I have racked up over 80k miles on my 93 Allante with the Northstar engine. Recently the fuel economy has dwindled to 13 mpg when the ac is on. It has never gotten such poor mpg. When the AC is off it returns to a respectable 23 mpg. In my attempt to understand what is going on I have driven around with the instant economy display on - it plainly shows dramatic increase in mpg with the ac off. I have also noticed that when accelerating with the ac on and you let off the accelerator the mpg does not bounce back at all - also while cruising the slightest amount of acceleration will cause the instant economy to plummet. I have replaced an exploded intake plenum, a radiator, and a litany of other items(I understand the car)- the ac compressor is two years old. Is there some sort of electronic control that disengages the ac clutch- I know this sounds weird and I am a reasonably accomplished Allante mechanic but I fail to understand what it going on.