Tiger07

My car, 2002 A6 3.0 with 69K miles has gone through a cylinder misfire before the thanksgiving holiday. The issue was a failed ignition coil on cylinder 5. Since than the computer shows the average mpg to be between 15 and 17, even with highway driving. Before the incident average mpg ranged from 18 to 21. Did some manual calculations and came up with 16.74mpg as an average. Have new air filter, spark plugs changed @ 40K, same power and acceleration as always. What can it be?

OOOOya

mass air flow sensor?

Tiger07

No codes are showing up...

OOOOya

I went through a month or so of similarly bad MPG in our A6 this past summer. In addition to asking the wife to drive as if there were an egg under her right foot, I checked all the normal things. None seemed to be malfunctioning. I remember running the basic settings/throttle adaptation before a road trip & filled the tank with premium + fuel treatment. I reset the trip computer MPG average (which in our car is actually accurate) and have spot checked. MPG has returned to 20.x in town.

Tiger07

Actually that is what I did 2 weeks ago, added fuel treatment and mpg seemed to get back to normal. However with the next fill up, the mpg dropped again. I always get premium gas. Just today, the average mpg shows 15.7 and after a 6 mile (50/50 city/highway) the instantaneous mpg showed at idle showed 13.7 and average stayed at 15.7. No "slow" gear or heave acceleration during that trip. I'm just worried that there was some damage to the cylinder or maybe a leak when it misfired.

OOOOya

Straight from the Home Depot in the paint dept. 4oz to a 20 gal tank. Toluene also works. Most off the shelf fuel treatment is mostly filler/fluff. The only treatment in a bottle that I've had success with is Gumout Regane.

Tiger07

This time it's cylinder 1. Maybe the ignition coil was slowly going bad on this one too and it was causing the low mpg? Any comments?