My car judders when accelerating - could this be down to previous misfuelling? | Ask Honest John

My car judders when accelerating - could this be down to previous misfuelling?

On odd occasions my 2013 BMW X1 suffers the judders as I accelerate through the gears. As if it doesn't know whether to go on - but you can easily just accelerate through it. BMW can find nothing amiss. My own thoughts lead me to think it could have been misfuelled by the previous owner or allowed to run to close to empty. I've only owned it for three months. I have fuelled at different garages to eliminate any possibility of it being one fuel. There hasn't been any pattern, but it is a vague possibility that it does it at a very specific fuel level - being the mark under the quarter tank, which is when I usually fill up. There has only ever been one occasion that I have felt it possibly faltering in sixth gear. What are your thoughts?

Asked on 19 January 2018 by Sharron Kyte

Answered by Honest John

If it has been misfuelled the petrol does not mix entirely with the diesel because it is of lower specific gravity, so tends to 'float' until it is gradually absorbed into the diesel. The diesel pick-up is at the bottom of the tank, so I think what is happening is that when your tank is less than quarter full you are getting a greater concentration of petrol that has less lubricity than diesel and is causing the hiccup. A diesel additive may help, but you have found the solution of maintaining the tank above quarter full and probably need to do that for another 1000 miles or so.

Tags: misfuelling technical issues juddering

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Related models

BMW X1 (2009 - 2015)

Compact off-roader available with two or four-wheel drive. Car-like to drive. Economical diesel engine with various power outputs. Usefully large boot.