This is an epistle.
A bit long, but describes how my ignition coil behaved before replacement. (2004 SV6 A5 sedan. 106,000km. Plugs done 15,000km)
With hindsight, this ignition coil has been faulty for some Kms. I was showing visitors around SEQ for about a week covering up to 200 - 400kms per day with 5 adults in the car. On one or two occasions I noticed a slight "wobble" in the car when under acceleration or cruising under light throttle. It lasted about 2-3 seconds and then disappeared. The day when the coil actually failed it started as a slight miss under acceleration then cleared as 100/110kmph was achieved. Slowly over about 4 hours driving the coil deteriorated. It would cause a wobble, like a driveshaft was playing up, then clear after about 4-5 seconds. Started to think the tail shaft coupling needed a looking at.
Stopped for fuel, about 6.00pm at night, started and the miss was there. It came and went spasmodically, then after about 25km at 80Kmph it was on full time. At one stage I wondered if I had picked up a bad load of petrol. If I got the rpm over about 2000, the miss was less apparent. The car didn't have full power; I think the output voltage was not full value, and probably the fuel air mixture was not as dense at the higher rpm so did not require full volts as at lower rpm.
Did about 80km on the motorway in third gear to keep the revs up and the miss down. Fuel economy fell away very quickly. In the suburbs to home the miss was very noticeable.
I ran my multimeter over the ignition coils' connectors, they were all the same. Had a mechanic's scanner hooked on the car's OBD socket initially and it suggested the Bank2, front O2 sensor had low voltage, and no other problems - coils/fuel injector etc. were indicated. I removed each ignition coil's power connector in turn and the engine miss did not change much for the better or worse. I took the easy way out and replaced the O2 sensor. The O2 sensor DTC cleared.
Things improved at idle and under load, but occasionally the miss was present over the next few days. The next scan did not show any DTCs for O2 sensors, coils or injectors. So I was looking for another fault?
It started perfectly when cold, then after about 30 seconds it would miss a little, a lot, or not at all. I checked out the injectors, finally decided they were OK. It looked like #2 cylinder was the culprit. Did a process of elimination by swapping coil positions, and found it to be the original #2 coil. I could not detect any burnt rubber smell on the coil from arcing or burn through.
With the new coil fitted the car is much more spirited, pulls strongly and is fun to drive again. Economy is a litre or so/100km better around the suburbs. I think the problem had been there for a month or two and slowly been getting worse.
Spoke with mechanic about any future problems caused by the dud coil. His opinion was, that since I had kept the revs higher than normal I had minimised damaging the catalytic converter by not flooding it with excess unburnt petrol. I guess the cat will play up if I have damaged it in the longer term.
Maybe this can help fellow members.
