Diesel cars have been getting a drubbing in recent months.
Tax surcharges on their purchase, use in inner-city areas contributing to pollution and parking costs have seen them rapidly fall out of favour with car buyers, with sales slipping by 17 per cent in 2017.
And new research is unlikely to help matters for the under-fire fuel type, with a new study finding them to be more than three times as likely to break down than petrol models - and &84 more expensive on average to fix when they do.
Diesel under the microscope again: A new study by MotorEasy found that used diesel cars are more than three times as likely to suffer a fault than a comparable petrol, and are &84 more expensive to repair on average each year
Car ownership and maintenance firm MotorEasy said it had reviewed 30,000 faults registered by owners of used vehicles that were three to eight years old across the last 12 months.
The data showed the average repair bill of &517 for a diesel car last year was 20 per cent higher than those for petrol vehicles.
Models fuelled with unleaded had repair receipts of &433 on average.
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MotorEasy said the higher rate of diesel engine failures was mostly down to the fact that diesel engines are under more pressure than petrol motors.
'Diesel units rely on self-compression, meaning that fuel is compressed to a much greater extent, putting more pressure on internal engine parts,' it said in its report. Some faults involving engine parts can be extremely expensive, the study revealed.
The most substantial diesel engine repair bill one owner received last year was a staggering &4,030.80.
Duncan McClure Fisher, founder of MotorEasy, said: 'Diesels experience many more small problems than petrol cars. They are less reliable and, when a big item goes wrong, it costs a lot more to put right.'If you're still considering a used diesel car, our advice is to avoid high-mileage examples, particularly if you are only driving low mileage or doing city driving.'
Diesel cars were hauled back into the limelight this weekend after a Federal Motor Transport Agency (KBA) investigation in Germany found that Audi models fitted with Euro 6 diesel V6 engines had emissions cheating 'shut-off devices' fitted.
Diesel units rely on self-compression, meaning that fuel is compressed to a much greater extent, putting more pressure on internal engine parts.
Some 127,000 models produced since the end of 2015 are affected, the report in Audi's home nation said, though it is still unknown how many of these are owned by UK drivers.
MotorEasy's study looked only at used cars, with the majority likely to be powered by older, Euro 5-compliant engines - those found to be fitted with diesel-emissions cheating devices by Audi's parent group VW in September 2015.
With millions of cars impacted by the industry's biggest scandal in modern history - which revealed some cars were coughing out up to 40 times the legal emissions limit - fears have grown over the harmful pollutants emitted by diesel cars.
In response, the government announced in the November 2017 Budget that, from April, all new diesels will face higher taxation unless they adhere to a standard called Real Driving Emissions, Step 2 – a certification that will not exist until 2020.
Drivers of older models in the capital have also been having to pay London's new T-charge since January, while some boroughs have increased the cost to park diesel models in their areas.
Which brands have the least reliable diesel cars?
MotorEasy found that diesel models were less reliable than petrol for 71 per cent of car manufacturers.
However, it wasn't Volkswagen or one of the other VW-owned brands that topped the chart.
That unwanted title went to Italian car maker Alfa Romeo, with data showing that its diesels were more than four times as likely to have a fault than its petrol-powered vehicles.
Of all the Alfa Romeos analysed in the data, almost half (47%) of diesel-powered models suffered a breakdown in the last 12 months
Least reliable diesel cars by make
1. Alfa Romeo - 47% failure rate
2. Land Rover - 41% failure rate
3. Mitsubishi - 36% failure rate
4. Saab - 33% failure rate
5. Mazda - 33% failure rate
6. Kia - 29% failure rate
7. Mercedes-Benz - 27% failure rate
8. Vauxhall - 26% failure rate
9. Mini - 26% failure rate
10. Audi - 26% failure rate
Source: MotorEasy
In fact, it was a VW Group manufacturer that was found to have the most reliable diesel engines.
Skoda, the Czech brand that now comes under the VW banner with others such as Seat, Porsche, Bentley and Lamborghini, had a failure rate of just nine per cent for its diesel vehicles.
Only seven of 25 car makers analysed showed fewer breakdowns for diesel models.
Petrol-powered BMWs experienced faults 60 per cent more often than its oil burners, while Audi, Skoda, Mini (also BMW) and Peugeot also bucked the trend.
Alfa Romeo, Land Rover, Kia and Mazda diesels had the highest average diesel engine repair cost versus petrol counterparts.
