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'99 E430 '98 C230
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Discussion Starter #1
My '99 E430 within the last 5k. miles has lost 6mpg highway driving and down to avg. 19.4 mpg. combined with the AC on or off. Was getting 23 combined.I have tried every thing I know to max the mileage..."egg between the gas pedal" coasting to time the red lights..ect. The car has 30k on NGK #7092 plugs,O2 primary sensors around 70k,fuel filter 30k, Air filter 30k,MAF insert 30k cleaned with MAF sensor spray every oil change (8k) Mobil 1 10/30,Can of BG44K every 10-15k, burn top teer primo gas.I have no codes and I have even removed and cleaned the EGR valve /tube with in the last 20k as preventive maintance. Also have alignment done and Michelin primacy mxv4 running at 32psi. Could it just be the Crappy Ethanol gas the goverment has watered down for the past 3 months? Car now has 159,000k. Sugestions welcome!
 

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I am supprised when a '99 E430 has a 23 mpg combined. A BRAND NEW car was set at 18 mpg combine for RWD and only 17 mpg for AWD by the factory
I used to have one 99 E430, and now I got a '01 E430 AWD, both car have a similar mileage as set by the manufacture
I think these mileages apply for almost of any V8 engine, over 400 cc , 275 hp or more ,running on premium gas like BMW 540, Lexus LS400, etc...
I thought the 23 mpg apply only for a V6 E320, 225 hp
 

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1999 E430; 2005 ML500
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I can get 23 when 'm extra careful, but 21 is the norm combined. 26 would only come into play on pure highway.
 

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2001 E430 4matic
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419 Posts
Good job on keeping up with all your maintenance.

To answer your question, Mine with 54k miles one it, on the highway with traffics around the major cities, gets me about 27mpg (during 1500 miles road trip). Combined city and highway with stop and goes gets me around 23 mpg or sometimes less. I have seen even 17mpg. Yours is not really out of range. If it was a road trip of 5K (interval of a week) and you had 23 mpg, I would look into it. But 5k of city and highways and traffic during an interval of 3 month, does not really representing what your fuel consumption is. You would only know what your actual fuel consumption is on the highway, and that is only during at least 800 miles highway trip within couple of days.

Been many discussions about the fuel, but mine gets this mileage on the long highway trip with ethanol gas. Its been warm this year, AC been on all the time and I have also noticed my fuel consumption is higher than normal.
 

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2002 E430 4-Matic
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1,334 Posts
I'm confused - your title refers to hwy mpg, but all of the information you provided is for combined. I agree with the other posters - 19.4 mpg combined is not out of range, depending on the mix of hwy/city. I typically see 20 mpg combined surbuban Chicago.

However, 26 - 28 is normal for pure hwy. I've seen as high as 30mpg under perfect conditions, Ohio Turnpike, 70mph - no traffic.

What are you getting hwy today?
 

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99 E430, 01 E430 Sport, 00 SL500
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The OP wants to find out why 23mpg dropped to 19.4mpg.
I only saw a possible problem, "Air filter 30k". I replaced the air filter much sooner than 30kmiles.
 

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'99 E430 '98 C230
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57 Posts
Discussion Starter #7
I can tell the hwy mileage in the past always showed 120-130 miles at the first quarter mark on the fuel guage..It now only manages 80-100 miles hwy. this is running 65-70 mph. AC on or off.
 

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'99 E430 '98 C230
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Discussion Starter #8
My car has been consistent within the 23 combined because I drive it to last and for max fuel efficiency. The AWD adds weight and friction affecting mileage figures so naturally yours might reach 23 only in perfect conditions (gas with little to no Ethanol in it,ect). Thanks for your input.
 

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'01-E320 & 02-ST2
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I can tell the hwy mileage in the past always showed 120-130 miles at the first quarter mark on the fuel guage..It now only manages 80-100 miles hwy. this is running 65-70 mph. AC on or off.
This isn't exactly the most reliable means of checking fuel economy, since the sender can lose it's adjustment. Mine has gotten off by more than a gallon (the light comes on and the system says 2.6, but there is actually closer to 4 gallons based upon the amount of fuel I add).

The best thing to do is to calculate your actual fuel economy based upon miles driven and fuel added. If you have actually not changed your driving habits and the calculated fuel economy has dropped by that much, I would try to get some live data, including FT values. The alternative is to just start replacing things, and that can consume a lot of time and money, as well as increasing your frustration level.

Good luck.
 

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'99 E430 '98 C230
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Discussion Starter #10
Funny you mentioned that,I just replaced it yesterday with a new Mann. In the meantime I would take out the filter and tap it out and take my blower and force air through it then vacumn out the air box with my shop vac. I do this every time I rotate the tires...6k. So I think the filter stays pretty clean...I also clean the MAF every 10k.I try to keep my baby running right.
 

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'99 E430 '98 C230
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Discussion Starter #11
Yes I try to keep the sensor accurate with the BG44k treatments in the tank (it works great by the way) every 10-15k. I have a idea it might be time to replace the primary O2 sensors again. They have around 70-80 k on them.
 

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Yes I try to keep the sensor accurate with the BG44k treatments in the tank (it works great by the way) every 10-15k. I have a idea it might be time to replace the primary O2 sensors again. They have around 70-80 k on them.
The sensor accuracy isn't affected by whatever is in the tank, be it fuel or additives. There's a sticky on how to adjust it, but I don't think it's really intended for fine-tuning. I mentioned it because if you were getting 23 and suddenly dropped to 19, that would certainly explain it, at least if you were basing that on just a range on the gauge.
 

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'99 E430 '98 C230
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Discussion Starter #13
Thanks...the BG has done a good job for me keeping the fuel guage sensor surface clean of fuel contaminates keeping it accurate. I always reset the mileage at every fill up and always divide the gallons to fill against the mileage on the clock..along with the visual on where the needle is when my car reaches certain mileage points.Yes the fillup and divide is the best way for measurement of mpg.
 
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