A few thoughts on sudden reduced MPG... (archive)

Tire pressure OK?
Same brand/octane gas as before?
Seasonally oxygenated gas is in season all of a sudden?
New tires (different brand?)
Wind drag from newly installed dubious ground effects/spoilers?
Gigantic American flag on antenna causes wind drag?
Heavy object in trunk?
Obstructed exhaust system?
Compression test?

I assume that your OBC is generally correct in its MPG reporting. That means there's no fuel leakage- The OBC monitors fuel injector duty cycle exclusively to determine fuel usage, so the problem is not a stuck cold start valve or fuel tank leak. Something is causing the car to require more gas. Either the gas releases less energy/gallon, the engine isn't as efficient, or there is some friction in the drivetrain.

I'm stumped and could use some help. My daughter had my 84 528e for a month and when she returned it, I only got 29.0 mpg instead of 32.2 mpg on a trip I frequently take. I call that substantial. I have verified these readings from OBC readouts to actual--the computer is right on. I checked the following per Bentley:

Temp switch OK
Temp sensor OK
Throttle plate adjustment OK
Throttle switch adjustment OK
O2 sensor OK within the limited testing Bently describes
Fuel pressure OK (about 36 psi)

Also it idles high after warm up. Sometimes it will kick down but only after a few seconds.

Any ideas on where in the hell to go from here!?

Follow Ups:

Ya But... Jamie 23:56 12/02/2001 (0)

[ Follow Ups ] [ 5-series (E28) Message Board ] [ Msg. Board FAQ ]

Questions, comments, or problems, please visit the Roadfly help desk.