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Write What You Drive

A showcase of independent motoring journalism and automotive travails

Look after your Diesel, and your Diesel will look after you (and everyone else)

The current UK Government recently alluded to a scrappage scheme for "older" diesel vehicles to encourage owners in particularly polluted areas to part with their dirty old oil-burner in favour of a cleaner new oil-burner or an as-yet-without-appropriate-stigma-prefix modern gasoline (gasoline, of course, also being "oil"...but let's not get bogged down in the detail). Over-simplification, yes - but the whole matter of diesel emissions has been simplified, often to the point of being incorrect, for mass-media distribution.

Much has been written, and will continue to be, about the apparent demise of diesel and associated emissions and air quality issues. A small amount was balanced and well researched; much was sensationalist, incorrect and designed to basically act as clickbait for those who thought they might be in line for a payout or some righteous indignation.

I don't intend to add to the existing volume of work, of either variety, however one very obvious improvement that could be made to older diesels hit me almost literally in the face recently.

Following a 1990s diesel 4x4 (although the type of vehicle is irrelevant), it didn't take a keen diagnostic eye to spot day turn to night every time the driver went onto the throttle. It would take a seriously oblivious driver not to notice an obvious fault in the enormous clouds of (carcinogenic) diesel particulate following them around.

This was one of the worst offenders in terms of unnecessary diesel particulate I'd ever seen, but it's certainly not an unusual sight on UK roads and extends to vehicles much newer than this example.

It's not fair to expect everybody to be able to diagnose vehicle issues or necessarily ensure their car is running at its sweetest at all times, but huge plumes of black smoke obscuring your rear view mirror might reasonably be expected to trigger a visit to the local garage. In fact, it might be considered your public duty to fix such an anti-social fault.

It's also not sufficient to fall back on the MOT as an indication of safety and roadworthiness; after all it contains only a basic emissions test and comes but once a year. If you know of an issue with your vehicle that could affect other road users, it is your responsibility to repair it - not wait for an MOT fail.

Despite the jokes, misconceptions and lazy library images - clouds of black smoke coming from the exhaust pipe does not indicate correct operation of a diesel engine. Particulate is the result of incomplete combustion of fuel and usually indicates a deficit of air during the combustion process. A good first guess is a boost leak, which could be caused by something as obvious as an intake hose hanging off so just looking under the bonnet could yield a very quick and easy fix.

The traditional diesel trait of a puff of smoke under heavy acceleration was caused by the delay in building up boost pressure from the turbocharger. Fuelling control is electronic so can be summoned immediately, whereas the required amount of air takes more in the order of seconds to be reached - due to delays in spooling up the turbo and pressurising the intake volume. During this time there is not enough air available to react with all of the injected fuel so particulates (and carbon monoxide) are produced as a result. This is also a handy indication of a poorly remapped diesel engine, where tuners lazily increase fuelling without considering how the extra air required to burn it will be obtained.

Modern diesels (that is, since 2009 when allowable particulate limits were cut 5-fold) render this sort of James Bond smoke screen trick almost impossible. Firstly, the electronic engine controller can accurately model the amount of air available for combustion and adjust fuelling appropriately; meaning much less smoke is created in the first place. The second measure, pretty much mandated by the advent of Euro 5 emissions standards, is the diesel particulate filter. Literally what it says on the tin, this is a brick of tiny passages in the exhaust, designed to catch as much soot as possible so it never makes it out into the world. All of which means there is something very seriously wrong with a modern diesel creating clouds of smoke - but whilst manufacturers can develop the emissions and diagnostic systems to the best of the technology's ability, they can't force drivers to maintain older cars properly.

The advances made by manufacturers (driven by legislation) and the recent public (media driven?) outcry regarding diesel emissions are somewhat at odds with users intent on removing their DPF (so much so that it is now tested as part of the MOT), using human urine instead of industry-standard diesel exhaust fluid (trust me, not even worth discussing) and finding ways to extinguish emissions-related engine warning lights when the management system picks up on their modifications.

It's worth remembering that, even if a scrappage scheme does come to fruition under a new government, much of the sizeable current population of diesel vehicles will be running for another 10-15 years, before penetration of the newest generation and beyond is anywhere near complete.

Before that current population can be vilified, it is at least fair and prudent to ensure they are operating correctly and not having maintenance neglected or emissions systems worked-around just because it might cost a few extra quid and the car doesn't grind to a halt without them.