Just Gizmo

Driving home tonight my car sputtered / throttled back, then I got a check engine light all in a matter of a SECOND, then, the car was fine. Didn't happen again check engnie light is gone.

Any ideas? Sounds like a misfire, but I have no experience with this.

rigor

How does OBD-2 determine which cylinder misfires?

JWG

The OBD-II system monitors almost every aspect of the engine. Our engines misfire regularly, but you never feel them, because they very rarely occur in succession. When you have a successive misfire, you will feel a rough idle. At speed, it will feel like a massive power loss. If you have a check engine light for misfire, it means the OBD-II counted a certain number of misfires (forty if I recall correctly) within 4000 revolutions. A check engine light for misfire does not go away by itself, you must clear it with a VAG computer.

In my case, I have dirty fuel injectors which cause misfires at warm idle only, due to failure to mist properly at low (i.e., idle) fuel levels. I can see and feel the misfires when they occur, because they cause a rough idle which I can monitor by watching the tachometer. Once I pour some additive in to the fuel tank, the injectors run well for a few thousand miles, then clog up again.

To answer the other person's question regarding how the OBD-II senses which cylinder, I believe it relies on the O2 sensors before and after the catalytic converters. If a particular cylinder's exhaust pulse comes through with a lot of O2, it means that the O2 did not get burned in the chamber. It may corroborate this with crankcase and cam sensors. Quite frankly, this is my best guess.

Just Gizmo

guess I'll have the dealer check for fault codes next time I'm in.