I bought my current car a little over five years ago, and for those five years, I dealt with a random multiple misfire that made it hard for the car to pass emissions. The stupid thing is that I knew the car had a misfire code when I bought it—the Service Engine Soon light flashed when I was test-driving it, and I pulled in to a Pep Boys to get the code checked. But I figured it probably just needed the spark plugs or spark plug wires replaced—no big deal—so I bought it anyway.
Little did I know that this is one of the most difficult diagnostic codes to troubleshoot. Strangely, the misfire had no noticeable symptoms—no jerking, hesitation, or obvious loss of power. But I went ahead and changed the plugs and wires, to no effect. Then I took it to a shop for a diagnosis. They confidently declared that I needed a new ignition control module and a new coil pack for cylinders 1 and 4. I ordered new parts from my dad, who manages the parts department at a GM dealer, and swapped the old ones out. Still nothing.
I threw a few more parts at it over the next few years, even though I hate throwing parts at a car as a diagnostic technique. I replaced the fuel filter. No luck. I replaced the crankshaft position sensor. Nope. I cleaned the EGR valve. I cleaned the fuel injectors and replaced the O-rings. Nada. I checked the compression and the fuel pressure. Everything looked good.
At some point I started a thread on the Saturn Fans forum to get advice. The people there mostly said what the shop had said: it was probably the ignition control module or coil pack or both. When I said I’d already replaced them, they shrugged it off—I’d probably just gotten bad parts. (The new parts were bad in the exact same way as the old ones? Curious.)
I bought a USB OBD2 code reader so I could not just read the codes but see the real-time misfire data from my car. Even though the code said “random multiple misfire”, the data showed that it was only misfiring on cylinders 1 and 4, which would certainly seem to indicate that it was a bad coil pack. Except that I tested the resistance on the coil pack, and it was within spec, and when I swapped the two coil packs, it kept misfiring on 1 and 4. It couldn’t be the coil pack. I also swapped the fuel injectors around, and the misfire stayed on 1 and 4. It wasn’t an ignition or a fuel problem.
Out of desperation, I replaced the intake manifold gasket. It didn’t cost me a lot of money, but it was a beast of a job, and I broke off a small coolant tube that came out of the intake manifold as I was putting everything back together. I had to buy a replacement part (thankfully not the whole manifold), so I essentially had to do the job twice. Still no dice.
I took it in again, this time to a dealership, because I thought they’d have someone competent enough to fix it. I explained everything I’d done and stressed that I wanted someone to do a real diagnosis, not just throw more parts at it. They said okay and started working on it.
They called back and said that the scanner said it’s misfiring on 1 and 4, so it needs a new coil pack. I was adamant. I already replaced the coil pack. I tested the coil pack. I swapped the coil packs around, and nothing changed. It doesn’t need a new coil pack. But they were equally insistent. They assured me that if it didn’t fix the problem, they wouldn’t charge me for the parts or labor. But if you assume that new parts are bad even when they test within spec, then you have no way of ruling anything out. If it still misfires, how do you know that the new-new coil pack isn’t also bad? I told them no thanks, I didn’t want to waste the time or money, even if it was just their time and money. They apparently didn’t know how to do anything else besides throw parts at it, so I took my car back. Thankfully, they didn’t charge me.
While I was dealing with all of this, my strategy for getting my car to pass emission was to clear the code and then go drive around until the car was ready for testing without inadvertently setting a new code. Since driving on the freeway set a code almost immediately, I’d drive around on the country roads west of town, since there was little traffic and the speed limit was mostly 45 to 55. It usually took two hours or more of driving before the car decided that it was ready for testing. It was a pretty big pain in the butt.
So this year I finally tracked down a diagnostic flowchart that was taken from the factory service manual. I went through it and found that I’d already ruled out all the major culprits—fuel pressure, compression, and the ignition system all checked out fine. The next thing to do was to do the crankshaft variance relearn procedure. This meant taking it to a dealer so they could plug it in to a computer and reteach the crankshaft position sensor what was and wasn’t a misfire.
I took it in, paid just over a hundred bucks for them to do the procedure, and drove it home. I drove it around until it was ready for emissions testing. It passed without a hitch. I’ve been driving it around for a few weeks now, and the Service Engine Soon light hasn’t come on once. I keep waiting for the problem to reappear, but it seems that after five years, a few hundred dollars in unnecessary parts, and countless frustrating and fruitless hours spent working on the dang thing, I’ve finally fixed it.
About freaking time.