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Maintenance Service Interval questions, oil change info.

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Old 12-20-2009, 03:35 PM   #1
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Default MPG potential problem and Fuel Filter question


Ok, after reading a few posts it seems like an average MPG should be 30 via the onboard computer. I have a '98 318ti with 112k miles. It had a Inspection II done last February.

I commute 500 miles a week on the highway. When I am on the highway my avg. MPG moves up to 25 miles per gallon. However in the city I get at best 14.

I remember once after I filled up the tank the MPG went all the way up to 18 as I was driving down the street but then once I hit some ice on the road it went to 11.

I use Premium 92/93 octane fuel. I do not have studded snow tires and this is an automatic transmission. The ASC is always on.

I will replace the spark plugs with NGK's [I think part number is 2288], change the oil with Castrol 5W-30 and replace the oil filter as well with Purolator or maybe Mann. I also want to replace the fuel filter.

Now, which brand of fuel filter do you think is best? Can I use the one from Advanced Auto Parts for $20?

Furthermore, is my MPG avg. normal and if not, what should I do or what could be the potential problems?


Also, what about
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Old 12-20-2009, 03:40 PM   #2
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14 is pretty low, I would also think about O2 sensors. Something is wrong with the car to get that low of MPG. Ice shouldn't make a difference. The things you are looking at are the right things to look at, but I would think that the fuel filter should still be good. It should be replaced with an inspection II, as should your spark plugs, so those should be good as well.

Have you actually calculated your MPG manually? It is possible for the OBC to be wrong. Record your mileage and the amount of fuel you put into the car for a month or two, and determine what your MPG is. If you are resetting the OBC MPG counter all of the time, it isn't giving you an average, but rather an instant reading. I like the newer OBCs because they have two MPG counters. So I can reset one to get more of an instant reading, and I leave the other so I have a long term average.

I use the Mahle-Knecht fuel filters. You can get them online for 15$. I tend to avoid the generic parts from the major auto parts stores.

Last edited by mohaughn; 12-20-2009 at 03:46 PM.
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Old 12-20-2009, 03:52 PM   #3
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As Mohaughn said, it's possible for the OBC to be wrong. I'd make sure you're actually getting mileage that low before spending money on replacement parts.

The OBC has a formula for calculating mpg that can be adjusted as seen in the following link.

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Old 12-20-2009, 04:15 PM   #4
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Sweet, thank you for your answers. I will do some manual calculations. Burning through one or two tank fulls of gas will take about two weeks for me. From my memory I calculated an Avg. MPG of 22 [very rough estimate] and that was 50/50 driving of city/ highway.

I guess I will look up on how to replace O2 sensors just in case.

Once again, thank you for your detailed answers.

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Old 12-20-2009, 05:33 PM   #5
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One thing that might help save money is the Ti calls for Mid-Grade. I put premium in mine for the first 6 months I owned mine since that is what every BMW I have owned has called for. One day I noticed on my gas cap that the car calls for Mid-Grade. I feel I am getting the same gas mileage and performance as I was with Premium.
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Old 12-21-2009, 05:37 PM   #6
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Premium when the outside air temps are 70F or above
mid grade when its cooler 40F to 70F
and if its really cold I drop to regular. When its below 40F out.

Dave - PDX
1995 318ti - Active Black and Tan.
2005 330xi - Mtech 1 - 6spd - Orient Blue/Black
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Old 12-21-2009, 07:11 PM   #7
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Hi Dave [pdxmotorhead],

I am confused as to why you would switch the fuels like that....could you please provide some reasoning?

I remember I filled up the car with regular once and it was 30F outside and it had a terrible idle and the engine felt like it was not performing well. Hard to describe...

Once I put some BP Premium gas in it later that week the car did a 180 in performance. Nice and smooth.

So, naturally, I am cautious about the fuel....
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