Toyota Repair: Toyota Previa 2000 VVTi Automatic, air fuel mixture, fuel mixture ratio

Question
Hello, i got a problem with my previa, my dealer don't have a clue about the problem.

Parts that have been changed, ignitioncoils, airmassmesure, catalyc, co2's.

The problem is when driving between 30-80 km/h, and release the gas, the engine rpm drops to 500, and the car kind off enginebraking for 2 sec, and the goes up to 1500-2000 rpm.. if i shortly push the gas pedal, the engine again goes to 500 rpm, and 2 sec after goes up again to 1500-2000 rpm.
This courses the car to enginebrake every time i push the gas. Just like i put it in 2nd or turn of the overdrive.

My old previa year 1995, didn't make this rpm drop, when releasing the gas it dropped tol 1500-2000 rmp, when releasing the gas.

My previa also got a button at the dashboard, for cold winter, this makes the engine run faster at the start, like a shokker, when this is activated, the problem gets even bigger 2 sec goes to 3-4 sec, and some times it drops to 500rpm, up to 1500 rmp, and down, up several times, when releasing the gas.

Sorry for my bad english, hope you can undestand my problem.

- allan  

Answer
Well, I think when you release the gas. At that time the air fuel mixture ratio is not correct. You better check the Oxygen sensor and EGR valve which are mainly responsible for sensing engine emissions. It gives feed back to the ECU which controls the whole system. You didnt mention, but if rich mixture is going to the intake then air fuel consumption will also increase and if a lean mixture is going to the engine then you will notice some increase in the engine temperature and as result increased NO2.