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Instant expert Decoking - techniques that do not demand an expensive engine strip. O ne might have...

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  • Car MechanicsMarch 2016 www.greatmagazines.co.uk/carmechanics6

    In making engines cleaner, they have become dirtier. This irony is not lost on Rob Marshall, as he looks at modern decoking techniques that do not demand an expensive engine strip.

    One might have thought that decoking had been banished to the history books long ago. At one point, it was normal

    for engines to require a semi-regular tear-down to remove carbon that had accumulated on the pistons and valve-gear. The procedure was necessary because carbon and sludge, both of which are byproducts of combustion, built-up on the engines operating parts, restricting either their movement or interrupting efficient airflow. Not only can these issues reduce reliability, efficiency and engine life, but emissions and fuel consumption can also increase.

    Is this progress?Over the years, cleaner fuels, enhanced lubricants and better engine design have negated the need for regular decoking.

    At the same time, engine designers have increased engine power, while decreasing fuel use and overall exhaust emissions. Heightened complexity is the main cost of these seemingly incompatible achievements, along with a greater tendency for combustion deposits to build within the engine, from its air inlet to the sump. Sadly, the situation even afflicts engines that have been well-maintained, although neglected units suffer to a greater extent. Consequently, decoking has made a comeback, albeit with several modern twists.

    Should you decide to dismantle an engine to deal with other faults,

    it is prudent to clean the internal components, especially as evaluating their condition will be easier, plus the risk of old contaminants shortening the life of a rebuilt unit will be reduced. Despite decarbonising being a straightforward process, when an engines parts are laid out on a workbench, stripping the unit to deal with the issues alone is time-consuming and not cost-effective. Thankfully, a number of recent developments enable you to remove deposits from certain parts of the engine without having to dismantle the engine block and tear down the cylinderhead.

    DecokingInstant expert

    t EGR valves and inlet manifolds can be cleaned manually, using solvents that can range from inexpensive brake cleaner to aerosols containing bespoke EGR cleaner.

    u If cleaning out the intake with solvents, ensure that all traces of the cleansing vapour have been removed before refitting the parts to the car.


  • Dr Helmut Leonhardt of Shell Lubricants revealed that car-makers require that lubricant manufacturers develop products that minimise inlet valve deposits.

    Viscosity improvers are a vital component of engine oil blends, yet they are responsible for starting the process of inlet valve deposits on many direct injection engines.

    www.greatmagazines.co.uk/carmechanics March 2016Car Mechanics 7

    Decoking modern engines

    Starting at the topOne of the major ways in which engineers have enhanced efficiency in small capacity petrol and diesel engines has been to install direct injection. In the case of petrol engines, non-direct injection (port injection) relies on the fuel injector being situated behind the intake valve and the continuous contact with the solvent keeps the area clean. Once direct injection design relocated the injector within the cylinder, this washing effect was removed.

    Although carbon results from burning petrol and diesel, engine oil can pose a greater problem. In theory, an engine should not consume its lubricant, but production tolerances, breather design and wear mean that all vehicle engines burn oil to a degree. As an engine is not permitted to vent toxic crankcase gases into the atmosphere,

    they are recirculated through the breather system and into the inlet tract. Inevitably, some engine oil vapour is carried along, too. Some enterprising enthusiasts try and negate the issue, by integrating an oil catch tank into the crankcase ventilation circuit, to separate the lubricant from the blow-by gases. Catch tanks (or cans) tend to have limited success, depending on the make and model of car. The tanks also require periodic emptying, which is another reason why they are not used on mainstream production vehicles. Valve stem seal wear exacerbates the problem, because oil seeps down the valve stems, then carbonises and builds on the rear of not only the inlet valve but also, potentially, the exhaust valve. Excessive oil consumption will also damage post-treatment devices, including diesel particulate filters (DPFs) and catalytic converters.

    t Poor engine design and excessively long oil change intervals can result in sludge accumulating within the lubrication system, including the sump.

    u One can remove carbon from pistons and cylinderheads manually, but the dismantling process is expensive. TerraClean removes carbon chemically.

