For some reason, my fuel effeciency has gone down alot. Im barely getting 150 miles on half tank. I noticed this after I took off my air intake housing to replace my light bulbs.
Connections are all good. Anyone else experience this before?

 

What are you averaging on a full tank?

 

I was getting around 385 miles on a full tank. That has dropped dramatically to about 290-300

 

how many miles? I ask because of spark plugs...but that wouldnt result in a sharp decline....unless one started misfiring. But that'd probably cause you CEL to go on,

Maybe you got some some dirt in your MAF your intake removal? Again, could have caused the CEL to go on, but might be worth hitting it with some MAF cleaner.

 

I wish I got 385 per tank! I only get like 280-270 per tank. Running 93 octane. I'm doing 100% city, and I fill up ever week and a half.

 

I average around 300 in mixed city/HWY spirited driving.

 

Im nearing half tank and i only hit 120 miles. While was swapping out my bulbs, I forgot the air cleaner was off an tried starting the car. well, it didnt start and I realized i had the airbox off. When I reconnected everything, I had a warning light on the dash, I forgot exactly what it was but after a couple minutes, it turned off. Could that have anything to do with it? If i remember correctly, I think the warning message was ECM or something like that.

 

385? holy crap that is amazing. I was getting 365ish on a tank, now I get between 340-350 with mixed driving. I assume the decrease is due to the cold weather gas mix.

To address your issue, disconnect the neg terminal on your battery for 10 minutes. It should reset everything.

 

Agreed, hopefully that helps. Double check the MAF sensor is plugged up in good too.

 

have you changed anything? Wheels, tires? Also any changes in driving patterns?

 

mine decline right after I got full synthetic oil

 

385?? holy cow! I average 310 mile a tank suburb driving