LB9 runs amazing. MPG dropped to 10.8
It always did between 12 - 14.9 MPG over the years.

It's 95% stock. I know I have 1 bad CAT, even though it's no longer rattling around. But it was rattling before I removed the AIR.
What else would make MPG drop ???

and no, it's not the tire pressure.

 

Improper timing, sticky fuel injectors, plugged EGR/PCV, bad plugs are all items to take a look at. A plugged cat certainly is likely to be the main culprit. The other cat is probably not far behind and can't keep up.

 

Any number of things can cause it. Get an ALDL to USB cable, get the free TunerproRT download and start looking at the ECM data. Otherwise we're all just guessing.

 

^That won't tell him much about the cats. If he already knows the cat(s) are bad, then he should replace the cat(s) and reassess.

 

I definitely had 1 rattling cat (N10). It stopped rattling when I removed the AIR.
I should pull the plugs and see if they are different from right to left.

Now without AIR, I can pull the plugs in 10 minutes