True...there are a lot of errors which will not throw lights. i find some when i diagnose. I would definitley check the MAFS and basic cheap maintenance first. Sparks plugs, air filter, for vaccum leaks etc. I change spark plugs about every 30-40K....you can see some DIY when I changed them at 20K when doing my DISA....not a pretty site.

 

When riding the brake to a stop light, the MPG should not drop to 13 MGP with the RPM bouncing between 500-800 RPMs. The car idles fine at a complete stop. Even if the automatic transmission is downshifting as the car approaches a stop, with the foot on the brake, this should not cause the MPG to decrease. Although it could be transmission related. But putting the car in neutral and gliding to a stop should not cause dancing needles.

I called the dealer to setup an appointment and the advisor blamed it on bad gasoline. I only use 91 octane from a reputable place (Conoco). I don't believe it is bad fuel. None of our other cars have ever had a problem with the fuel in our area.

I will check the MAF sensor and air filter. The car only has 32k miles. If I can't find anything, I'll take it to the dealer.

 

Well definitely not gas. They alwasys do that man. ALWAYS. RUN 87 and you're still not going to have this issue. As far as the bouncing RPM's happened to me with a VANOS seal issue and also from a vacuum leak. As far as the MPG I have to look because I think several of my cars go toward 0 and then back to 50. I will check and let you know.

 

I think I figured out the mystery behind the MPG needle jump while braking for a stop light. While braking and the speed drops to about 20 mph, the MPG needle suddenly sweeps over to 12 MPG. I believe it is due to the automatic transmission. I suspect the torque converter "locks up" at speeds greater than 20 mph to increase fuel economy. When the automatic transmission is not in "lock-up" mode, which I suspect is speeds of 0 - 20 MPH, the built in slippage that allows smooth starts, stops, and shifts, decreases fuel economy. So when I slow down for a stop light and the speed drops to 20 MPH, the torque converter decouples out of lock-up mode and I get less fuel consumption. This shows up as a bump on the mpg needle and sometimes the RPM needle bounces.

Since my first three BMWs were manual transmission, this torque converter and "lock-up" feature are new to me. And since most other cars do not have an instant analog MPG display, we don't notice when the transmission changes in and out of lock-up mode.