Originally Posted by jfitch
My main problem is that the car will randomly shift out of overdrive and won't go back in. This causes the car to suck gas. I'll be going down the road at 65 and ill feel a slight jerk and the car'* tach will read 3100 rpm. With overdrive the car will be at about 2000 rpm at 65 mph. I've been getting terrible gas mileage because of this. When i get home and shut the car off it'll work right the next morning for a little while and then do the same thing again.
A partially-blocked catalytic convertor could be causing this. Excess backpressure kills intake manifold vacuum, and the slightest touch on the gas pedal causes TCC lockup disengaging and kickdown as you describe. Mine did this exactly as you describe, right down to the way it would be fine for a while when the car was cold, then do the same thing again after the car had warmed up.
The symptoms will get worse soon enough if that'* your problem. If the car still has its original convertor, I'd say it'* on the way out now.