The importance of clean air

We learnt the hard way about the real-world importance of air filtration when our LandCruiser ute suffered ‘dusting’, caused by an ill-fitting air cleaner element. An important rubber sealing ring had been accidentally misplaced during air cleaner element replacement and that allowed unfiltered air to get around the element and into the engine.

It took some time for the effects to manifest themselves, but eventually the engine needed a teardown and the worst was revealed: scored bores were obvious and subsequent dismantling showed big-end and main bearing scoring as well. The only cure was a complete recon job.

Sure, our disaster was with a 4.2-litre, small-capacity diesel, but the risk of engine dusting is common across the whole engine spectrum. As little as 100 grams of silica ingested into a heavy truck diesel can cause a noticeable drop in engine performance, ‘blow-by’ pressure in the crankcase and increased oil and fuel consumption.

The best protection for an engine from dust ingestion is a high-set air intake – to minimise the entry of dust particles – and an appropriate, well-fitted air cleaner element. In high-dust environments a dual-filter and pre-cleaner may also be required.

When it comes to air intake filters, major truck engine makers and filter suppliers suggest dry air intake filters, rather than oiled-foam, aftermarket options.

Those of us with grey hair can remember oil-bath air cleaners, in which oil-coated wire gauze was employed to capture dust particles. When the modern dry air filter was developed we sighed in collective relief, because it filtered more efficiently, never ‘dried out’ and was easy to inspect and replace.

Dry filters typically give the optimum combination of high efficiency, low initial restriction and long life, according to filter specialist Donaldson. The company says that oiled filter elements have the potential to provide long filter life, but some have issues with overall particle separation efficiency.

Another issue is that a sticky-oil filter has an open-pore foam medium that relies on the optimal oil content for filtration efficiency. If the oil dries out, or is absorbed by dust particles, filtration ability is greatly reduced or non-existent. In contrast, a neglected paper filter clogs with dust, which triggers the restriction alarm, but doesn’t allow dusty air into the engine.

An advantage of sticky-oil foam filters is that they can be washed out and re-oiled, but care needs to be taken to avoid damaging the plastic pores. In the case of dry filter elements, it is always recommended by engine OEMs and filter suppliers to replace filters rather than clean them.

We suspect most people have done it: tap a dusty air cleaner element against a truck tyre, or blast it ‘clean’ with an air hose. However, filter makers strongly urge against these practices, because there’s a risk of damaging the filtration media, or causing separation of the filter from the filter end caps.

Some dry air cleaner elements can we washed, but this needs to be done with great care.

Filter makers also point out that many truck operators and drivers ‘over-service’ air cleaner elements. A fine layer of dust on the element is actually beneficial in achieving optimal air cleaning and helps it filter air more effectively than a new element does.

An air filter restriction indicator or dashboard gauge is a much more reliable way of knowing when a filter needs replacing than does a visual inspection. Every time an air cleaner can is opened and closed there’s a risk of filter misalignment or damage, as well as the risk of dust entering the engine inlet.

Any air cleaner can that regularly holds a lot of dust and debris is a clear indication of the need for inner and outer elements – and possibly a larger air cleaner – in this particular engine operation. The inner element need not be disturbed when cleaning out the dust and serves as engine protection while cleaning is happening and when installing a new outer element.

Donaldson points out that its inner filter does not increase the overall efficiency of an air cleaner and it’s not a secondary filter – it’s ‘engine insurance’ – and recommends changing the inner filter every second to fourth replacement of the primary filter.

Another function of the inner filter is as a ‘check’ on the operation of the main, outer filter. Any dust or debris on the outside surface of the inner filter is a sure sign of a problem with the outer filter or its fitment.

Because the inner filter has a much smaller filtering surface area, it will plug rapidly if contaminants pass through a damaged primary filter. The restriction alarm will then be activated. If that situation has occurred, both filter elements need to be changed.

Meticulous care is needed when fitting any new element, to ensure that the gasket surfaces are clean and everything mates precisely. Improperly mated or damaged gasket seals are among the major causes of engine dusting.

Related reading:
The real cost of mismatched tyres
Truck air compressors explained
The importance of brake hose inspection

At the same time, it’s wise to check all pipe unions and clamps between the air cleaner housing and the engine air inlet. Turbocharger suction will pull unfiltered air through any gaps in the plumbing.

The ‘flat-panel’ air cleaner elements used in virtually all new diesel utes and wagons are well known for being difficult to seal tightly. Well publicised LandCruiser V8 diesel engine dusting incidents have resulted in an aftermarket range of metal-box air cleaner housings, to replace the warp-prone standard plastic ones.

Fortunately, heavy truck air cleaner cans are much larger and seal more effectively, but plastic ones age and may not be as long-lived as steel ones.

An important part of any pre-cleaner design is that it doesn’t create an obstruction to the incoming air, even if its dust collection chamber is full.

Some truckies have theorised that if a pre-cleaner is desirable to keep excess dust out of the main air cleaner element, why not use a screen of some sort on a non pre-cleaning snorkel scoop?

They opted for ‘socks’ made of simple cloth, or foam plastic soaked with sticky oil, to act as dust-screening pre-cleaners.

The one major problem with this ‘solution’ is that these socks can clog up with dust and insects that restrict airflow into the engine. The ‘strangling’ effect on air flow into any engine can cause a loss of power and excessive fuel use. It’s also likely to cause turbo overheating.

Another quite nasty and dangerous situation can occur if the sock is rinsed clean with petrol; we’ve heard of an engine ‘runaway’ when petrol fumes sent the engine revs soaring!

The best way of keeping dust out of an air cleaner element is by not driving in other vehicles’ dust clouds. If you can’t avoid prolonged driving through clouds of dust, fit a proper cyclonic-type pre-cleaner.

There’s a belief that a high-mounted intake provides ‘water-proofing’, allowing a truck to ‘wade’ through deep water if necessary. That’s true, to some extent, but it’s important to check your inlet trunking to see if water could be sucked in.

The classic possible water entry point is the vacuator ‘flapper’ valve fitted to some air cleaner canisters. It’s a rubber venturi that’s held closed by engine suction, but opens when the engine isn’t running. It’s placed at the bottom of the housing to allow dust and dirt particles to drop through the valve when the engine is switched off.

If that valve is under the water surface during a wading exercise and the engine stops for any reason, water can find its way into the air cleaner canister. That water may be ingested by the engine when it restarts and a cupful may be enough to cause ‘hydraulic’ engine damage.

Wading preparation involves taping up any possible points of water entry, including vacuator valves.

It’s not possible to over-emphasise the importance of correct air cleaner fitment and servicing. Lack of care and precision can result in an unscheduled engine rebuild and the pricing for an in-frame rebuild starts at around 20 grand.