Hi Folks, car (2.5v6 35k miles) has started to feel slightly lumpy when cruising at low revs.

Any suggestions? Thanks

I would have though as a rule a diesel couldn't misfire?

There's no spark plug to missfire, diesel just combusts under pressure.

So it's either going or it isn't?

The only thing I can thing of that would cause similar symptoms to a missfire is maybe injector trouble or compression being off causing the diesel not to combust?

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Edited 18 August, 201213 yr by Alan16ac

If one cylinder or more isnt firing then that is a misfire.

Need a fault code check, does tank level make a difference.

If one cylinder or more isnt firing then that is a misfire.

Need a fault code check, does tank level make a difference.

My first diesel car so put misfire? as that what it feels like.

I will get a fault check done (sadly I have wrong VAG-COM).

No difference with fuel level, usually fill up when half tank showing.

Thanks

If you've been using cheap supermarket diesel I would suggest trying full tank Shell V or BP Ultimate.

If you've been using cheap supermarket diesel I would suggest trying full tank Shell V or BP Ultimate.

Thanks for the tip. This may be the cause, I have been using supermarket fuel and recently changed to Morrisons.

I thought these big lumps could run on anything.

All I know is when I moved over immediately noticed my 2.5 more responsive and smoother. Don't touch anything else now. If car press right should also help engine longevity and maybe keep dodgy VAG sender unit/fuel pumps going.

I have used supermarket fuel (diesel) for all my cars for over the last 15 years with no trouble at all. It is the cheapest and I can use my 5p off vouchers and get points.

Its all a myth. There's nothing wrong with supermarket fuel!

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Its all a myth. There's nothing wrong with supermarket fuel!

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This is one of those each to their own things. All I can say is when my fuel tank sender unit failed at 32,000 which then caused fuel pump failure the Skoda dealer's first question was did I use supermarket diesel. Then answer was yes. Also Daily Telegraph Motoring section has regular supermarket fuel stories. Not the mpg benefits as these are neglible but engine smoothness, engine/fuel system longevity. All I know will never go back and keep on enjoying that extra creamy smoothness http://www.honestjohn.co.uk/askhj/answer/24490/supermarket-fuels---are-they-that-bad-

Put Shell V in today.

Car felt more responsive but may be placebo effect. I will know better with some motorway runs (at legal speed) over the next few days.

Supermarkets just buy off excess stock from mainstream refineries, as the refineries are geared up to run at certain daily fuel volume and this is hard to adjust and makes more economic sense to just sell-off excess at short notice to supermarkets, sometimes even at loss.

So you do sometimes get Shell fuel (which is actually most of the time Esso) branded as Tesco, JS, or whatever. The problem that MIGHT come out of this is that different refineries/ grades of fuel use different chemistry for the additives. These over time settle down, and if tanks run low they can get picked up by the pumps - this is worsened by the fact that different fuels use different additive packages and there MAY be long term compatibility implications between the different additives (more relevant for petrol heads than diesel drivers).

Talking of diesels, what makes the BIGGEST difference to perceived fuel quality is the cetane index improver additive - most fuels need boosting up to EU cetane levels, even more so in winter times. "Non-fossil" diesel (GTL - gas to liquid) are naturally high in cetane number, and contain no sulphur so are good with emissions, but lack of sulphur then presents other issues related to poor lubricity - again easily resolved with an additive. All of these fuels are good, it's just that supermarkets buy whatever and whenever so the different stuff gets mixed up with its inherent variability you get inconsistent engine behaviour and hence poor perceived quality. This weekend I stopped at a fuel station (ARAL) in Germany, and they have pumps where you can see the fuel going through a little tell-tale window, and normal diesel was yellow and super-diesel (probably also GTL) was all like water clear. But at 1.70 euros/litre (normal was 1.60) all a bit too expensive to try out - but the pump claimed 4% better mpg, which accounts for around 1/2 the additional cost.