MOTORING: Filth that destroys engines

Sunday June 2 2019


By Gavin Bennett

Ask motorists what fuel their car runs on, and most will confidently answer: “petrol” or “diesel”.

For this knowledge, they deserve a mark of five out of 10. Because no car runs on petrol or diesel. Ever. Engines need a mixture of fuel and air — in fact, 15 kilos of air per litre of liquid fuel.

In contrast to the exceptional purity of petrol supplied at most pumps, the air that is sucked in through your radiator grille is a maelstrom of muck; not just the larger bits you can see and euphemistically call “dust”, but also the smaller particles of natural and human detritus that we bizarrely refer to as “fresh air”.

Just above the road surface these include particles of rubber from vehicle tyres, soot from their exhaust pipes, wood from a nearby sawmill, iron filings from the local machine shop, burping beetles and flatulent flies, and various other cocktails.

In engine-speak, fresh air is in fact “see-through sandpaper”. All that stands between this whiff of grapeshot and your delicate carburetor jets and the high-pressure fast-moving parts in the engine is the air filter, whose job is to keep all that gunk out while letting plenty of genuinely fresh air in.

Failing to clean the air is akin to pouring the contents of your kitchen and garden bins into your petrol tank.


Using a poor quality air filter lets too much muck come through with the breeze. Using a clogged air filter doesn’t let enough air in, with adverse effects on performance, fuel consumption and engine wear.

Most air filters these days are the paper element type, designed to be regularly thrown away and replaced (others are oil-bath filters usually found on trucks and older model cars, and the oiled-sponge type common on motorcylces and lawnmowers, which can be “serviced”.)

The life of a paper element filter can be extended slightly. It can be cleaned by blowing pressure air through the paper fins from the inside outwards.

If the air-blowing is too vigorous this might cause minute tears in the fabric which will then let damaging particles of dirt through into the engine. So don’t blow directly — put the filter on clean ground and blow into the centre — and don’t take the “extended life” remedy too far.

With blowing, the visible dirt (larger particles) will whoosh out and the filter might look clean.

To ensure it is not clogged with less visible particles, hold the element up around a light bulb — you should be able to see light through the paper mesh. If you can’t, then it is clogged and should be replaced.

It will pay you, in every respect, to buy only good quality fuel, oil and air filters, and keep them clean. If your vehicle has a diesel engine, double the importance of that principle.

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