I have had a 2008 Mazda 6 diesel for a year. The fuel consumption in general is very good until the car decides to rev higher than normal. I keep the computer set for average fuel consumption because of the infuriating problem the car has. On a long journey the mpg gradually increases and then all of a sudden it starts to drop. This can occur after a hundred miles or so and there is nothing I can do when it happens. If I drop the engine into neutral the engine will tick over at approx 900-950 rpm. If I set the computer at present fuel consumption the car is shown as running at 27-30 mpg at 60 mph. This happens about twice a week. When I took the car back to Mazda the car was checked out and no faults were found .... because it behaved itself (typical).
The car is driven fairly steadily in general. Has anyone experienced a similar problem?
Thanks in anticipation. Bill Jones.
Edited by Pugugly on 23/04/2009 at 00:42
|
Could it be the DPF regenerating? On my new Mondeo, tickover when the DPF is regenerating is slightly higher than usual, and sounds more "thrummy" than normal.
Also when "actively" regenerating, the ECU injects more fuel to initiate and sustain the process. This would account for the dip in economy when cruising.
A full regen. can take around 5 minutes, and will normally happen every 200 - 250 miles or so, but this will depend on the type of usage, driving & load.
One thing I have found with my Mondy is, although the car WILL pull 4th gear at 30mph, this equates to 1400rpm which puts the motor under load = more soot = more buildup in the DPF.
I have had far fewer renegerations by staying in 3rd gear within 30 limits. This puts rpm at around 1900, which is nicely in the peak torque band, which means exhaust gas temps stay high, which in turn supports continuous "passive" regeneration.
As a result I have adapted my driving to keep the revs as much as possible when cruising within the engine's peak torque range of 1800 - 2400rpm. With a 6-speed gearbox it isn't hard to do this.
Below this rev band, I don't believe the exhaust gets hot enough to sustain "passive" regeneration, which in turn leads to the "active" process that you are seeing. The active process demands extra fuel.
Edited by craig-pd130 on 22/04/2009 at 09:15
|
Hi all
Bill not sure if you have the the model later than me I have 06.
To be honest I don't even bother looking at what mpg says but I agree with Craig keep your rev's up and don't try to get into high gears to soon.
These cars with DPF need work and rev's the harder, longer you drive them the better they run !
Make sure your oil levels not to high it could need changing.
Engine rev's on my 6 at tick over never appears to change.
|
Thank you for your replies. Very helpful. My model is the 08 new shape. The engine is basically the same but tweaked for CO2 emissions.
I'm pretty sure that you are both spot on with your replies. I have already started to change my driving techniques and hopefully it will pay off.
Many thanks. Bill.
Edited by Bill Jones on 23/04/2009 at 23:51
|
|
Also check oil levels. They rise on these engines. Yours in the new car is a slightly less powerful version so might not be affected quite the same. I assume you know there is an Empty, Full and X on the dipstick.
|
|
|
|
Back to square one. I drove to Bournemouth this weekend. Long stretches of fast roads. 180 miles covered. The car averaged under 30 mpg for the first 14 miles and the same problem occured half way back in the evening.
Edited by Bill Jones on 27/04/2009 at 09:24
|
|
|
|
|
|
Is it perhaps nothing more serious than a quirk of the trip computer, which are notorious for being inaccurate? The only way to calculate fuel consumption is by recording the total amount of fuel used over a reasonable length of time/mileage, say one month or a thousand miles minimum.
|
What is fuel consumption is the car actually giving you, ignoring what the computer says?
It is irritating if the computer is showing erratic figures, but the real test is how much fuel it's really using.
|
Hi all
On my 6 diesel it's about 40 to 43 mpg brim to brim most of the time but if I go on a long run towing van it's nearly as good.
All the other car's I've ever had towing dropped it to 25-30 mpg admit they were petrol.
Must be due to not going much over 60 towing but I still keep rev's over 2000 even if it means dropping a gear or two.
|
|
|
What is fuel consumption is the car actually giving you ignoring what the computer says?
But it's not only what the computer is telling me. Even after 60 or 70 miles, when the car is stopped, the car will tick over at 950 rpm, or 200 rpm above normal. This cannot be correct.
I have checked the accuracy of the computer. I filled the tank to it's maximum, drove 500 miles and then refilled the tank once again. I calculated the figures and they tied up with the computer within a gnat's whisker.
|
|
|
|
|
