93 4X4 3/4 ton xtnd cab, after having new Y-pipe made, new Cat(high flow), new exhaust(flowmaster), my mileage dropped from 10.5-11.0 to around 8.3mpg.

Also at the same time I replaced fuel filter, plugs, plug wires, rotor, rotor cap, air filter, and ran pint of injector cleaner thru gas tank.

It has loads more power now with the new exhaust, but mileage sucks.

I even drove like an old granny for a couple tanks to make sure it wasn't just me liking that power and the new sound.

Right now it's in the shop getting a new vacumm chamber that mounts up on the heater somewhere. Shop said that's why A/C Max would divert air to Defrost sometimes while driving. Could this cause lower mpg?

They hooked up the computer and said that they are getting a "full data stream" and everything looks good. Not sure why my mileage went way down.

I know 10-11 mpg is pretty bad but, 8 mpg, somethings gotta be wrong.

Have any of you guys experienced lower mpg by reducing exhaust back pressure like I did. I didn't think it would go down, if anything I thought mpg would go up just a little.
Confused, any comments, suggestions, empathy would be greatly appreciated. I know the turbine heads need to eat too, but damn. 8 mpg.

 

What size engine? Everything tight and in place? Any other things that are out of the ordinary like a high idle, smoking, ect?

 

Where did you put the o2 sensor?
Tex

 

Did your weather take a sudden turn for the cold at the same time? The cold weather here in SD is sure killing my MPG.

 

Sorry, was in a hurry to go to my sons B-Ball game.

It's a 460, yes everything appears tight and in place.(not sure what ya meant by that though).

No smoking or fast idle, been driving this truck for ten years now. Has 91,000 miles. Been driven in all kinds of weather, but has never gotten this low of mileage, unless I have the camper on and pulling the boat going over the pass.
The 8 mpg was completely empty, but , yes the weather was a little colder, nothing extreme though. Maybe 50's to lows of mid 30's.

The O2 sensor was relocated but is still upstream of the CAT. A couple inches from original spot, except at about 3:00 or 9:00 instead of 12:00.

Sorry, it took me a while to get back.

 

Have you pulled your codes? If not go to fordfuelinjection.com and you will see how to pull and read codes. If you have Haynes manual #36058 you can look them up. Check your fuel pressure regulator and see if there is any gas present in the vacuum line, sure sign of failed regulator. Look at your ignition system: cap; rotor; plug wires; etc. Check your air filter. You have several options from there, but let's baseline it before spending money! Oh, have you checked your tire pressure, should be 50# cold. Sometimes the onset of colder weather can affect true tire pressure. What kind of tires are you running? Let us know where you are from the codes and we have a starting point. I'm getting an average of 11 to 12.5 at this time and as it's a 460 4wd I'm okay with that. Last improvement change was 285/75R16 tires which helped my final gear ratio for mileage. Best mileage improvement under load, replaced steel 26' gooseneck with a 16' Featherlite aluminum three horse trailer.
Tex

 

Just got rig back from shop, mechanic said he took it ouit for a drive with the computer hooked up, checking the data stream.
The water temp dropped like a rock when he started driving, but would be fine in the shop at a rest. Replaced the thermostat, will check a tank or two and see if my MPG goes up. He also replaced a vaccum tank up around the heater somewhere. Said this was causing my A/C to stop blowing sometimes when on Max A/C.