Hi,

I bought a 2015 Mazda 6 2.2 D Sport in July 2021, and over recent months have seen some strange behaviour in the MPG readings, which suggests my car is intermittently using much more fuel than usual for no obvious reason.

I probably average around 50-55MPG between city and motorway driving. I feel like I have adjusted my driving to get the best economy from my car.

However, every now and then (I'd guess every 150-200 miles), no matter how efficiently I drive, for about 10-15 miles, the car absolutely drinks fuel. I probably get consumption as low as 30-35 MPG. coincidentally, when this happens, my i-stop also stops working.

Could this be a sign of my battery beginning to die, given it could be charging itself up again, or perhaps a faulty alternator? I'm not sure how old the battery is, as I only bought the car 7 months ago. It's been regularly serviced at Mazda since birth, and I had it fully serviced as recently as a month ago.

All of my journeys are motorway based and I use BP fuel at every top up.

I found a similar thread below, but it seems dated and nobody has since posted a useful answer.
https://www.mazdaforum.com/forum/maz...rt-2-2d-39199/

Any ideas folk?

 

What are you using to calculating your MPG?


Fuel fill up v's mile travelled? or the dashboard readout?

 

I am referring to the dashboard readout. I am reading both the overall average MPG calculator, and the "live" MPG usage display. Both drop dramatically during this high usage period that seems to come every 150-200 miles.

 

Most likely a DPF burn.

The dashboard display isn't accurate, too many influences. Measuring MPG or L/100km via a full tank tank of fuel and distance travelled is the only accurate measurement.

 

Your car has a Diesel Particulate Filter (DPF). This is a ceramic honeycomb in the exhaust which filters the particulates (soot) to lower emissions. Like any filter, it becomes choked after a while so there is an automatic cleaning function known as Regeneration. The system can detect when the DPF starts to become blocked, this is usually every 150 to 200 miles as you have found and will vary with your driving conditions. When regeneration is triggered, the injection system shoots some fuel into the cylinders during the exhaust stroke. this is carried by the exhaust gas into the catalytic converter, which usually runs at around 900 degrees Celsius. Cats love raw fuel - they burn it ferociously and the internal temperature rises to around 2000 degrees. The superhot gas then passes to the DPF further downstream in the exhaust and burns up the soot deposits over the next 10 -15 minutes, until the DPF pressure readings confirm that it is clear for another 150-200 miles. That's why the onboard MPG display plummets at regular intervals, it's telling the truth.
These regular gas-guzzling sessions do bring down the average MPG. I ran two Mazda3s as company cars, identical apart from a DPF on the second one. The consumption was 4 or 5 miles per gallon worse on the DPF-equipped car.
Disagree with the previous comment about the accuracy of the onboard MPG display. It's been spot-on in all the cars I have run and tested when compared with tank-to-tank checks.

 

Yes I have an identical issue in a diesel Mazda from 2015. Identical symptoms, istop stops working for no reason whatsoever (it’s not the aircon, battery, windscreen heater, steering wheel position etc etc) and the mpg goes super low for a while for the exact time that istop is out for. I’m actually averaging 77 mpg normally but I’m down on 25 or 30 for the duration of the issue.

I’ve not figured it out yet but it might be the engine heating up and burning off the soot from the diesel combustion. Mazda do list that as a reason for istop to stop working for a time on the diesel version.

https://www.autobutler.co.uk/wiki/dfp-regeneration

Did you resolve it?