Need help diagnosing bad mpg

Fresco

Contributor
Well guys according to fuely im getting between 12-15 mpg city, before when i filled up from where the first line is by the F on the fuel gauge I would get between 55-60 mpg (highway) once the needle got right under the first line and between 127-139 mpg (highway) by the second line....Today I filled up and between the first and second line I got 77mpg on the highway no where near what I use to get.

My 02 P5 has 158000 miles, 205/50r15 hancooks all around, a simota CAI, OBX header with header wrap, NGK wires with NGK plugs (ZFR5F-11), new coil packs, a vibrant performance stealth muffler, and an oil catch can between the pcv valve and intake mani. I have no codes, no misfires, no sputters, no hesitations, no fuel smells and no idea where to begin with trying to figure out why my mpg's are so bad so if anyone has any idea where I should begin, I honestly think I have a leak but with no fuel smell its kinda hard to say thats what's going. I did notice that the needle did move quick between the the first two lines which gave me that 77 miles (in my trip meter) but as soon as I hit 80 miles (trip meter) it slowed down drastically. So please guys don't be shy with your input.
 
Seafoam Air Filter

14 Poor fuel economy
DESCRIPTION • Fuel economy is unsatisfactory.
POSSIBLE CAUSE
• Contaminated air cleaner element
• VICS malfunction
• Tumble swirl control system malfunction
• Engine cooling system malfunction
• Improper automatic transaxle fluid level (ATX)
• Weak spark
• Poor fuel quality
• Erratic or no signal from CMP sensor
• Improper coolant level
• Inadequate fuel pressure
• Spark plug malfunction
• PCV valve malfunction
• Brake dragging
• Improper valve timing due to jumping out of timing belt
• Contaminated MAF sensor
• Improper engine compression
• Exhaust system clogging
Warning
The following troubleshooting flow chart contains fuel system diagnosis and repair
procedures. Read following warnings before performing fuel system services:
• Fuel vapor is hazardous. It can easily ignite, causing serious injury and damage. Always
keep sparks and flames away from fuel.
• Fuel line spills and leakage are dangerous. Fuel can ignite and cause serious injuries or
death and damage. Fuel can also irritate skin and eyes. To prevent this, always complete
“BEFORE REPAIR PROCEDURE" and “AFTER REPAIR PROCEDURE” described in this
manual.

Service Manual Link: http://www.floptical.net/mazda/

I find it interesting that the manual doesn't list "clogged or contaminated injectors" as a possible cause. Seafoam takes care of that (or any injector cleaner,... I use some in every three or four fill ups)
 
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Well guys according to fuely im getting between 12-15 mpg city, before when i filled up from where the first line is by the F on the fuel gauge I would get between 55-60 mpg (highway) once the needle got right under the first line and between 127-139 mpg (highway) by the second line....Today I filled up and between the first and second line I got 77mpg on the highway no where near what I use to get.

My 02 P5 has 158000 miles, 205/50r15 hancooks all around, a simota CAI, OBX header with header wrap, NGK wires with NGK plugs (ZFR5F-11), new coil packs, a vibrant performance stealth muffler, and an oil catch can between the pcv valve and intake mani. I have no codes, no misfires, no sputters, no hesitations, no fuel smells and no idea where to begin with trying to figure out why my mpg's are so bad so if anyone has any idea where I should begin, I honestly think I have a leak but with no fuel smell its kinda hard to say thats what's going. I did notice that the needle did move quick between the the first two lines which gave me that 77 miles (in my trip meter) but as soon as I hit 80 miles (trip meter) it slowed down drastically. So please guys don't be shy with your input.
The gas gauge markings give you a ball park idea of how much gas you've used. Most companies modify the fuel gauge sender's arm so that it appears to stay near F longer but when it gets near the middle, it goes down faster. When the fuel gauge is linear, driver's think their car consumes more fuel so they think their car isn't fuel efficient.
 
Fuel trim numbers would be real helpful here. A cheap OBDII scan tool (<$100) that can give you data stream will really help in narrowing it down. Besides fuel trim numbers, I would look at your Mass Air flow readings at idle and under load. Typical readings will be 1-1.5g's per Liter per second at idle, so a "normal" P5 should have a Mass air flow reading around 2 - 3 g/sec at idle and should be north of 80g/sec when under wide open throttle. If the Mass Air flow looks good and the trim numbers are positive (indicating a lean condition the PCM is adding fuel to compensate for), then i would start looking for air/vacuum leaks. Short of smoke testing the intake, a cheap and dirty method is to spray 2+2 (carb-clean) around the intake and see if there is an rpm change. And as the other posters said, almost all fuel gauges are "buffered" so the rate at which the needle drops is never consistant from full to empty.
 
