I put a quarter of a million, near trouble free, miles (water pump, alternator & 4 batteries) on the truck when, after leaving it set for about 3 months, it started to fast idle and dropped from 18 mpg to 15 mpg. The check engine lite comes on and goes off randomly. I've had 2 mechanics look at it and they both said that if the lite doesn't come on they don't know what to do. I changed the temperature sender. That's all I know to do....help!!