Fixing two misfiring cylinders in a 2006 v6 Mustang?

November 17, 2010 4:46 PM

MustangFilter: 2006 v6 Ford Mustang giving cylinder misfire codes. Crank sensor and pulley recently replaced, the shop tells me to replace the coil pack. Looking for other root causes before I take the plunge?

About two weeks ago, my Mustang started to stall out in idle, so I took it to my shop. They replaced the crank sensor and pulley, and all was well. For about an hour. The check engine light came back on, I experienced a pretty severe drop in power, and the check engine light began to flash (indicating a misfire). Having the error codes read showed 0302 (misfire in cyl. 2), 0306 (misfire in cyl. 6), and 0300 (random misfire).

The garage initially told me to replace the coil pack and all would be well. Since I don't have a ton of money to my name, I'd rather be sure that the coil pack is the cause before I turn to them again. (I prefer to do small maintenance myself, so that's part of the hesitation.) I've done the research on these codes, but seem to get conflicting opinions about what could cause the misfire, and specifically why ONLY those two cylinders are experiencing it. Any thoughts, mechanics of the Green?

posted by ASoze to travel & transportation (5 answers total)

Are those two cylinders on one coil pack? That's a good indicator.
posted by notsnot at 4:53 PM on November 17, 2010

Coil pack sounds likely. Plugs or wires could be it, although it's only a 2006, you never know if there was a bad batch.

Anecdata: my plugs started going bad at 85k, on a 10-year old car, not 4-5 year old.
posted by Mister Fabulous at 5:40 PM on November 17, 2010

It would seem to me that the random misfire error means that the problem isn't necessarily limited to #2 and #6. I would also doubt that those two cylinders are on the same coil pack; that arrangement is usually 1-2, 3-4, 5-6. (Or some other combination of odd and even)

Going back to basics: three things are needed to make it go: fuel, air and spark. Air is pretty easy to diagnose, make sure the filter isn't plugged up. Fuel is a possibility- specific cylinders would lead to those specific injectors, random could be bad fuel or a clogged filter.

Spark-based misfiring issues, in my experience, tend to manifest differently from what you relate. The car will be hard-ish to start, idle will be OK, and the car will even drive ok to begin with. But whenever the car is under load, like going up a hill or if you floor it, the engine will sputter and/or cease running. As soon as you lift your foot from the pedal, it will leap back to life.

I would personally hold off on the coil pack, and try switching the plug and wire from #4 to #2 or #6, depending on how long the wires are. If the misfire changes, you know your culprit. Or just change the plugs and wires- that should be a pretty easy job in that car. Make sure you get double platinum plugs because cars with coil packs fire the spark in both directions and there needs to be a platinum button on both electrodes of the plug. (Called the waste spark system.) Use Motorcraft or Autolite plugs. Don't use Bosch unless that's what came on the car originally. I know it seems like voodoo, but they just don't work right otherwise.

Which makes me wonder- were the plugs replaced semi-recently? (Like 5,000-20,000 miles) If they were, and double platinum (or iridium) plugs weren't used, they might well be shot. Well, half of them would be, whichever side of the engine gets the waste spark. It would be #1,3,5 or #2,4,6.

Unless it has over 100,000 miles, I would doubt the factory plugs are bad enough to cause this issue. I changed the ones in my Ford at 85,000-ish and they came out looking like they could do another 85.
posted by gjc at 6:39 PM on November 17, 2010

I've seen many Ford coil packs go bad. That's usually what I check first when I get a misfiring V6 Ford. If I remember correctly, cylinders 3 and 6 are companion cylinders so to find them both misfiring is a good indicator of a failing coil. What I mean is that in that coil assembly, there are three individual coils each of which outputs spark to two cylinders respectively and when one goes bad, it's common to develop misfires in two cylinders.
posted by Jon-o at 2:34 AM on November 18, 2010

Just an update, have swapped out the coil pack, the plugs, and wires, with no change. Swapped the wires as gjc suggested, still misfired in 2 and 6.

Took it to the dealer after my normal garage gave up on the problem, the dealer wants to keep it until they figure it out. Compression tests showing low compression in 2 and 6, but not terrible low, just low-er. Misfire only happens above 3k RPMs, too. Confounding problem to have, to be sure.
posted by ASoze at 2:47 PM on December 16, 2010