[snakey5138999]

well my gf has a 99 mustang with about 85k on it. got about 17 mpg and then she changed her spark plugs, wires and fuel filter and ever since then it seems it has actually dropped a little. now i think shes getting a little less than 16 mpg. im going to school so i have access to alot of tools and scanners also. im going to be bringing it in on tuesday and was wondering what i could check. ive done things like seafoam, different intake because we though the k&n one was crap because the boot kept coming undone,inflate tires to reduce drag. nothing really has done anything so what could i check for when i bring it to school on tuesday. i was thinking fuel trim to check o2's.

[stroker408]

tire pressure is the fiest thing leading to bad mileage

my wife 99 v6 vert got 16 in the city last time i checked it. appx 175 miles and light came on.

[snakey5138999]

wow these cars are that bad on gas mileage? i could have sworn they should get at least 20, im mean its only a 3.8 v6. i have a 5.7 v8 and im still getting 13, and i have 150k on it wilh alot of improvement to do. so that being the case anyone wanna buy a 99 v6 mustang with about 85k new tires,plugs,wires, fuel filter, ebay intake and flowmaster exhaust???????

[whitemustang02]

mine gets a little over 22mpg and that is wit hme getting on it a lot, so i would say 25mpg is very posiable in these cars.

[Blainestang]

Originally Posted by snakey5138999 wow these cars are that bad on gas mileage? i could have sworn they should get at least 20, im mean its only a 3.8 v6. i have a 5.7 v8 and im still getting 13, and i have 150k on it wilh alot of improvement to do. so that being the case anyone wanna buy a 99 v6 mustang with about 85k new tires,plugs,wires, fuel filter, ebay intake and flowmaster exhaust???????

I just calculated my mileage today... 25.xx mpg over the last tank of gas... and I have heads/cam and about 60whp more than stock.

[GreenFaleen]

clean/ check your air filter, o2 sensors

[snakey5138999]

yeah i was thinking o2's because i know there original and would affect mileage greatly. how much some new o2's cost anyways? she really doesnt want to put anymore money into it so.......FOR SALE IT IS!!!!

[whitemustang02]

i just replaced one of mine for $50

[Hourofpain]

Seafoam the intake and the gas can
Replace the spark plugs
replace the fuel filter
Oil change
clean your maf sensor
clean your Iac sensore\

[GreenFaleen]

Originally Posted by snakey5138999 yeah i was thinking o2's because i know there original and would affect mileage greatly. how much some new o2's cost anyways? she really doesnt want to put anymore money into it so.......FOR SALE IT IS!!!!

about 60 a piece

[mustangjeff1997]

yea, if you wanna replace them all it's gonna be a LOT...if she doesn't wanna put a little bit of money into it then i say let her buy a new car for more money than to fix this one with...i've had some experience with O2 sensors...not a whole bunch of fun

[El_Cangri]

its going to be between 40-100 sensors depending on brand..

[whitemustang02]

Originally Posted by mustangjeff1997 yea, if you wanna replace them all it's gonna be a LOT...if she doesn't wanna put a little bit of money into it then i say let her buy a new car for more money than to fix this one with...i've had some experience with O2 sensors...not a whole bunch of fun

i repeat my previous statment i just did one for 50 bucks, so as far as car parts go that is relitivaly cheep.
also 02 sencers r not hard to work with at all. ive puled all 4 on mine out and back in twice in the last cuple months, as well as doing the rear ones a few more times. other then having to get under the car and get ur hands in to some kind of tight spaces it isent hard at all.

[MustangMo]

Im gettin bad mpg's to, but i think its cause i need an oil change in a bad way. But if its not that, what else might it be?

[whitemustang02]

Originally Posted by MustangMo Im gettin bad mpg's to, but i think its cause i need an oil change in a bad way. But if its not that, what else might it be?

every thing that is listed in here

[Juggalo_X]

A bad thermostat that is stuck open will cause the car to get stuck in an open loop because optimal closed loop temperature cannot be achieved. you might find at idle it can barely hit closed loop and at highway speeds it gets stuck in open loop because of over cooling. if you get stuck in open loop you will have very poor gas millage because Open loop ignores o2 data. check your PT trims at highway speed and make sure they are ok and its not falling into open loop. WOT will always be open loop so make sure u test this doing PT. just throwing this idea out there...

[Hourofpain]

you should tell them how to test that

[Juggalo_X]

Originally Posted by Hourofpain you should tell them how to test that

a stuck open thermostat is hard to diagnose. all u can really use to sell this is the fact that ur temp gauge is a bit lower then usual and that ur heat isnt as hot as it used to be. he said he has the tools to check the trims for the 02 so he should be able to tell if its getting stuck in open loop at highway speeds. because the trims will stop fluctuating and depending on the tool it will tell you when open or closed loop is active.

[Hourofpain]

Ghetto way for the thermo stat
unplug the lower house and see if coolant ever returns
it will atleast tell you if its always closed.

[Juggalo_X]

Originally Posted by Hourofpain Ghetto way for the thermo stat unplug the lower house and see if coolant ever returns it will atleast tell you if its always closed.

better way is to touch the upper rad hose. this goes directly to the thermostat housing. if its stuck closed the hose will be cold and soft. if it has opened the hose will be hot.