Sproradic very poor MPG/iffy running

[Rodders901]

Hello

I drive a 2010 Mazda 6 Estate 2.2 ts2 diesel manual. Its high mileage at 71k, but has been running tip top with an average MGP of over 46mpg, with with both daily town and motorway running. In the last couple of weeks, I have noticed that it sometimes sounds very rough and on the motorway at steady 70mph, I am getting instant 20mpg instead of around 50-60mpg when it was running fine. After a while it seems to go back to good running and expected MPG. There is no obvious mis firing. I had wondered if this was some passive DPF regen thing, but I have no DPF lights on or flashing at all and would have expected this if it was a DPF issue. Before I book it in, has anyone else experienced this? Is it worth putting in some additives to clear it out or that expensive diesel stuff-or will this knacker the DPF as the manual suggests it would.

Look forward to any knowing replies....

P

[678hug]

I never look at the instantaneous fuel consumption, but my running average dropped a fair bit at around the same mileage.
The diesel fuel filter is replaced at the 72500 service  and my fuel consumption went back to normal after this.
It was pretty dirty when i replaced it.

Hopefully it will be a similar easy fix for you

[gfitches]

Agreed - I had a significant drop in fuel economy and thought I'd change the filter as a quick and cheap test (look at 'car parts 4 less' or 'euro car parts' and they start around £5).

The fuel I poured out of the filter I left in a jar and after a couple of days there was quite a thick layer of sediment at the bottom.

Once changed the economy went back to normal.

Could have been a coincidence, but it seemed to run smoother too.

The job is quite simple, but you will need to take the battery off to get at the filter and it's a bit fiddly to get to. Given the effort it took to get the filter off the casing I would say it had never been changed (although I'm sure I have paid for it to be done twice!)

Good luck

Garry

[678hug]

My post above should have said it's on the 75000 service not 72500.
Anyway, if you are interested in doing it yourself, here's a guide i did a while ago

http://www.mazda6forums.co.uk/index.php?topic=1667.msg6478#msg6478

[Rodders901]

Thanks for all the advice. changed the filter. A week later (today) nice hot day, while going down the motorway at normal cruising speed in 5th, slight cough and back to 20mpg on the motorway and much rougher run.....I am sure its the dpf regen kicking in, but on Weds I did a 40min motorway drive and back again and it ran like a peach. V quiet and my av mpg went up to 47...in just one run home tonight its using so much in one 25 min run its knocked the av back to under 46...

Very very odd. Puts me right off the car!

[markiboy44]

have you sorted this out yet?

mark

[Rodders901]

No. Once a week or so it does this...wondering if its a turbo issue? Runs badly but does run. Getting it serviced and diagnosis next week......

[markiboy44]

interested to see if they find whats up. good luck.


mark

[markiboy44]

did you find your problem?

mark

[Dave467]

Sounds like dpf regen to me.  When it happens on mine I feel a slight judder the mpg goes right down and if I put the clutch in while driving revs don't drop all the way down, instead they sit just below 1000. 

[Rodders901]

Hi all

Had a basic service done last week. No faults at all registered (good news) and had oil done, etc. In terms of running, just that made my little car run so smooth it was amazing. Then a week later, slight cough and back to silly mpg and rougher run. Stopped after about 30miles then fine again for 4 days. Did it again today....really annoying.

My mazda independent chap says he has heard there is a little tiny spring somewhere in the depths of the engine that gets stretched and thinks this could be the issue. Its not a DPF thing. I have never seen the DPF light on and I am on the motorway everyday, abiet not for terribly long.

Google research seems to turn this up as a fairly regular issue for the 2010 era diesel car. Maybe I will have to live with it......

[Dave467]

Definately sounds like the dpf, the dpf won't come on if the regen is successful.  When theses dpf's get loaded with soot the regen seems to clear it a little but only for a while.  Perhaps take it to a garage and get a forced regen done on it then see how it goes.

[Rodders901]

Hi Dave, thanks for that; I often wondered if it was the dpf, but thought it wasn't due to no light flashing. On reflection, I am starting to think my sort of mileage is not good enough....on m/way every day, but only for 10-15 mins....that may explain the nearly weekly need to have a quick regen? Though three times in one week is taking the mick!!!

[Rodders901]

Another good point Dave is what you said about idle....mine does just that. Idles a little faster than @700rpm more around 800-900.

[Dave467]

what I found was that my car was doing a regen a lot when I was trying to get in as high a gear as possible as quickly as possible in a bid to help MPG.  I was putting it into 4th gear at about 30 - 35 mph, i have changed my driving slightly to stay in 3rd a bit longer and 4th etc as well.  I also found that driving motorways, instead of moving into 6th I would stay in 5th to keep the revs at about the 2500 range.  I found that it doesnt really hurt the MPG that much either.  Having the revs a little higher seems to keep the DPF running a little hotter and burning off the soot.  I have heard about the Wynns EGR cleaner helping the soot burn off at lower temperatures that would help drivers not doing a lot of miles but I havent tried it.