In TECHMAIL, we answer your motoring queries…

I own a 2009 Mazda CX-7, which I was very happy with until I had it serviced at a Mazda dealership in December 2015. Since then, the vehicle "runs on". In other words, when I take my foot off the accelerator, the vehicle does not slow down immediately. It appears the engine receives fuel and starts to wind down only after a few seconds.

My fuel consumption has also increased quite a bit and, despite the three occasions I have returned the vehicle to the dealership to remedy the situation, the problem still persists. From the digital readout on the dashboard, I have also noticed that the vehicle is still using fuel while slowing down in certain instances – even going downhill.

In modern vehicles, does the ECU not shut off the fuel to the engine when your foot is off the accelerator before coming to a stop? Your assistance in this matter would be greatly appreciated.

John Rodger - Table View, Cape Town

On modern drive-by-wire systems, to improve the smoothness of the drive during acceleration and deceleration, the electronic throttle input can be slightly modified via the engine control unit (ECU). To lessen jerkiness, this calibration essentially ramps-in the fuelling to the engine when it receives a sharp accelerator input (step input). This ramping-in of fuel takes less than half a second, after which the full fuelling demand is met.

Similarly during deceleration, the fuelling is not immediately cut after the driver has taken their foot off the accelerator, but rather ramped-out to again lessen driveline shunt. This process is also completed in less than half a second. This calibration technique, however, should not be detectable and definitely not last for more than half a second. You are correct that a modern engine cuts fuelling on the overrun, but also remember that the digital fuel-consumption readout in the dash may not update quite fast enough (so it may still show fuel usage when there is none).

You mentioned that the vehicle started to behave this way only after the initial service. Find out whether the dealership downloaded a new version of software to the ECU that could have altered the tip-in and tip-out calibration. If not, there is another problem.

We would investigate the signal from the accelerator during the tip-out event to ensure that it indeed goes to zero in less than half a second. With the correct diagnostic tools, it is also possible to evaluate the fuelling signal (injection quantity) of the ECU and, if it responds in correlation with the accelerator signal, should allow half a second for the calibration technique.

If all appears in order, focus on the injectors themselves to make sure they shut off in time, as per a command from the ECU. The vehicle cannot accelerate (or continue at constant speed) if no fuel reaches the combustion chambers.