Yaris fault???????

[StuartM]

Hello all.
I have been experiencing a problem with my 1.4 D4D TR since new.
Frustratingly, returns to the dealership have not provided any kind of explanation.

The "problem", if it is a problem, is cyclic. For the first 6 or 7 months, about every 300 miles, usually per tank of diesel, the instant economy figure drops, it will not get above 50 mph, whereas it would be running happily on A roads at 75 mpg. This phenomenon will last for exactly 15 minutes and has the effect of reducing the overall mpg figure, for the fuel used to date, from maybe 70 mpg to possibly 59 mpg.
The other symptom of this problem is increased engine idle speed, up to 1000 RPM from the usual 950(?).

Despite a couple of returns to the dealership, I cannot get any explanation for this fault.

I had it serviced recently, and the cyclic event is now occurring every 200 miles! Plus, the service light developed a life of its own, and decided to light up sporadically.
Again I returned the car to to the Dealership explained the problem, complete with emailed explanation.
They found that the oil had seemed to have been over filled (or increased somehow) drained and refilled it & asked me to try it for another 500 miles.

It is no better, the diesel guzzling continues in the 200 mile cycle.

I had a D4D Auris for 3 years prior to this, no such problems with this car.


Can anyone please throw any light on this, before it drives me completely mad???