Originally Posted by skEwb (Post 1253116)

I'm experiencing some very poor gas mileage with my 2004 525i. Just for a quick overview the car has 86k miles and I've performed an oil change on it with full synthetic, has a K&N oil air filter that's brand new and has had the microfilters replaced as well. I reset the computer and took it for a very careful zen driving session to try and see what my MPG would be. As you can see it's a whoping 13.5 (air conditioner off)(tires checked and filled for proper pressure). It gets about 9-11MPG with careless fast driving.

I know for a fact this car has original spark plugs. Will it need a new O2 sensor? New spark plugs? What else do I need to look at to see what can improve the mileage? When I bought the car at 78k miles it was getting about 15-17MPG and now it's dropped severely, even with VERY careful driving. It has 20" rims on Nexen tires (TPMS goes off, but no worries all tires are fine, just bad sensor I need to replace after crushing it in an LA pothole).

It's worth mentioning that at idle if the AC runs it makes the engine quiet down, whereas if the AC is off the car rumbles more at idle. Any input would be appreciated, I'm getting tired of paying $200-250 a month in gas just to drive to work that is 15mins away.

Originally Posted by gotabyte (Post 1253233)

I heard that the oil from the K&N filter can get on the mass airflow sensor(MAF) and cause it to misread to the computer. There is cleaner for the MAF at auto parts stores. Do NOT attempt to clean it with other solvents or some other device. It is easily damaged.

Originally Posted by pjinca (Post 1253371)

Let's get back to the discusiion:
To the OP, try getting some new iridium spark plugs and replace the ones you have. I found NGK Iridiums for $6/ea at Kragen (O'Reilly).
Next, get a can of MAF sensor cleaner and use it. Luckily one of our members posted a DIY on how to clean it: MAF Cleaning :thumbsup:

Originally Posted by AP Sales (Post 1253409)

I bet it's the flat tire :P

Originally Posted by skEwb (Post 1253414)

Good to have a guide, I like guides. I have to get a hold of the dealer CD that has all the instructions on taking things apart. I used to have it but misplaced it.

Originally Posted by balbs (Post 1253453)

Here's a DIY for cleaning the MAF:

https://5series.net/forums/topic/980...aning-diy-n52/

DOH!!! Nevermind, you don't have an N52... :doh: