O2 sensor good gas mileage still bad

[pepper]

going through the paces trying to track down the source of an 18-20 mpg problem in my 168,000 mile 318is. Used to get 30-33 mpg but now its way down. Reading the O2 sensor could be a source I went and tested it via the Bentley method checking the resistance and it was right in spec. In the print it says that old sensors react more slowly to changes in O2 than a new sensor and even when the old sensor has the right resistance. The thermostat is an 88C, Air filter pretty new. Even though the O2 sensor tested ok should I replace it?

[Brew]

Does it look like its been done recently? If not, definitely replace it to rule it out. A bad coolant temp sensor could also be causing a rich fuel mixture.

[gearheadE30]

Wouldn't hurt to run injector cleaner in the fuel for a few tanks either, although I don't think that is your only problem. How do the plugs look?

[pepper]

brand new plugs. its strange because it runs beautifully, I would never know there was a problem without looking at mileage figures. It does better when I run injector cleaner but I think because of the higher BTU's of the additive. first I'm going to remove the injectors and see what they look like because it doesn't cost anything and then go to the O2 sensor. I had the injectors out about 50,000 miles ago, not seeming like long enough for major crud buildup but who knows.

[pepper]

update... found an O2 sensor from Advance Auto for $77 cheapest direct fit replacement I could find by far. Bosch #13108 Stripped threads off old sensor taking it out, had to re-tap the bung M18x1.5. New one went right in. Hope to report back increase in gas mileage.

[pepper]

no increase in gas mileage, still at about 20 MPG. Coolant temp sensor tests OK. Engine temperature fully warmed up is on the center engine temp gauge mark. Added some injector cleaner to this tank of gas. Next I may pull out injectors for a full cleaning.

[tjts1]

Your car is rated 17 city, 23 highway by the government.

http://www.fueleconomy.gov/feg/2008car1tablef.jsp?id=7641

Your mileage is normal. Most of the people claiming 30-35mpg are either full of it or don't know how to calculate their mileage. It takes a lot of effort to get these cars to 30mpg. Definitely not stock.

[pepper]

I'm looking at the previous 3 years of driving this vehicle which was always near or above 30 MPG which is why something seems to be underperforming. In my experience EPA standards are easy to beat.

[Gray S.]

I've been around the 24mpg range with alot of highway driving also. I cruise around 70mph most of the time so I feel like I should be a little higher. I've replaced and done the exact same things you have. One area we could check is the coil/plug wires. I was going to convert to COP and see if that fixed the issue but the seller backed out. It might be worthwhile checking into your ignition system, thats going to be my priority when I get back stateside.

[wolfgangstbd]

i made 350miles on a full tank once. what would my average mpg be?

[pepper]

new related information- odometer just stopped/siezed- perhaps as it wore out first slowed down before stopping, consistently relating approximately 2/3 of the correct miles to the miles tally making me think I'm getting 20 mpg but really getting 30 mpg. going to do a search and read about the tach now.

[stevofthahill]

Your car is rated 17 city, 23 highway by the government.

http://www.fueleconomy.gov/feg/2008car1tablef.jsp?id=7641

Your mileage is normal. Most of the people claiming 30-35mpg are either full of it or don't know how to calculate their mileage. It takes a lot of effort to get these cars to 30mpg. Definitely not stock.

I call BS on this. I average between 27-30mpg per tank. I generally get ~350 miles before my fuel light comes on. Supposedly 318's have 14.5 gallon tanks. I have never filled up more than 12.5 so I could probably go further but don't want to risk it.

P.S. I drive my car hard...

[twistednut]

I found a vac leak and my mileage went from 20-22 to 26-28.

[Gray S.]

Hm maybe I should look for some vac leaks then. How did you identify the source of the leak?

[romeomike]

Eyes for cracked or disconnected hoses, ears for hissing sounds. Fuel injector o-rings are a tougher case due to their placement under the intake manifold. Spray carb cleaner around suspected trouble spots. If the idle improves, you've found your culprit.