    If you are considering having your engine tuned, it might be a pointless exercise, unless you can be sure the engine is clean.


    Deposits build behind the intake valve and within the intake port (pictured on this cutaway illustration in light blue).

  • Car MechanicsMarch 2016 www.greatmagazines.co.uk/carmechanics8

    Decoking modern engines

    petrol, the issue of intake valve deposits disappeared. However, the advent of direct injection and the various demands to reduce oil viscosity for efficiency and emissions reasons have seen the problem resurface.

    Crushed walnut shellsDr Leonhardt says: Today, intake valve deposits are caused primarily by viscosity improvers within the oil blend, which become baked onto the hot inlet valve and are burnt subsequently by incoming exhaust gases from the EGR, causing the deposit to carbonise and trap further deposits. Here at Shell, we develop lubricants for car-makers and many of those engine oils have to pass an intake valve deposit test, to comply with manufacturer-set specifications.

    As intake valve deposits are a consequence of many direct-injection petrol engine designs, knowledgeable owners can reduce the problem. As it is illegal to disable pollution control equipment, we do not recommend removing the EGR valve; even if you

    The French direct-injection petrol engine, used in MINIs from 2006, as well as various Citrons and Peugeots, is particularly prone to inlet valve deposits.1

    Exhaust gas recirculation (EGR) valves pose an additional complication. The system was introduced as an effective means of increasing engine efficiency (by reducing pumping losses), while driving down NOx emissions. Unfortunately, as hot, untreated exhaust gases flow from the EGR valve and into the inlet manifold, the existing sticky oil film attracts certain exhaust gas deposits, including carbon. The build-up restricts not only the ports diameters but also inlet manifold swirl flap movement, on engines thus fitted. Fortunately, the deposits can be scrubbed out with a suitable cleaner, which necessitates removing the inlet manifold. Despite being messy and time consuming, the procedure is a fairly straightforward DIY task.

    The intake tract, within the cylinderhead of a typical direct-injection engine can be even harder to clean. Not only does a narrow port present an access problem, but the higher temperatures involved also bake the oil deposits onto the back of the intake valve, which acts subsequently as a fly-catcher of unburnt particles from the incoming exhaust gases. The result is a rubbery accumulation that is incredibly difficult to remove.

    Dr Helmut Leonhardt, Shell Lubricants team leader for the development of engine oils in Europe, revealed to CM that inlet valve deposits are not a new problem. He tackled the issue for his PHD in the mid-1980s: Over 30 years ago, lead oxide built-up on the rear faces of inlet valves, which is a porous deposit that absorbed the incoming fuel; the biggest deposit that I ever removed from the back of single inlet valve in those days weighed a whopping 4.5 grammes.

    Helmut admitted that, when indirect injection was used in conjunction with new additive blends of unleaded

    were to do this, it would not provide a solution, because oil can still enter the inlet via the breathers. Rerouting the system and venting the gases straight into the atmosphere is also an offence and may lead to unpleasant odours permeating the interior of the vehicle.

    A potential option is to select a lubricant with fewer viscosity improvers, the cause of the initial build-up, which will be present in oil with a bigger differential between its cold and hot viscosity grades. For example, this means that a 0w30 oil is likely to contain more viscosity improvers than one with a 10w30 grade. This is not an excuse for you to stray outside of the recommended oil viscosity for your engine always stay within the car-makers parameters.

    Unfortunately, post-combustion fuel additives, water injection and even solvent cleaning tend to be ineffective. Firing abrasive particles under pressure into the inlet may also cause damage, especially if grains of the fine grit stray into the combustion chamber. Fortunately, crushed walnut shells have provided a solution, as they are tough enough to remove the deposits, without scoring the metal surfaces.

    While the procedure is not a DIY task, it is a significantly cheaper option than dismantling the engine. As the PSA/BMW Prince series of 1.6-litre direct injection petrol engines seem to be particularly prone to inlet valve coking, this operation is common for MINIs built around the R56 platform even some main dealers carry out the work.

    Given their experience of dealing with the problem on both MINIs and BMWs, we approached SWSS MINI of Cardiff for its insight into what the operation entails and how effective it can be. This is

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