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Old 01-09-2013, 08:11 PM   #21
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(see below)

Last edited by PWGuy; 01-10-2013 at 09:54 AM.
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Old 01-10-2013, 09:32 AM   #22
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Im running 215/45r17 tires they are only .5" over all od bigger? Would that kill the mileage? But i was running that before the mileage dropped from 23-18
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Old 01-10-2013, 09:54 AM   #23
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Quote:
Originally Posted by Insaniac View Post
On my old '02 ZTS I was having a similar issue, where my MPG was steadily declining until it got as low as 21-23 MPG. My issue turned out to be my DPFE sensor (EGR pressure feedback).

I found an easy way to test it as well, simply go to your firewall and unplug the wiring pigtail from the DPFE sensor. If, after you unplug it, your MPG improves then you know it was the problem. >Here< is a photo showing the DPFE sensor near the brake fluid reservoir. After I unplugged mine, my MPG immediately went up to 25-27.

Another recommendation, if you do find your DPFE sensor is bad, don't cheap out on the replacement. I bought a generic chinese cheap-o replacement on eBay and it was worse for my MPG than the OEM one that went bad. Go with a genuine Motorcraft part, it will be more expensive but its worth the difference. After replacing mine with a quality part my MPG returned to the normal 27-29 MPG.
Was your Focus throwing a code to indicate DPFE failure?
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Old 01-10-2013, 01:43 PM   #24
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Quote:
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Was your Focus throwing a code to indicate DPFE failure?
At the very end of the process it gave P0401, but my gas mileage started dropping a few weeks prior to the CEL.
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Old 01-10-2013, 02:18 PM   #25
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Tire size can do cause different readings in MPG but will not cause a drastic difference "over night".

A larger diameter tire: 1 rotation = further distance traveled. So you will have actually traveled further than the OD reads. 0.5" difference not sure how much... their was a chart available that shows the MPH differences vs tire size. Larger tire may provide better freeway MPG, could have a slightly lower RPM to maintain a specified speed. City driving will cause higher MPG due to starting and stopping a tire with a larger rotational mass.

A smaller tire: 1 rotation = Less Distance. So you will have actually traveled less than the OD reads. Your actual MPG will be less, also freeway you will need to maintain a higher RPM to maintain the same speed.

Have you checked to see if you might have a dragging caliper or partially stuck drum? if you hit a pot hole and somehow had a huge change in your alignment that could be another possibility.... try testing the DPFE its easy to test.
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Old 01-10-2013, 06:43 PM   #26
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I unplugged my DPFE this afternoon. The CEL came on when I pulled out of the garage this evening. I assume that's a good thing? Should I leave it disconnected to see if MPG decreases further to see if its 100% working or not?
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Old 01-10-2013, 06:54 PM   #27
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does it run any better?
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Old 01-10-2013, 07:42 PM   #28
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Quote:
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does it run any better?


This


It's going to throw a code for what you unplugged, question is did it fix the problem?
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Old 01-10-2013, 09:53 PM   #29
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How will I know until I drive it for awhile and fill up with gas?

I think I noticed a little more vibration while stopped at lights but I can't say for sure. Honestly, the car "feels" the same when I drove it to a store and back earlier tonight.
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Old 01-10-2013, 09:54 PM   #30
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Apologies but my at doesn't misfire or anything - just consistent 17-19 mpg.
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