need advice, sudden drop in mpg

[BlackBeauty]

Hey everyone, it's time to turn to this board for help. I've been consistently getting 16.5mpg's in the city, 19hwy. I track my gas mileage for the heck of it with every fill. Starting this past Dec. I've been recording 12.5mpg city. I did not make any changes at the time with tuning or modifying. An interesting thing is, my gas mileage has always been consistent granny driving vs. driving hard. During one tank, I took the car to the track twice and drove around with 18psi in the DR's the whole time, and still got 12.2mpg!
Anyway, I did a search on here and couldn't find anything to help me. I'd appreciate some advice. Car has a Walbro 307, afpr, and bluetops for fuel. I was running an RA93 chip, in Jan. I installed the Trans+, LT1 maf, and extender chip. The drop in mpg started before the maf/chip change, I was hoping I could wait it out and that these new parts would help it. Didn't work.

-O2 cc's avg. 20 updates on the ScanMaster (~30sec. to go from 0-255)... o2 sensor was replaced last year
-BLM's seem ok, I think they stay around 119-125.
-injectors have about a year or two of use, so they are somewhat new
-fuel filter was replaced sometime last year
-fp jumps up when I pull the line off the afpr, so I'm guessing (hoping) no vacuum leaks
-engine reads vacuum (~15 in park, ~10 in drive with brake)
-maybe mpg's are ok, and it's my odometer that's wrong??? I have a second car though, so I wouldn't be able to recognize more or less frequent fill-ups
-NGK UR5 spark plugs, put in last year

Thanks for the help.

 

[BlackBeauty]

TTT.

Just calculated 12.9mpg for this past tank.

 

[gokada]

Try a solid tune up. Change the plugs, wires, and fluids and filters. Make sure your tire psi. is up to spec too. I would suspect your tire psi to be the culprit. If all else fails, stick a block under your gas pedal. That could take care of it too.

Post and let us know if your milage gets better.

 

[azgn]

try another 02....you are using a Delco or Denso, right?

 

[BlackBeauty]

As mentioned in my first post, I still get 12.x mpg with granny driving (granny's don't boost at all). I believe the o2 sensor is a Delco, I got it from GM discount parts some time in 2002 or 2003. I've never run race gas. From the above info, do you think the o2 cc's are too slow? Thanks.

 

[scottyb]

Is the torque converter still locking?

 

[azgn]

maybe you have a bad injector?

pull the dipstick and take a whiff of the oil......smell gas?

02's are cheaper than gas...just replace it

tune it up as suggested

 

[cathan]

Regardless of cross counts......change the o2 sensor. Let us know if there's a difference

 

[87gninpa]

definately check the 02 sensor and if the converter is locking properly, that will really kill your mpg. also mine did the same (about 12 mpg from 17) when my ign module and coilpack were on the way out. best bet is to have them tested.

 

[BlackBeauty]

Thank you for the replies everyone. You are right, a new o2 sensor will quickly pay for itself if that's the problem.
I was hoping it was not a leaky injector, since they have low miles on them.
Coilpack and module were recently replaced as well.
And yes, the convertor is locking right around 45mph during cruise.