Fuel trim numbers would be real helpful here. A cheap OBDII scan tool (<$100) that can give you data stream will really help in narrowing it down. Besides fuel trim numbers, I would look at your Mass Air flow readings at idle and under load. Typical readings will be 1-1.5g's per Liter per second at idle, so a "normal" P5 should have a Mass air flow reading around 2 - 3 g/sec at idle and should be north of 80g/sec when under wide open throttle. If the Mass Air flow looks good and the trim numbers are positive (indicating a lean condition the PCM is adding fuel to compensate for), then i would start looking for air/vacuum leaks. Short of smoke testing the intake, a cheap and dirty method is to spray 2+2 (carb-clean) around the intake and see if there is an rpm change. And as the other posters said, almost all fuel gauges are "buffered" so the rate at which the needle drops is never consistant from full to empty.

Megazuel,... Any opinions or advice in this thread would be appreciated: http://www.mazdas247.com/forum/showthread.php?123809881-ultragauge-shows-my-p5-stuck-in-open-loop.

Frezco,... My mileage suddenly dropped from 38 to 31 MPG (Canadian MPG,(... 4.54 liters per gallon)), so I changed my air filter (and cleaned my MAF, but it wasn't dirty) and my mileage went right back up. It paid for itself with one tank of gas.
 
Well guys according to fuely im getting between 12-15 mpg city, before when i filled up from where the first line is by the F on the fuel gauge I would get between 55-60 mpg (highway) once the needle got right under the first line and between 127-139 mpg (highway) by the second line....

12-15 mpg is low for city driving, but it depends on what kind of city driving. Stop and go in one block hops put my car nearly that low a few times. Besides the losses from braking, the automatic transmission is super inefficient when used that way - it never gets to torque converter lock up.

Your other MPG numbers make no sense. Did you mean at 55-60 mph the car got 27-39 mpg? That would be about right. Pretty sure no P5 has ever managed 127-139 mpg!!!

After you drive the car walk around it and feel the wheels (carefully). If one of the brakes is dragging your mpg will fall and it will make that one wheel hotter, sometimes much hotter, than the other three.
 
I've noticed that after refueling, I go between 25 and 45 miles before the needle starts to move. I look for the bottom left corner of the F on the gauge to be covered by the needle. I think most of the difference is from how much fuel I put in after the nozzle shuts off. I usually round the cost up to the next full dollar. Things like the gas warming up in the summer and expanding affect the reading too.
 
are you running premium fuel and have a mp3 ECU with those upgraded plugs? if not, thats maybe whats causing your bad MPG (i have the same damn problem, returning to stock plugs)
 
well guys with the combination of a sticky caliper, a dirty air filter, and a questionable gas station, (all fixed/changed) i am happy to report my mpg's are up dramatically, and ZHY i dont think you need an MP3 ECU to run the long reach plugs but i just might go back to stock and see what happens, THANKS to everyone that chimed in..
 
the ONLY real way to calculate mpg's is by filling the gas tank, monitoring miles driven to empty the fuel tank, and divide by the number of gallons needed to refill the tank. i reset my trip meter at every fill up, and compare the miles driven by the number of gallons added at the next fill up. any other method is really just an estimate.
 
Don't go by the fuel gauge, they're not even close to accurate

the ONLY real way to calculate mpg's is by filling the gas tank, monitoring miles driven to empty the fuel tank, and divide by the number of gallons needed to refill the tank. i reset my trip meter at every fill up, and compare the miles driven by the number of gallons added at the next fill up. any other method is really just an estimate.

I use the fuelly app
 
14 Poor fuel economy
DESCRIPTION • Fuel economy is unsatisfactory.
POSSIBLE CAUSE VICS malfunction? how would u test for that?
 
It's in the service manual 01-3b-57

VICS Operation Inspection
1. Start the engine.
2. Verify that the rod of VICS shutter valve actuator is pulled.
• If the rod is pulled, proceed to next step .
• If the rod is not pulled, inspect as follows.
— Loose or damaged vacuum hose and vacuum chamber
— Shutter valve actuator (See 01–13B–11 VARIABLE INERTIA CHARGING SYSTEM (VICS) SHUTTER VALVE ACTUATOR INSPECTION [FS].)
— VICS solenoid valve inspection (See 01–13B–12 VARIABLE INERTIA CHARGING SYSTEM (VICS) SOLENOID VALVE INSPECTION [FS].)
3. Inspect the rod operation under the following condition.
• If the rod operation is not as specified, inspect as follows.
— Shutter valve actuator (See 01–13B–11 VARIABLE INERTIA CHARGING SYSTEM (VICS) SHUTTER VALVE ACTUATOR INSPECTION [FS]
— VICS solenoid valve inspection (See 01–13B–12 VARIABLE INERTIA CHARGING SYSTEM (VICS) SOLENOID VALVE INSPECTION [FS].)
Note
• The shutter valve actuator rod extends for five seconds after the engine is started.
 
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