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sudden mpg drop - winter blend fuel?

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Old 12-10-2012, 06:37 AM   #1
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Default sudden mpg drop - winter blend fuel?

im big on keeping track of my mpg in my jeep.

ive noticed a drop in mpg ever since i put a new fuel pump, sending unit. and cleaned the throttle body

i was getting around 17-17.5 mpg calculated on paper

now im getting 15-16

i keep track b/c when i see dramatic drops like this, i know something isnt right

my drop in mpg has been for the last 2-3 weeks

i thought a new fuel pump/sending unit/throttle body clean would improve it

plugs,wires, cap,rotor, new o2 sensor all done about 4 months ago...and like i said mpg was up

now i have a drop

could it be winter blend fuel?
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Old 12-10-2012, 07:34 AM   #2
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well winter blend fuel starts in sept around here anyway....How is it running? How many miles on your tune up stuff? did you get a brass cap and rotor? good wires? and if you have a 2000 are there not 2 o2 sensors? did you do both?
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Old 12-10-2012, 08:23 AM   #3
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How many MPG samples have you taken? Get at least 4 before coming to any conclusions. Gotta do the math; miles driven divided by gallons consumed.

And yes, fuel blend can be in play here. As well as slightly richer mixtures during startup in cold weather, idling, etc.

It would not be unusual to see what you are seeing all things considered. I wouldn't be concerned.
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Old 12-10-2012, 08:50 AM   #4
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I'd lean towards an o2 sensor as well. They can have a big play in mpgs. Get a scan tool hooked up and check sensor functions.
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Old 12-10-2012, 09:07 AM   #5
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cap, rotor are brass

all tune up stuff was done about 3 months ago...along with a new o2 sensor


i did the o2 sensor upper one (my jeep is a 99) so it has a upstream and a down stream

i replaced the upstream one only and saw my mpg go up...

my mpg was up until about 2 weeks ago when i replaced the fuel pump, filter, sending unit and cleaned the throttle body

ever since then my mpg has been down about 2 mpg

and i have taken about 12 fillups or more since i bought the jeep

started out at 16, tune up got me almost 18

now, for the past 2 weeks im in the 15ish range

the only thing that changed other than the fuel system was the weather..it got colder...now i do let me jeep warm up for a few minutes...but that shouldnt bring me down almost 3 mpg

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Old 12-10-2012, 09:19 AM   #6
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The downstream o2 sensor will affect gas mileage, not as much as the upstream, but it affects it nonetheless. the downstream sensor measures the effectiveness of the cat and a bad sensor will tell the ECU to compensate for the lack of a cat, thus making fuel/air mix different. Will it be noticeable,maybe if you are paying attention like you are, might only be 1 or 2 mpg off , but it will affect it. hopes this helps
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Old 12-10-2012, 09:26 AM   #7
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the only thing that changed other than the fuel system was the weather..it got colder...now i do let me jeep warm up for a few minutes...but that shouldnt bring me down almost 3 mpg
Well.. Cold air is denser than warm air. So when your jeep is breathing in cold air, the computer sees that it's bringing in a lot of air, so it starts dumping more fuel to compensate. This will cause a (noticeable) decrease in mpg's. So it's possible that this has some bearing on it..
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Old 12-10-2012, 09:47 AM   #8
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The downstream o2 sensor will affect gas mileage, not as much as the upstream, but it affects it nonetheless. the downstream sensor measures the effectiveness of the cat and a bad sensor will tell the ECU to compensate for the lack of a cat, thus making fuel/air mix different. Will it be noticeable,maybe if you are paying attention like you are, might only be 1 or 2 mpg off , but it will affect it. hopes this helps
i was told the downstream sensor has nothing to do with mpg

hmm...i will have to research this now

and yes...it has got colder...so i did expect it to go down some..but now quite this much....oh well...not much i can do i guess

i will research this more
thanks
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Old 12-10-2012, 09:52 AM   #9
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Doesn't downstream measure cat function? It reads the exhaust gasses and determines if they are being properly processed. If there's a leak or poor cat performance. I think... I've also been told its important to have a working downstream sensor, too..
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Old 12-10-2012, 09:52 AM   #10
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vt you are correct and the computer changes the fuel mix on this info...
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Old 12-10-2012, 10:08 AM   #11
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vt you are correct and the computer changes the fuel mix on this info...
So the downstream is important.
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Old 12-10-2012, 10:36 AM   #12
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So the downstream is important.
not directly to mpg..but cat function...i will look into it
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Old 12-10-2012, 12:36 PM   #13
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Quote:
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the only thing that changed other than the fuel system was the weather..it got colder...now i do let me jeep warm up for a few minutes...but that shouldnt bring me down almost 3 mpg
This could certainly be the case...idling fuel consumption is horrible for MPG. Also the ODBII system will stay in Open Loop mode until a certain engine temp is reached so your A/F mixture is going to be richer for longer in the winter. Get a partial grill block, an engine block heater or park your car inside.

Most importantly get some sort of instant ODBII feedback gauge such as Scangauge or Ultragauge (cheaper). This way you can see instant fuel economy feedback so you can see if your cruising mpgs are abnormally low or if the warm-ups and open loop mode along with the lower quality winter blend gas are the issues.
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Old 12-10-2012, 12:48 PM   #14
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This could certainly be the case...idling fuel consumption is horrible for MPG. Also the ODBII system will stay in Open Loop mode until a certain engine temp is reached so your A/F mixture is going to be richer for longer in the winter. Get a partial grill block, an engine block heater or park your car inside.

Most importantly get some sort of instant ODBII feedback gauge such as Scangauge or Ultragauge (cheaper). This way you can see instant fuel economy feedback so you can see if your cruising mpgs are abnormally low or if the warm-ups and open loop mode along with the lower quality winter blend gas are the issues.
here the thing tho....i do park it inside every night...i dont let it warm up now, any longer than what i do in the summer months

my 2 guesses on what it could be:

colder weather/winter fuel

more shorter trips...never heats all the way up
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Old 12-10-2012, 01:52 PM   #15
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My dad believes that gas companies are watering down their fuel more, making it cheaper for them to produce and create a larger profit. Check the O2 for starters.
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