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Car: 2005 Dodge Grand Caravan

In the past few days I started to feel the engine stumble. Yesterday the engine light came on and the error was a that cylinder 4 was a misfire (there was also a "running rich" error, but I'm guessing that was because a cylinder wasn't burning the fuel from the misfire).

So I then proceeded to change the plug and the plug wire, but I'm still having the misfire.

I'm a bit at a loss here. Could this be fuel problems? If so, would that technically constitute a "misfire", as the plug would technically still be firing? I would guess a misfire should be specific to a non-firing plug. But I'm assuming here. And also if it was a fuel delivery problem, I'd be surprised that I'm getting a rich mixture error as well.

Any additional thoughts on this? Thank you in advance.

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2 Answers 2

up vote 3 down vote accepted

There are two things I can think of for you:

First - Are you sure you changed the plug/wire for the #4 cylinder? I know this sounds elementary, but you never know. When you pulled the plug out, did it look wet, or tan as it should? If it was wet, you've changed the right plug. If not, you probably want the middle cylinder (assuming a v6 engine) on the other side of the engine.

Second - If you are completely sure you changed the correct plug, you probably are having an issue with the coil pack. It appears, from the pictures I just looked at, that it is a single unit, with three coils firing two cylinders each (again, assuming a v6 engine). If one goes bad, you'll probably have to change the entire assembly.

At this point I wouldn't think it's a fuel delivery issue, seeing as how you are getting a rich (too much fuel) condition. You are right in that if you have a misfiring cylinder, you'll get rich readings.

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It turned out to be the ignition coil, as you said. I was a little thrown off, as it is an individual coil for all 6 plugs. I was surprised that is was just malfunctioning for one plug. Thanks! –  Thomas Stringer Feb 25 at 21:04

I'm not so sure its a fuel issue. If the ecm detects a rich condition (too much fuel) then id think fuel supply is okay. If the plug isn't firing at all (dead cylinder) then all of the unburnt air and fuel from the cylinder will be sent through the exhaust to the 02 sensors. 02 sensors can only detect the oxygen levels in the exhaust gases, they cant detect fuel. So they will begin telling the ecm that too much oxygen is present in the exhaust, but don't account for higher fuel levels. The ECM will try to correct the air/fuel ratio by adjusting fuel trims (among other things) to add more gas to the mixture even if the fuel supply is fine. This will cause other cylinders to run rich and begin misfiring as well. Eventually the fuel trims exceed factory preset limits and can set a code for a rich running condition. I'd start by verifying fuel pressure is okay and replaced all the plugs at the same time. Then clear the OBD2 codes and reset fuel trims. The car may run rough for a day or two after trims are reset, but should even out as you drive and the ecm relearns new fuel adjustments.

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It would be a good idea to do a compression test while you already have the plugs removed to make sure it's not an internal issue –  user4546 Feb 23 at 20:30
    
The most common issues are from plugs, wires, and ignition coils, but compression is definitely a possibility you should check for. –  Nathan L Feb 24 at 20:27

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