new cats now bad mpg

any ideas y this would happen

 

If you haven't yet, pull your cables off your battery and leave them off for a couple of minutes. Then reconnect them. You might get lucky and just needed to reset your computer. Takes a few days of driving to really calibrate your fuel system. And changing your cats really affects it for some reason. So start over when you change a big component.

 

this is the right idea, but a better way is to just remove the EFI fuse for 10-15 minutes so you don't have to reset all the radio stations again.

reset the computer and after a few driving cycles it will readjust the fuel mixtures to where they need to be and it will have good mileage again

 

Speaking of mileage, what is typical "good" mileage in a 3.4? I have an '03 dbl cab 4x4 that I recently changed plugs, plug wires, air filter, syn oil, Seafomed intake and cleaned MAF; best mileage I've had is just under 16mpg mixed city/highway. I'm not an aggressive driver but am not going 65 in a 75 zone either. Truck is weak on hills and I've read this can be an indicator of bad/plugged cats.

 

I get 19/23 with my 4 banger so im guessing a 6 would be slightly less, maybe 17/21

 

Well I'm getting horrible mileage myself. Pretty sure it's my cats. Just haven't got around to testing them yet. I'm getting under 13 now!! Keeps going lower and lower. I do have a lift with 33 12.5 15's and a bit of wind resistance. So I know that's playing a part, but not all of it. Last fill up was 12.63. Horrible. But you can test your cats a few different ways. Back pressure tester or even a vacuum test. Or read the temps with a laser thermometer I think can give a clue.

 

Thats about where you should be. Its an older truck so it's not going to get what the EPA rated it back in 01. I've been getting 16.x mpg average per tank using no-ethanol fuel. Original fuel filter and dirty air filter. Just got my fuel system cleaned a few months ago but it didn't really do much. I also have a heavier foot.

 

i was getting 21 mpg easy b4 cat replace ment but check enigne light was on

 

just checked today only got 16.2 if battery or fuse trick dont work. f150 here i come lol