Causes and cures of a ‘coked’ engine

Many years ago, a de-coke was a common repair job for cars.  Not anymore, it seems. What caused it, and why is it so rarely mentioned now? ER

Because it is very rarely necessary, thanks mainly to higher quality fuels and improved engine designs that reduce “coking” by ensuring more complete combustion. 

If the combustion of the air-fuel mixture is not complete, it generates sooty smoke, which leaves carbon deposits in the combustion chamber, on spark plug tips, on valve seals and rods, and in the exhaust system. 

These progressively build into thick coatings (cooked hard by extreme heat), which exacerbate combustion by disrupting the gas flow in the combustion chamber, weakening the spark, and reducing compression because the exhaust valves don’t close properly. 

More smoke, more deposits ...further reducing performance and increasing fuel consumption, possibly causing hot spots that lead to pre-ignition, and eventually debilitating the engine to “no go”.

When the condition is still mild, there are branded additives that can sometimes help dissolve the deposits. When the condition is severe, the only solution is to dismantle the fouled parts, scrape the deposits away, reseat the valves (by grinding), and replace the plugs.  In either case, the initial cause of incomplete combustion needs to be diagnosed and remedied. 

Causes can include dirty or sub-standard fuel, the wrong air: fuel mixture (carburettor/injector fault), wrong ignition timing (settings, timing belt) weak spark (plugs, coil, points, condenser), engine running too cold (thermostat faulty or removed), lots of short stop-start journeys so the engine does not reach operating temperature, etc.

Modern engines are less susceptible to these faults, and many have computerised engine management systems that trigger warnings or make adjustments to remedy them. Modern fuels have additional additives to help reduce or remedy the problem.

Read: What mechanics know... and sometimes don’t

An early indicator is the colour of the inside of your exhaust tailpipe. On petrol-engined cars it should be medium grey, going darker if used at low speeds in town traffic, light grey after a long, high-speed journey. Although black exhaust is normal for diesel engines, on petrol engines, it indicates incorrect settings that warrant attention.

If most of your motoring is around town in heavy traffic, the inside of your exhaust tailpipe is likely to be darker or black. If you have an opportunity to take it on a longer trip at sustained higher speed and revs, the colour should get lighter. 

This “blow-out” journey will have cleared some deposits inside the engine, too. If the colour is still black after a long/fast trip, have the mixture and timing settings checked at the next service. 

If the colour is very light grey even after an extended period of driving in slow traffic, there is a possibility that the mixture is too lean, and the timing is too advanced. This can result in a loss of power and an increased risk of overheating. A service check is therefore advisable.

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