The gas mileage on my '99 GT suddenly went from about 15-18 down to about 11, and the engine is bogging down when I come off the throttle and I'm still in gear. I changed the fuel filter and ran some fuel system cleaner through it and no change. The problem seemed to start about the same time my alternator went out and did a number on my electrical system. I think it may have done something to the control chip... is this possible? Has anyone had a similar problem? She sounds fine, and has plenty of pickup... but the gas is killin' me. Any ideas or help would be greatly appreciated.

~ Glenn

 

try looking around here: www.mustangforums.com

 

yeah it could have done a number on your ECU. i would go and have the car scanned for any dtc codes and see where that gets ya

 

I had a similar problem, but it was on my Ranger, I was getting about 18mpg, then out of no where I was getting like 15mpg. I eventually found a disconnected wire. A wiring harness had some how come loose from the bottom of my fuse block. I put it back, and the truck is running up to par again. Just out of currosity, has your check engine light come on?