then it idles rough.
 

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Those are the times the alternator is putting out its lowest voltage so that would be the perfect time for a weak battery to show itself.
 

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You say it will give ghost codes but will it also make it not run correctly? It runs great until you are idling or in park and then it idles rough.
Almost like a snowball effect. "Let's say your throttle body is a bit dirty and makes your car idle rough, then your RPM fall below the alternator thresh hole then voltage drops, then MAF sensor loses it's calibration making the engine run rougher and the ECM trying to compensate by opening throttle which in turn revs up RPM which kicks in the alternator which changes the MAF calibration again because voltage came back up so ECM cuts fuel and engine RPM drops again and the vicious circle starts over." A good battery will keep all the sensors at calibrated voltage and save you a lot of headaches.

Try borrowing a known good battery and do the exchange and see what happens but make sure your throttle body is nice and clean first so your not chasing your tail.
 

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Discussion Starter #18
Well, it's not the battery or alternator. Voltage when off is 12.69. Voltage with no load, 14.920 and loaded 14.580.

Any other suggestions? Someone mentioned I may have a burnt valve in #4. Thought about this, but want that to be a last resort if so because of the expense.
 

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You said you did a compression test...Burnt valve would have showed up. But you never answered whether or not you cleaned the throttle body or if you even checked it.
 

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Discussion Starter #20
Haven't done that yet, sorry forgot about that. That will be next then. Yeah, did a compression test. That was fine. But this mechanic that I spoke to said it still could be a valve with a good test. Didn't sound right to me though.
 
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