Ya But... (archive)

How would I check for an obstructed exhaust system?
I suppose I could do a compression test but I don't have a previous one to check it AGAINST. the engine runs strong and smooth.
No change on the rest of your suggestions.

I like your reasoning. It had not occurred to me and I'm going to check it out as best I can. But why couldn't something be telling motronic to extend the duty cycle of the injectors without matching it with air, thus causing a rich mixture? Bently suggests that O2 sensors loose their edge with age and that this subtlety is a bench check item for a specialty shop. I'm wondering if mine could have crossed some such threshold and is claiming there is more O2 in the exhaust than there really is.
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!?

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