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#1
07-27-2019, 01:31 PM
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Doing maintenance decreases fuel mileage now?
This one has me baffled, so maybe someone can explain it to me. My 99 E300TD was getting between 29 and 30 mpg during my routine travels for work/personal around town. Had it that way for a few months. Then I decided to change the oil from a Mercedes branded 10w30 diesel oil to Liqui Moly 0w40 diesel oil with an OEM filter, and replaced the air filter and both diesel filters at the same time with OEM filters as well as putting a quart of BG diesel fuel system cleaner through the tank. Over the past couple tanks I've noticed it I wasn't getting right at 500 miles on a tank anymore so I did the math today on a fill up and now I'm only getting a tiny bit over 25 mpg. Since when does routine maintenance drop your fuel economy? lol
Also I've noticed, it seems when I'm taking a "road trip" where I'm staying at one speed in cruise control on a highway or interstate and no red lights, starting and stopping, speeding up and slowing down, etc...I get the same or slightly less fuel economy than my normal commute around town. Are all diesels this way compared to a gas engine? 
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#2
07-27-2019, 01:35 PM
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After you run the oil through try and switch back to the TriStar oil .
I've seen some crazy mpg changes on oil weight before. Though, it was when I went lighter to 0w40 that had happened.
__________________
-Typos courtesy of my mobile phone.
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#3
07-27-2019, 01:54 PM
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diesels go by rpm's,keep them under 2500 and amazing mileage
__________________
1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt
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#4
07-27-2019, 02:25 PM
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I figured it had to be heavily RPM dependent instead of just a constant throttle position my gas cars seem to prefer. Around town it usually hovers about 2200-2500 RPM where as on the road trips it's been hovering around 3000. I guess I'll just have to sacrifice a little fuel on my upcoming 400 mile road trip so that I'm not on the road all day.
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#5
07-28-2019, 10:11 AM
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Check to make sure you are not leaking any fuel from the fuel filter connections since they were messed with when the filters were changed. Probably not the issue but can't hurt to check.
__________________
2012 Mercedes ML350 Bluetec 102K (hers) 2005 Corvette 55K (fun car) 2002 VW Jetta TDI 238K (mine) 1998 Volvo S70 T5 Turbo 211K (kids) 1994 Ford F150 4WD 246K (firewood hauler) 1983 Mercedes 300D 384K (diesel commuter)
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#6
07-28-2019, 01:54 PM
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It is possible the increase in airflow from air filters cause a boost in power and you are driving slightly quicker to speed between stops than before.
You also increased the viscosity of the motor oil, this increases oil pressure on the oil system consuming slight power to build...
__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread "as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do! My drivers: 1987 190D 2.5Turbo 1987 560SL convertible 1987 190D 2.5-5SPEED!!!  1987 300TD 2005 Dodge Sprinter 2500 158"WB 1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!
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