Mike263

Over the last week or so I've had a big drop in mileage. Both a traditional check - fillup to fillup and the computed mpg on the dash show a 2 mpg decrease, driving around town. I went from 15 mpg to 12 mpg.
Can't figure out what caused it.

Since the drop I had to replace the MAF, O2 sensor, coil, dist cap, and rotor (due to other issues) None of this affected the reduced mpg.
The drop occurred just after I installed a new rear y-pipe and MAD "Monza" mufflers. I don't see the connection.

Any ideas?

Oh, and no codes, no black smoke - just a blue gray puff at startup.

drive it

Opening up the exhaust will usually lean it out a bit-of course that shouldn't affect mileage in closed loop. Are you sure its going closed loop? I've seen 'em stay open loop and not set a code. Check the fuel press.-if its too low it will give you a lousy spray pattern-droplets instead of mist and that can drop it. Also how many miles on it? If the injectors are getting clogged that will also louse up the spray pattern-try some injector cleaner. Hope this helps....