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Project ThunderBunny: Fixing An Unstable Idle

By  May 28, 2015
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Over the past few weeks, our daily driver 2007 Volkswagen Rabbit, known as the ThunderBunny, started developing a rough idle. Driving around town, it felt completely normal, but once you stopped at a red light, you could feel the engine misfiring, giving you a frustrating little kick every few seconds. This weekend, while trying to work on the Miata, the idle issue worsened and so I decided to first take a look under the hood. Luckily, before I’d even gotten under the hood, on the way to pick up some silicone spray and a coolant hose pick for the upcoming Miata coolant hose change, ThunderBunny finally threw a check engine light.

Now for some, that might be a normal sight in a Volkswagen or something to be frightened over, but in this case it was a relief. I had not been looking forward to swapping the one good original coil we had with each one until the idle stabilized, so I was thankful when the car finally threw a check engine light. It was confirmed, a bad ignition coil, as was suspected.

Close up of ignition coils

With the bad coil changed on cylinder number 1, I decided to label them all with a cylinder number and installation date in month/year format… This is a German car after all, so it’s only fitting that the parts are meticulously logged. With it all buttoned up and back together, I started it up… only to be greeted by a blown parking lamp! At this point, my patience was wearing thin. I pride myself on being a Volkswagen owner who does not let a check engine light or any other light stay lit for very long, so I took another trip to the parts store. The lamp change wasn’t too bad, it was just the fact that it had to blow right then that bothered me, when I needed it to just be fixed and get out of the way, to start work on the Miata’s hoses. And then, with the bulb changed, I started it up, only to find a very slight, much less prominent, but still present occasional misfire. Argh!!!

This wasn’t the first time this coil issue had happened. We had just changed all five coils at the end of January 2014 after one had died while we were away on vacation in St Augustine, Florida, four hours away from home. Unfortunately, at the time I was afraid to trust the remaining four to get us home and just bought what was locally available off the shelf from a parts place, which meant non-OEM parts. While with many cars, that might be okay, Volkswagens can be a little more particular about certain parts, and this seems like it might be the case with the coils as I have not had much luck in finding information on aftermarket coil longevity. Most everyone seems to only replace them with factory parts or “better” (debatably better Audi coils) unless under an emergency situation far from home.

So, it looks like I’ll soon be either living with an annoyingly rough idle, doing the hokey pokey with the engine cover again and again to find the bad coil, or just changing all of them with new OEM ignition coils. Perhaps it might even be all of the above. Such is the case with learning a car’s quirks. I could trade it in, but to fix everything wrong with it would be much less expensive than going into car payments again. It still starts every day and doesn’t overheat or anything too serious like that. Besides, I’d feel like a huge jerk trading my wife’s car in when I still haven’t installed all of the upgrades we’ve acquired. So, I hope to get the AC running right soon, the coils replaced, and the grille and antenna changed soon and then we may start looking at fun upgrades like an intake and exhaust. Stay tuned!

Text and Photography by Bryce Womeldurf