Huge drop in MPG after BMW service!

[torq]

I recently brought my Z4 into the dealer for service. They did an oil change and replaced the wiring harness to the sensor that allows you to check the oil level (which was not working). Right after the service visit, I noticed that the mileage has dropped significantly. I'd been averaging 21-23 MPG before service and now I'm only getting 14-16 MPG. That's a hell of a drop! I've had the car for a year and I have never seen such low readings. The previous oil change was done last June at a BMW dealer and was the same type and weight of oil (5W30). Car has upper 20K miles on it. Any ideas what could cause this?

Before I call and revisit the dealer (long drive), I'd appreciate any input.

[paris1]

If the car is running alright, my guess is they messed up something electronic in the process of changing the harness that is affecting your mpg readout. Is your gas gauge dropping precipitously? If not, your fuel mileage is probably unchanged, no matter what the readout says.

[robertd33]

i have found the computer to be HIGHLY inaccurate for MPG.
or are you doing the math?

[torq]

Originally Posted by paris1

If the car is running alright, my guess is they messed up something electronic in the process of changing the harness that is affecting your mpg readout. Is your gas gauge dropping precipitously? If not, your fuel mileage is probably unchanged, no matter what the readout says.

Thanks for the quick reply, Paris.

No, the gas gauge does not appear to be dropping faster (I'd wondered about that).

One other thing they did was to clear faults and reset CBS data. Not sure what that entails or if it has any effect.

[SpikeMilligan]

If they were doing electronic work, they probably disconnected the battery, which could have done a throttle body reset. Some of the settings like the accelerator response adapt to your driving style over time and a reset will put them back to the original factory settings which in my case usually results in a snappier response (and consequently more fuel burned). If that's the case, it should adapt back to your driving style within a few weeks.

[Frank Costanza]

That's a big drop. I avg 20 mpg and my OBC is pretty accurate. Something's not right.

[rking117]

if you are going by the gauge on the dash then it could be they reset it while at the dealership. this means its memory is empty and is calculating new values from scratch. your driving habits could have changed or you may have driven the car on a long trip before it was reset giving you the higher MPG rating. nevertheless there are two actual test you can do that will help you determine the real facts. one fill er up at the pump. reset you ODO drive er empty filler er up and divide miles by gallons used. that is your true method of determining MPG. the other fast method is using the gauge on the dash albeit less accurate it is a simple gauge. turn er to instant MPG and drive off to a normal speed on a highway (65 - 70) and on a flat road you will likely be around 30mpg on cruise control. if it is much less than that i would definitely take it so the shop for testing.