99 Q45 with 180000 mostly highway miles in South Central Texas. Ran well until about 4 months ago and I started to notice an occasional engine miss at idle. No service lights; no impact on fuel economy. Had new plugs, plug wires installed by my long time mechanic but slight miss still occasionally there. Over the course of the next two months, engine miss became slightly worse, but still mostly at idle, however under hard acceleration, it was VERY noticeable. Still no impact on fuel economy. Ran OBDII codes on home scanner and got PO330 and PO400. Mechanic fixed the Exhaust Gas sensor code but said the knock sensor was an expensive item, might want to delay on that.
Miss got more persistent, fuel economy started to suffer (still no check engine light) so I took it in. They had it for two days and couldn't find the source of the miss (I asked them to check the plugs and wires and they said they did). They also said the knocki sensor wouldn't cause this miss.
After yet another day, they now say it looks like a coil might be bad. Now there is a check engine light coming on. My OBDII scanner says PO302 and PO330 now.
Any ideas? The car REALLY runs rough so I don't want to be driving it very far.