Original

Guys I need your advice.
Have a 08, 528i with 108k and getting an average of 20 mpg and I use the car for every day commute driving it 70% freeway.
6 months ago took the car to the dealer complaining of rough idle and they fix it by raising the idle, I do not know what is the normal idle I used a scanner and my car the lowest is 758. Do you know what the normal lowest rpm is?
I think I should be getting a better gas millage. I also think that I should replace the fuel filter and before doing that wanted to get your opinion.
When do you replace the fuel filter do you change the fuel filter pump too or just the filter? Please advice.
Thank you.

pcy

Fuel filter and the pump can be replaced separately. Under the rear seat, pump is on the passenger's side and the filter is on the drivers side.

Mileage depends lot on various factors - type of fuel used, type of driving (fast acceleration vs. moderate acceleration), tire inflation, etc. While 20 MPG is on the low side given that 70% of driving is on highway. What is the average speed?

At 108K miles, you may want to look at replacing spark plugs and possibly ignition coils.

Original

My average speed is 65 to 75mph and the fuel I use is from the 76 I also replaced 2 of the original tires from 2 used ones.
If the fuel filter and the pump are separated I do not need to replace both? Correct?
I want to replace the fuel filter since the car has many milles and hope I can get better on gas too.
Please advice.

pcy

Unless you've been using gas from no-name gas stations, I don't think fuel filter is really blocking fuel flow. Hence, I wouldn't expect better mileage from fuel filter replacement. On the other hand, if the car is not starting properly or running properly (due to fuel flow issues), then I would definitely look at replacing fuel pump and/or fuel filter.

I'd highly recommend you replace the spark plugs. Check and clean DISA valve. Clean Mass Air Flow Sensor (aka MAF).

Original

I forgot to mentioned that I did replaced the spark plugs 7 months ago but not the coils and I used to fill the car on the no-names gas statios in the past, but from the time the dealer raise the idle they also did the fuel injector cleaning I only been using gas from station the dealer recomended me. Appreciate your help.
Thank you