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Fit filters right first time, every time

By autotech-nath on March 12, 2020

Like lubricants, filters are fast turnover service items that are critical to optimum engine performance but, as Rob Marshall finds, workshop mistakes undermine their effectiveness.

Filtration manufacturers have a tough job. Not only do they offer an increasing range of part numbers, but every quality aftermarket filter must also perform to increasingly stringent manufacturer-set standards for longer. After all, engine performance and reliability rely on decent filtration and an OE quality filter in good order can make the difference between an emissions pass and a fail at MOT Test time. Considering that relatively high-value precision parts, such as those within a fuel injection system, can be damaged severely by an errant particle that has snuck past the filter, technicians must not only select trustworthy brands but also fit the parts correctly.

We are grateful to a number of high-quality filtration brands, including Blue Print, Champion, Comma, HELLA Hengst, Mahle and UFI for sharing their experiences of the most common fitting errors and their potential consequences.


As engine lubricant passes through the filter from the oil pump directly, before reaching any of the engine’s moving parts, any installation issues can result in oil starvation, with potentially catastrophic results.

Advanced corrosion can cause leaks. Apart from the filter not being replaced in a timely manner, using a removal tool to tighten it (instead of hand pressure) might be responsible. This can scrape away the filter’s protective paint, exposing the bare steel to atmospheric moisture. Naturally, overtightening a spin- on oil filter makes it harder to unscrew at the next service.

Failing to lubricate a spin-on filter’s rubber contact seal prior to fitting can result in deformation and subsequent leaks. Yet, a faulty oil pressure release valve can reveal itself as an oil filter seal that looks as if it has been forced away from the filter’s metal body. Cross-threading the spin-on filter can cause it to be installed at an incorrect angle, affecting the seal’s ability to seat correctly. The damaged threads also heighten the risk of the filter working loose in service.

As many oil filter cartridge caps are manufactured from plastic, overtightening risks cracking the moulding. Many caps have a torque value printed on their tops, so there is no excuse to ignore it. Again, where rubber seals feature, they need to be replaced and the new items lubricated. Some engines (such as the PSA/Ford DV6 range) have square seals that can twist easily when replaced, so make doubly sure that the seal has seated correctly, before fitting the cap to the oil filter module on the engine.


Never think that air filters are unimportant, just because they are relatively straightforward to install. While they prevent airborne abrasive particles from damaging the cylinders, piston rings and valve-gear, they also help to reduce intake deposits and protect delicate sensors. For example, a defective/poor quality air filter can result in a damaged, or clogged, mass air-flow meter. Traditional results of blocked air filters remain relevant today, with the reduced airflow causing excessive exhaust emissions, high fuel consumption, and even illuminated engine management lamps and high DPF pressure differential readings.

Even if filter renewal is not scheduled, consider removing it at service time and inspecting it for either dirty, or distorted, pleats. Remove any solid particles that have accumulated at the bottom of the air-box. Subject to the application, ensure that a tight seal exists between the air filter and its housing. Before tightening the hose clips, inspect the hoses themselves for cracks, or splits – this is not always obvious. Should you encounter a ‘performance’, or ‘high-flow’ air filter, it may be worth notifying the owner that some types require special cleaning operations and subsequent re oiling. Should the car’s performance be unsatisfactory, or suffers an MOT failure on lambda results, it might be worth fitting a standard-specification, OE- quality filter (although this may be tricky, if the owner has discarded the airbox) to see if it makes a difference.


While you might resolve fuel starvation that results in poor performance and difficult starting by changing the relevant filter(s), neglect can threaten expensive fuel injection components, such as the pump. For simplicity, the following advice focusses on in-line fuel filters that are fitted to many older petrol vehicles. These tend to be mounted to the floor-pan/chassis box sections and are plumbed directly into the fuel line. Many recent models rely solely on a gauze-type filter that is attached to the petrol pump that is immersed within the fuel tank, which is worth checking periodically, especially if there is a fuel-flow problem. Apart from particles developed within the car’s fuel system, originating from corrosion of steel fuel tanks, for example, petrol fuel filters must guard against fine deposits that originate from underground fuel storage tanks.

If left in-situ for too long, metal in-line filters can corrode externally. Certain designs possess aluminium housings that react with the steel retaining housings, which make them even more vulnerable. Therefore, should you find that a protective rubber/plastic strap insert is either damaged, or missing, you might need to order a new one.

Also, take extra care, when undoing corroded unions and apply an anti corrosion coating to them, after you have completed the work. While corrosion causes leaks at the line connection, not replacing any O-rings, or insufficient tightening, can also be responsible.


Diesel fuel injection systems are more likely to encounter not only water but also the waste products from bacteria that are enjoying the food source, resulting from the ~7% bio-fuel content that pump DERV contains. Therefore, checking diesel filters at every service, or even renewing them prematurely, is not a bad idea. It may be worth questioning the customer from where he/she fills the tank. Non-forecourt pumps (such as a diesel storage tank on a farm, or industrial unit) can be more at risk of contamination, therefore, paying extra attention to the diesel filter is not a bad idea. After draining any water that has accumulated at the filter housing bottom, clean-out any remaining residue but ensure that you do not leave any particles behind. Do not forget to plug the ends of the exposed diesel pipes. Otherwise, you risk not only particles from the workshop entering the fuel system but also diesel leaking-out and damaging parts, such as the clutch. Some filters come with new blanking plugs for this purpose. Much fitting advice, however, is equivalent to that of cartridge type oil filters – replace and lubricate any seals and heed the cap’s correct torque figure.


While the heating/ventilation and air conditioning system’s (HVAC) ability to capture particles, pollen, bacteria, exhaust particles and moisture has obvious health (and interior cleanliness) benefits for the occupants, a clogged filter affects airflow within the cabin. Poor quality demising may be a complaint, but the HVAC components can also suffer. As the blower motor may have to work harder, its current draw can increase. This not only courts overheating and shortens the motor’s operating life, but can also make the resistor pack work hotter and fail prematurely.

Locating the housing into which the filter fits might also be a challenge. Some designs can be very inaccessible, such as behind the fascia panel, or mounted against the bulkhead, within the engine bay. Therefore, ensure that the new filter is not damaged during the replacement procedure and take extra care to note the air-flow direction, which tends to be indicated by an arrow printed on the filter.

Do not ignore the opportunities that cabin filters present for up-selling either, primarily if your customer drives in urban conditions frequently. Check with your supplier exactly which benefits more advanced filtration media can offer. Most common advantages include extra protection against ultra-fine gases, such as particulates, benzene, ozone and odour absorption.



About Autotechnician
Autotechnician is a magazine published nine times a year, delivering essential information to independent garage owners and technicians in the UK. Delivered both digitally and in print, autotechnician provides readers with technical, training, business advice, product and news, allowing our readers to keep up to date with information they need to run and work within a modern workshop.
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