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crappy fuel mileage

7K views 57 replies 14 participants last post by  curtisparker82 
#1 ·
i got a 2002 zx5 automatic 106k new plugs, new wires, new coil, new air filter, new fuel filter,...it misses at idle in neutral, almost dies when you first start it when it's real cold out side...and no mater how i drive it it gets 18-19 mpg...plugs and wires are motocraft, coil is bwd, and i run 5w30 castrol full syn oil....any help is greatly appreciated thanks
[ffrocks]
 
#4 ·
Coil is bwd is that saying the coil pack is bad? Well you should fix that....

The the PCV tube and the connection on the coil pack.
 
#6 ·
Why are you using 93 octane? Have a tune or some other reason? Unless you have modified your car with a tune going with a higher grade fuel us just wasting money.
 
#7 ·
What numbers are you using to figure fuel economy?

Usually when I see numbers this low, someone is filling up, and using gas tank capacity- 14 gal- to divide and figure fuel economy. The proper method is to fill up, hit the trip odo, then when you fill up again, divide the mileage on the trip odo by the number of gallons displayed on the gas pump.

Nearly every Focus we've asked about shows E (light comes on) when the tank has used about 10 gals leaving approximately 4 gals in reserve.
 
#8 ·
I am going to guess that for some reason the engine is never moving to closed-loop operation. Drive it a while and get it nice and hot then take it to an auto parts store and have them scan it and look at all the sensor readouts and see if the engine is in closed-loop. If any of them look strange, then you may have a bad sensor.

The factory coil is going to be superior to any aftermarket coil. Get a factory coil from a wrecker and I'd bet the ilde miss goes away.
 
#55 ·
i am going to guess that for some reason the engine is never moving to closed-loop operation. Drive it a while and get it nice and hot then take it to an auto parts store and have them scan it and look at all the sensor readouts and see if the engine is in closed-loop. If any of them look strange, then you may have a bad sensor.

The factory coil is going to be superior to any aftermarket coil. Get a factory coil from a wrecker and i'd bet the ilde miss goes away.
i have a gopoint cord and a ipod touch that works as a scanner how can i tell if it's in closed loop?
 
#9 · (Edited)
Im running 93 cause it was getting better than 89....BEFORE WINTER I WAS GETTING 18 ON 89 AND 23-24 ON 93 THEN IT JUST DROPPED TO 18 NO MATTER HOW I DRIVE....I DROVE IT FOR A WEEK AND DIDNT LET IT REV PAST 3K AND IT ONLY GET'S 18MPG...I DRIVE THE GUTS OUT OF IT AND REV IT TO 6 BETWEEN SHIFTS AND IT GETS 18MPG...DONT MAKE SENSE ...i fill up the tank and use the miles i drove/the amount of fuel i put in the tank....
 
#10 ·
you use the same station, same pump, and let it auto kick off with out topping off?

going from 250 miles and putting in 11 gallons vs 250 and putting in 12 gallons is a difference between 22.72 and 20.8 basically a 2 MPG drop because you put a little more or less fuel on each fill up.

If you saw that much of a change in your fuel economy from going from a 89 to a 93 octane (my altitude I live has offset octane levels, 89 is still a mid grade right? what were you getting on 87?) then something is wrong. the motor it self does not run any differently as it can not tell the octane nor is it tuned to advance timing to make use of higher octane. So the only reason why you would see better fuel economy is that the octane is "fixing" another problem.

What could cause this? I am not sure. Another thing to note is that most fuels have detergents in them, some more than others. you can typical get better fuel economy from Costco gas because they typically have less additives like the Chevron or Shell gas.
 
#14 ·
EGR should not impact your MPG in any way. Its an emission controlled device. Its to make your tail pipe emissions cleaner not reduce your MPG. When was the last time you checked tire pressure? Fuel filter? Possible partially clogged CAT? I know sometimes after sea foam in the intake or other aggressive quick intake cleaners could clog up the cat....
 
#16 ·
have you checked the PCV hose or the PCV it self?
 
#17 ·
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.
 
#25 ·
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.
 
#31 ·
ok if you were going to buy a new coil would you go stock or

MSD REPLACEMENT 4-TOWER COIL PACK, RED -- 4.097 in. L x 3.472 in. W x 3.349 in. H, 0.53 ohms primary resistance, 13.7 K ohms secondary resistance, 40,000 V maximum voltage, 3.9 mH inductance, 83:1 turns ratio
OR
ACCEL SUPER EDIS COIL, BLACK/YELLOW, SOLD INDIVIDUALLY -- 0.5 ohm primary resistance, 11.6k ohm secondary resistance, 85:1 turns ratio, 42,000 V maximum voltage,

both say Does not include mounting bracket, coil wire and ballast resistor;
 
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