2008 Mazda5 Fuel Economy not as expected!! LTFT +(7-14)%

haidar

Member
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Mazda 5 GS Auto 2008
Hi,

I have 2008 Mazda5 Auto (5 Gears) GS model. I purchased the van with 75,000 miles on it. It is running good but the fuel economy is not that good:

23 MPG: Highway Only
17 MPG: 50% City and 50% Highway
15-16 MPG: City Only

I have an OBDlink-MX and I am monitoring the PID'S of the van and I always noticed that my LTFT are always high positive specially at idle around +(7-14) % and STFT are also positive around +5% except when the van is cold. I have an excel sheet for important sensors reading live as a log file..

Why my van is adding fuel ? Any ides to improve my economy?

Thanks
 
Do you have any codes? This may prove useful http://www.easterncatalytic.com/education/tech-tips/fuel-trim-can-be-a-valuable-diagnostic-tool/ and http://longislandcatalytic.com/understanding-fuel-trims/

Apparently the sum of the LTFT and the STFT should be < 10% and they both should approach zero for a static engine condition. I don't know much, but my suggestion would be to clean your Mass Air Flow sensor and trouble shoot your oxygen sensors.

This should help with diagnoses http://www.aa1car.com/library/what_is_fuel_trim.htm

Tell us what you find.
 
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Yes, your mileage is low. I have 2007 GT with the 4 speed, and mine is much better than that. Sounds like a couple of O2 sensors are playing with you. As KW-DA said, inspect and trouble shoot them to see whats up. Also, if you have not done it, Plugs, wires, and filter would be a good thing to change out too.
 
Some recommendations: clean MAF sensor, check all intake and vacuum connections, listen for any hissing noise around intake. Many possible causes. Intake seals may be leaking, pcv hose located under intake may be cracked, or possible stuck open evaporative purge solenoid. At idle and steady cruise, stft should be switching positive to negative but around zero and ltft should be no higher than 5-7% on a healthy running engine.
 
Also check the tranny fluid. Friction in the transmission is often overlooked. How many miles/km are you putting on the car in a day? Short trips in the winter are going to be bad for mpgs no matter what car you have. You realize that the defroster engages the AC compressor, right? That is easily a 2 mpg hit alone - personally, I see mpgs dip from 24 mpg all city (5mt) in the winter when we don't need to engage the compressor to 17.5 in the summer when it is on every second the crank is turning.
 
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