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Discussion Starter · #1 ·
So lately the gas mileage on my mustang has been pretty terrible. It's a 2003 3.8 5 speed. 197000km. To all full tank i am getting a little over 300 km to a tank if just city driving. When i did mixed driving i got 393 km to a full tank/ 18 mpg mixed. Only mods done to the car are :pacesetter h pipe, flow master dual exhaust, sr performance cold air intake, GT rear sway bar. Lately i have done some maintenance things to try to bring the gas mileage back up but so far no luck. I have done : high mileage fuel system cleaner, fuel filter, spark plugs, spark plug wires, cleaned maf sensor, cleaned air filter, cleaned throttle body. I use synthetic oil in both the engine and transmission. Things that i know that are wrong with the car : has an exhaust leak, tires aren't the best but always kept at the right pressure. Any help is appreciated. Thanks

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Discussion Starter · #3 ·
How bad is the exhaust leak?
And where is it located?
Sounds like its where the manifold meets the downpipe on the passenger side. Its not extremely loud, the sound coming from the pipes definitely overpowers it, but its noticeable

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It is possible that the exhaust leak is fooling the O2 sensor into believing that the A/F mixture is too lean, so it is telling the computer to enrich mixture.
Assuming that there are no trouble codes, I would work on eliminating the exhaust leak first.
 

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Discussion Starter · #6 ·
It is possible that the exhaust leak is fooling the O2 sensor into believing that the A/F mixture is too lean, so it is telling the computer to enrich mixture.
Assuming that there are no trouble codes, I would work on eliminating the exhaust leak first.
Okay thanks I'll try that as soon as i get a nice day to work on it.

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Discussion Starter · #7 ·
Try a new thermostat--if it is staying open, the sensors will keep the mixture rich. Cheap and easy to change.
I forgot to mention that i put a new water pump and thermostat in a few months ago because it was overheating, so i don't think its the thermostat

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Low mileage

Still could be the thermostat if you put in the wrong temperature range. Take it out and put it in water with a kitchen thermometer. Should not open until near boiling (195-210 degrees).
 
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