Random MPG question

[Recon427]

I have a 1998 318ti 113k miles, auto trans.

I commute 50 miles [both ways] on the highway everyday and I have been watching the computer mpg calculator. I went the posted speed limits like 65 mph and to see the effect on the mpg. I got 27.3 on average. However, recently my friend and I were both driving [him in a 1993 300E mercedes] and we decided to have some fun. When the car was going 80mph + the mpg calculator started increasing to 27.5 then 27.6.

So, my question is this: What is the most optimal rpm for the M44 engine in the 318ti? By optimal, I am aiming for fuel efficiency.

Also, off topic, but is it really necessary to use Castrol 5W-30 oil? I mean, I spent $42 on 6 quarts of oil at Advanced Auto Parts. I could have bought Mobil1 5W-30 for about $28 [same quantity, and yes all synthetic].

Thanks!

P.S. Please no smart al answers like "the most optimal rpm is zero"

[tiFreak]

I've used Mobil 1 and Castrol Syntec, no real difference

[BigDaddySmooth]

Your driving style will probably affect your mileage more than trying to find a proverbial sweet spot RPM. Unfortunately you have an auto which limits when "you the driver" can shift the car. If you avoid jack-rabbit starts, coast to red-lights, keep your tires properly inflated, you'll do more to improve your mpg. 27 is the worse I ever averaged w/my 96 manual. That was during the winter in stop-and-go DC traffic. Compare that with 33 I got while traveling from Utah to NY going 65-85 carrying about 400#. The best ever was 44 when I toured Yellowstone NP...never going above 45mph. Altitude helped there, no doubt. I think 27mpg w/an auto at 65mph on the interstate is 3-5mpg less than what you should be getting. Have you changed your fuel filter lately?

[THAT318]

at 80 was your foot down on the petal? i used to cruise at about 80 in my dads e39 getting about 30-32mpg... id gun it to 80 and let off just enough for it to stay at 80 with out a suckin all the gas... worked for me.

[Recon427]

Ok, thanks for your replies.

I have not changed the fuel fitler, however Inspection II was done last February and they should have changed it. I will check the records. I will take an OBDII scanner and see if there are any error codes in the system...a friend of mine who works on BMW's suggested that something could be faulty and BMW sometimes has a tendency to not show the check engine light when something is wrong.

This weekend I will replace the air filter and oil/oil filter to see if things change. If not, I will replace the fuel filter the following weekend and then observe any changes. I guess if nothing happens to the fuel economy then I am sending the car to the shop and have them run a diagnostics on it to tell me why it is operating so inefficently. Of course I could just do a complete Inspection II myself again.

[atlanta318ti]

My fuel mileage was usually about 32MPG, and then I noticed in the last 3 months it dropped gradually down to 31, 30, 29, and finally 28MPG. I changed the fuel filter last month, and no change.

But in my case I think the plugs need to be changed as well as the plug wires based on a misfire that came up when the weather got cold and brought the symptom to bear. I guess it's really true when the old timers say watch your fuel mileage because it's one of the best indicators that something has gone astray. Had I paid heed to that advice I would have checked my plugs and plug wires and probably changed them 3 months ago and would have saved both money (on the lost fuel mileage) and aggravation.

[Recon427]

I guess I will just tack on the replacement of spark plugs to my list of things to do tomorrow. I was supposed to do it last weekend when I got back from Europe but I haven't had the time..

So, things to be done tomorrow:
1) Air Filter
2) Fuel Filter
3) Oil & Oil Filter Change
4) NGK Spark Plugs

I think the wires are fine. I did not notice anything bad about them.

I will post my results probably some time next week once I drive it on the highway.

By the way, is it really necessary to torque the spark plugs to the 18 ft/lbs that is stated by BMW?

I have heard people saying they just hand tighten them and then turn 1/4 of a turn with a wrench. Some people say 18 ft/lbs is too much and they torque them to 11 ft/lbs.

BMW is a precision vehicle. I will operate by BMW specifications unless somebody lets me know better.

Any thoughts???

[THAT318]

u should probably use o.e.m bosch sparkplugs. i think i heard the car runs better with the ones it came with... but then again it depends on what you have done to your car, and what kind of nkg's you get.

hand tightened is fine. over tighten and you break them. and with the deep plug wells in our cars i dont want to know what kind of trouble it is to pull them back out broken.

[tiFreak]

BMWs use NGKs stock

I've been a mechanic for a few years now and I've never actually torqued spark plugs, I just tighten them by hand

[Recon427]

Ok......

So, 5 quarts of oil is all that my engine needed of Castrol 5W-30 synthetic.

The air filter was Mann and it was fine but I replaced it with a brand new Purolator air filter. I was surprised to learn that Mann filters are made in Mexico! I was going to do things by the Bentley manual and quite honestly they have the worst instructions on how to change an air filter. Not that one needs instructions for this, but all the same.

The spark plugs had an absolutely worn out electrode / firing tip. It was non existant. Apart from that they looked fine. Regardless I replaced them with NGK's part #2288.

I spoke to the shop and they said that everything was replaced in Inspection II last February [everything that needed to be replaced per the Inspection II guidelines]. So, since the Mann air filter was of good quality I am going to presume that the fuel filter will be the same. Therefore I did not change that part. I might next weekend if I don't see an improvement in my mpg this week commuting.

[THAT318]

oh my bad, i could have sworn the book called for bosch plugs but i might be thinking of something else i replaced recently.

[Recon427]

Bosch plugs are ok, in my opinion. However you sure don't want to use Platinum ones. Copper is the only one that is fine. Rather than worry about all of that I just stuck to NGK's. I was recommended Pulstar's BE1 but they are fairly expensive....about $40 for two new ones. Where as the NGK's cost only $20 for all 4. According to my friend, he said they improved his mpg by 15%. Granted he drives a 1996 Lexus LX450 and not a BMW, I am sure the effects would be similar.

[Recon427]

Hey guys, can you tell me what causes a spark plug to wear down to this level in just 13,000 miles of use?

http://s788.photobucket.com/albums/y...3.jpg&newest=1

[Recon427]

Ok, well for those of you that are interested:

After the oil & oil filter, new air filter, and new spark plugs my mpg went from 27 to 31-32 on the highway.

I will replace the fuel filter sometime soon and see if that impacts it even more.