huge drop in mpg, anyone know why?

MustGT98allday · October 23rd, 2010, 09:37 PM

so heres the thing
I bought my 98 GT a little over two years ago. Its a 5 speed. also my mods are in my garage. Now my gas gauge has never really worked so i used to go off my miles and I used to get around 20-21 mpg the first few months I had it. then my odometer stopped working. And now I finally got a new cluster in with a working odometer and I have calculated my mpg for the first time in forever. It came out to be 16

what makes it even worse is I drive mostly highway and my rpms are lucky to see 3k. Now before when i first got the car i had fun and got on the gas sometimes and definitely more than i do now. So I am confused and pissed as to why my mpg dropped so drastically. Now since then I have had the intake manifold replaced by a local shop, new plugs, new wires, and a new purge canister, purge solenoid and vaccum line. Can anyone help me out here.
Its been about 8 months or so since my intake manifold was replaced and I wasnt happy with their job at all. SO i am thinking they screwed something up. Now I am starting auto school in two days and after maybe a week i will be able to do anthing i want to the car in the shop with anytool possible. so anyone know what i should do?
Sorry about the huuuge post
THANKS

Hotshot_98GT · October 23rd, 2010, 09:53 PM

Go to autozone and get them to scan the computer. Or if you have an OBD2 scanner scan it yourself. might be O2 sensor.
Also the EGR valve may be stuck. Sometimes it wont show up in the computer. Havent had that problem on my stang but my trucks mpg dropped horribly like that. All i did was take the EGR valve off, clean it with some carb cleaner ,and put it back on with a new gasket ($2 @ autozone). Might not be whats wrong with it but hey its a cheap guess, and only takes about 20 minutes.
Hope this helps

tfofkyle · October 23rd, 2010, 10:07 PM

have you changed the fuel filter? common gas mileage robber...

MustGT98allday · October 23rd, 2010, 10:12 PM

what and where is the egr valve and I changed the fuel filter a little bit after i bought the car
I even notice my car being alot slower than it was about a year ago.
IDK it seems like my car is just going down hill

MustGT98allday · October 23rd, 2010, 10:12 PM

and forgot to mention it has abotu 120000 miles

Hotshot_98GT · October 23rd, 2010, 10:28 PM

Its the pancake looking valve attached to the passenger side of the upper intake plenum. i'll try and find a picture or something

MustGT98allday · October 23rd, 2010, 10:34 PM

ah...i think i know what ur talking about

Hotshot_98GT · October 23rd, 2010, 10:41 PM

The blue circle is the EGR valve and the 2 red circles are the 2 bolts that hold it on oh and the hose on the back may have to be unhooked but i aint sure

Uploaded with ImageShack.us

MustGT98allday · October 23rd, 2010, 11:09 PM

sweet...Ill check it out thanks
I was going to replace my fuel filter but i couldnt get the old one out while laying on my back so ill do it when i can get it on the lift.
Also i might be getting a pi intake and do that so that will give me a chance to make sure its done right then for christmas hopefully i can get a tuner

October 23rd, 2010, 09:37 PM	#1
MustGT98allday Hardcore Enthusiast 1998 Mustang GT Join Date: Aug 2010 Posts: 1,282 iTrader: 2 reviews	huge drop in mpg, anyone know why? so heres the thing I bought my 98 GT a little over two years ago. Its a 5 speed. also my mods are in my garage. Now my gas gauge has never really worked so i used to go off my miles and I used to get around 20-21 mpg the first few months I had it. then my odometer stopped working. And now I finally got a new cluster in with a working odometer and I have calculated my mpg for the first time in forever. It came out to be 16 what makes it even worse is I drive mostly highway and my rpms are lucky to see 3k. Now before when i first got the car i had fun and got on the gas sometimes and definitely more than i do now. So I am confused and pissed as to why my mpg dropped so drastically. Now since then I have had the intake manifold replaced by a local shop, new plugs, new wires, and a new purge canister, purge solenoid and vaccum line. Can anyone help me out here. Its been about 8 months or so since my intake manifold was replaced and I wasnt happy with their job at all. SO i am thinking they screwed something up. Now I am starting auto school in two days and after maybe a week i will be able to do anthing i want to the car in the shop with anytool possible. so anyone know what i should do? Sorry about the huuuge post THANKS __________________ pi swapped and dropped sn95

October 23rd, 2010, 10:07 PM	#3
tfofkyle Hardcore Enthusiast 02 Mustang GT Join Date: Dec 2009 Location: SE Michigan Posts: 1,682 iTrader: 0 reviews	have you changed the fuel filter? common gas mileage robber... __________________

October 23rd, 2010, 10:12 PM	#4
MustGT98allday Hardcore Enthusiast 1998 Mustang GT Join Date: Aug 2010 Posts: 1,282 iTrader: 2 reviews	what and where is the egr valve and I changed the fuel filter a little bit after i bought the car I even notice my car being alot slower than it was about a year ago. IDK it seems like my car is just going down hill __________________ pi swapped and dropped sn95

October 23rd, 2010, 10:12 PM	#5
MustGT98allday Hardcore Enthusiast 1998 Mustang GT Join Date: Aug 2010 Posts: 1,282 iTrader: 2 reviews	and forgot to mention it has abotu 120000 miles __________________ pi swapped and dropped sn95

October 23rd, 2010, 10:34 PM	#7
MustGT98allday Hardcore Enthusiast 1998 Mustang GT Join Date: Aug 2010 Posts: 1,282 iTrader: 2 reviews	ah...i think i know what ur talking about __________________ pi swapped and dropped sn95

October 23rd, 2010, 09:53 PM	#2
Hotshot_98GT Regular 1998 Mustang GT Join Date: May 2010 Location: Gastonia, NC Posts: 259 iTrader: 0 reviews	Go to autozone and get them to scan the computer. Or if you have an OBD2 scanner scan it yourself. might be O2 sensor. Also the EGR valve may be stuck. Sometimes it wont show up in the computer. Havent had that problem on my stang but my trucks mpg dropped horribly like that. All i did was take the EGR valve off, clean it with some carb cleaner ,and put it back on with a new gasket ($2 @ autozone). Might not be whats wrong with it but hey its a cheap guess, and only takes about 20 minutes. Hope this helps

October 23rd, 2010, 10:28 PM	#6
Hotshot_98GT Regular 1998 Mustang GT Join Date: May 2010 Location: Gastonia, NC Posts: 259 iTrader: 0 reviews	Its the pancake looking valve attached to the passenger side of the upper intake plenum. i'll try and find a picture or something

October 23rd, 2010, 10:41 PM	#8
Hotshot_98GT Regular 1998 Mustang GT Join Date: May 2010 Location: Gastonia, NC Posts: 259 iTrader: 0 reviews	The blue circle is the EGR valve and the 2 red circles are the 2 bolts that hold it on oh and the hose on the back may have to be unhooked but i aint sure Uploaded with ImageShack.us

October 23rd, 2010, 11:09 PM	#9
MustGT98allday Hardcore Enthusiast 1998 Mustang GT Join Date: Aug 2010 Posts: 1,282 iTrader: 2 reviews	sweet...Ill check it out thanks I was going to replace my fuel filter but i couldnt get the old one out while laying on my back so ill do it when i can get it on the lift. Also i might be getting a pi intake and do that so that will give me a chance to make sure its done right then for christmas hopefully i can get a tuner __________________ pi swapped and dropped sn95

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