Ok I need some moderate help
 

Ok, first of all I have an 89 mustang LX 5.0L with an AOD tranny. I use it as a daily driver. I replace the whole exhaust system with Flowtech full length headers and Flowtech off road h-pipes. I have Flowmaster catback exhaust also. Now for my problem. I first install the catback exhaust. It ran fine and got better gas mileage. As soon as I put in the rest of the parts I went from 17mpg to 14mpg. What went wrong? I had the computer tested for errors and none came up. I think it is running too rich because it starts alot rougher almost as if it were carburated. It has never stalled though. I will say that it sounds great. I have many other aftermarket performance parts on it but they were installed way before my exhaust. What do u guys think?

Whats the diameter of the pipes if you went bigger that 2.5 you may not have enough back pressure.

They are 2.5" Catback and 2.5" h-pipe with no cats. Also have 1 3/4" or 1 5/8" full length headers. Could it be the oxygen sensors? or what

If it was the o2 it should give you a code. Take it to a dyno shop and have them look at the fuel and O2. Have you been stomping on the gas more than usually to hear the exhaust? You may just need a tune. Take a look at the spark plugs.

Hmm I will take it to a Dyno shop. Do u think they will help me because I have no emissions on my vehicle? Or should I just buy new o2 sensors because it will run me the same as going to get it tested. The o2s are supposed to be changed every 100k. Whats your thoughts? By the way what color am I looking for on the spark plugs? There Bosch Platiums.

Check this out http://www.centuryperformance.com/spark2.asp

This will probably get some arguement but when you removed your cats you sped up the flow past your o2 sensors that in turn will let your air fuel ratio go up a little especially if you have a high flow air filter or cai because the o2 sensor does not get the same reading it used to get also it doesnt heat up as fast.its not unusual for gas mileage to go down a little after heavy exaust mods,but the extra horses are worth it.

Do you think that there is a simple fix to this air fuel ratio problem. I'm having a friend at a Ford Dealership put in a reastat so I can manually change this. Whats your thoughts?

Best thing is to stop guessing and see whats going on.

A chip tuned for your mods is the only thing i know of.I'm not familiar with the device you mentioned but if you can manually adjust your air/fuel some that will definately help.That should get your gas mileage back.be carefull not to get to lean make adjustments slowly and make sure you run the engine under load (a/c on for instance)and listen for ping.did you disconnect the negative cable from the battery when you did your mods????