A mystery problem with diesel is causing a wave of breakdowns across the UK, according to the RAC.

The issue is more prevalent in eastern parts of England and Scotland, with the greatest concentration of breakdowns occurring in the North East, and can arise from diesel purchased from any type of fuel retailer. The problem is not specific to any particular vehicle age or make.

The RAC says it is working with the fuel industry and motor manufacturers to help find the root cause of the issue which results in diesel fuel filters becoming blocked by the formation of a gel-like substance in the diesel. This can block the fuel filter thus starving the engine of fuel and may result in the engine not starting or in a loss of power. The amber engine malfunction indicator light might also show.

While the primary cause of the issue is still under investigation, one of the areas receiving closest scrutiny is the up to 7% biofuel content which by EU law has to be added to all road diesel.

The RAC says the new problem is not “waxing”, which used to be an issue in very cold weather. Modern diesel produced for UK winter use is formulated with additives to prevent waxing. The symptoms of the current problem are similar to waxing but the cause appears to be different. And, unlike waxing, when the weather warms up the gel does not dissolve back into the fuel which means a new filter has to be fitted.

The issue affected a small minority of motorists throughout last winter with the largest number in March, but the problem disappeared over the summer. However, in November the RAC attended almost 600 such incidents, the highest number of blocked fuel filters so far this winter.

RAC technical director David Bizley said: “Motorists were often led to believe that there were differences in the quality of fuel sold at supermarkets compared to other retail outlets which is just not the case as all diesel, wherever it is sold, is produced to the exact same standard specification. Having diesel fuel filters changed at the right service intervals is clearly important because a failure to do so can lead to starving the engine of fuel; but from the number of breakdowns we have attended this cannot be the primary cause.

“The industry is working extremely hard to find a solution. Neither the fuel producers, nor retailers, nor the motor manufacturers saw this problem coming last year. The current specifications for all fuel sold at the pump have been developed over many years and continue to evolve based on a combination of test programmes and field experience.

“Specifications have been further tightened since the problem was first reported, but it’s clear that we still don’t fully understand all aspects of the underlying cause. Motorists will share the hope that progress is rapid so that the associated risk of inconvenience and expense is removed.

“The fact these issues are far more prevalent in the east than the west suggests that supplies to these areas have characteristics that are not common to the whole country. We urge the fuel industry to continue its efforts to identify the source of the problem and find a permanent solution to it.”

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