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I drove off from fueling my car minus the gas cap. I kept forgetting to stop and replace it for over a month. My car started running rough (like the gas or spark was being turned off and on) after a few minutes drive. It would then clear up in 2 to 4 minutes and run fine until the next time I started up from a cold start. I did replace (finally) the gas cap but the problem did not go away. Now the Service Engine light is constantly ON. I know the fuel system is pressurized and wonder if I got some trash in the lines. Now the car runs bad all the time, and I get a error code of P0302 on my Error Code Scanner (INNOVA 0303e) which indicates a cylinder 2 problem. Could be electrical (coil, sparkplug, or sparkplug wire), fuel, cat, related. Any ideas out there regarding running without a gas cap?

GasCapDumb
 

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Now the car runs bad all the time, and I get a error code of P0302 on my Error Code Scanner (INNOVA 0303e) which indicates a cylinder 2 problem. Could be electrical (coil, sparkplug, or sparkplug wire), fuel, cat, related. Any ideas out there regarding running without a gas cap?

GasCapDumb
Yea find out what is causing the P0302 error.

I don't think it is related in anyway to a gas cap that was missing or replaced.
 

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First - you need to provide the vehicle details - it's hard to help without those.

Second - a P0302 is a misfire on the #2 cylinder - it's a "localized" problem (ie not affecting all four cylinders) so the cause is unlikely to be something that can affect all four, such as the gas cap, a fuel line, or the ignition coil.

Yes - it could be a spark plug or a plug lead - those are "consumable" items that need to be replaced at intervals, when last were yours changed?
 

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Has it rained or snowed since you left the cap off? If so then you might have water in the gas. The filler door is far from a perfect seal. I'd dump in a drygas or two to absorb the water.

This may not fix the code but addressing possible water in the gas should be done regardless.
 

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Your gas cap basically deals with the EVAP system, which is pressurized, so it's always a good idea to have the proper gas cap and have it on tight, as well. As Fordem says, p0302 is misfire in cylinder #2, so that it is something specific only to cylinder #2 - a fuel problem, for example, would affect all cylinders. You may want to try to switch out the spark plug wire or plug to a different cylinder to see if the fault follows the wire, etc., or stays at #2.
 

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Since your scanner says cylinder #2 I assume your car is a late model with individual power/coil packs. I would check the pack for #2, you can always swap it with the coil pack for a good cylinder and see if your scanner reading changes. These are just guesses since no car is identified.
 

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The only conclusion that can be drawn from the fact that a P0302 code calls out a cylinder #2 misfire is that it's an OBD2 vehicle - you can get that code on an engine with a distributor and a single coil, and an ignition issue is not the only possible cause.
 

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The only conclusion that can be drawn from the fact that a P0302 code calls out a cylinder #2 misfire is that it's an OBD2 vehicle - you can get that code on an engine with a distributor and a single coil, and an ignition issue is not the only possible cause.
Since the poster did not give information on the type of vehicle it is easiest to start with the basics. Yes any OBD2 can give a misfire indication caused by spark plugs, wires etc. The posters needs to clarify.
 
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