Your Guide to Servicing: Filters

Posted on November 29th, 2013 by GEM Motoring Assist

An essential part of car servicing is routine replacement of filters but, because they are often neglected, Rob Marshall reveals what they are and why periodic replacement is so important.

Keeping the various fluids and fuels clean is essential to prolonging your car’s life and enhancing its reliability. Unfortunately, the ‘out of sight, out of mind’ attitude to maintenance can prevail, which is very much a false economy. Clogged filters can not only increase fuel consumption and make your car run less efficiently but a breakdown might also result.

Consult the car maker’s service schedule closely, to ensure that the stipulated service work has been completed and do not be worried if a garage decides to change a filter prematurely. Yet, by leaving a filter in place for too long, you risk voiding the manufacturer’s warranty.


One does not need to be a technician to deduce that an oil filter helps to keep the engine oil clean. The lubricant is filtered, prior to it being pumped around the engine, which removes larger particles that might cause damage should they reach the engine’s vital moving parts. This includes carbon and even small metal shavings that the oil filter traps until it is replaced, which should be scheduled at the same time as an oil change.

Should an oil filter become blocked, which can be caused by not changing it on time, most types contain a bypass valve. If activated, dirty, unfiltered lubricant is then permitted to travel round the engine.

Many engine types require that an oil filter contains an anti-drain-back valve, which prevents lubricant from draining out of the filter, when the engine is switched off. Should your engine oil pressure light remain illuminated for more than a few seconds following a cold start, you may have a filter fitted in which the anti-drain-back valve is missing. The result is increased engine wear.

Choosing the best quality oil filter is tricky and, without a laboratory and a team of scientists, it is almost impossible to guarantee that you are buying a filter that matches or exceeds the specification of the part that is recommended by the carmaker. Therefore, consider purchasing the filter from a franchised dealership. Surprisingly, the main dealership price is often between £6.00 and £15.00 and it can be cheaper to buy an oil filter this way than from a generic car parts store.

Car engine

Spin-on oil filters can be tricky to unscrew and so a dedicated tool will be useful. Tighten a new filter using hand pressure only

Replacement filter


An engine is like a giant vacuum cleaner, which sucks in air all of the time. Paper filters are fitted to almost every car as standard and are quick and easy to change. As the air filter blocks, the engine has to work harder and the restricted air flow results in less oxygen being burnt with the fuel. Leaving a clogged air filter in-situ will not only increase fuel and tailpipe emissions but impurities could enter the engine that might damage the MAF (Mass Airflow) sensor, a unit that, on modern cars, relays air flow information back to the engine’s electronic control unit.

Air filter

Air filters are often very easy to replace but ensure that the air filter housing is free of larger particles, such as dead leaves, prior to fitting a new filter.


Petrol can be contaminated by sediment from the bottom of fuel depot storage tanks, some of which might enter your fuel tank upon filling up. Steel fuel tanks on old cars can rust internally, causing flakes of corroded metal to be sucked into a fuel line. As most cars, built since the 1990s, have plastic petrol tanks, this has been less of a problem.
On most models, the larger particles are trapped by an integral gauze filter that is situated within the fuel pump, which is submerged within the tank itself. However an extra filter is often provided that captures many of the smaller contaminates.
The fuel filters on the majority of petrol cars tend not to be situated beneath the bonnet but close to a fuel line beneath the car, often towards the rear. As many of them have long service intervals, they tend to be forgotten and it is common to find cars, aged ten years or older, that still wear their factory-fitted fuel filter.

Petrol filter

Petrol filters are often neglected and corrosion can affect the unions. Beware that fuel will be discharged under low pressure and so be wary of any fire and health  risks, if you are undertaking the task at home


Diesel engines, particularly the latest high-pressure common-rail types, tend to be considerably less forgiving to neglected fuel filter changes than petrol engines are. Even the smallest particle of grit can wreak havoc on the high-pressure fuel pump and fuel injectors, which can result in a four-figure repair bill. Routine diesel filter replacement is essential but care must be taken to ensure that no foreign matter is allowed to enter the fuel system, while the filter is replaced. Additionally, air must be purged from the system afterwards and the system must remain airtight thereafter, to ensure optimum running and reliability.

Diesel filter

Generally speaking, replacing a diesel filter is more complicated on the latest engines. Cleanliness is vital and so ensure that the housing is drained of fuel and is cleaned out, prior to a new filter being fitted


Cabin, or Pollen Filters, were introduced during the late 1970s by Saab and almost every car, made within the last fifteen years and equipped with air conditioning and climate control, has been equipped with one.

Located within a housing either behind the fascia or close to the bulkhead, the cabin filter prevents airborne contaminates, such as bacteria, seeds, dust, exhaust fumes and of course, pollen, from entering the passenger compartment. Not only does it help keep the interior clean but it also is beneficial for occupants with allergies.

Like the engine air filter, the element becomes choked and requires renewal. If it is not replaced, airflow into the interior will be reduced and unwelcome bacterial growth will be encouraged within the ventilation system. Replacing the filter is often straightforward but it must be installed the correct way around, and the direction of airflow noted.

Cabin filters are designed to prevent contaminations from entering the passenger compartment and, because elderly filters will harbour bacteria, routine replacement is a prudent idea 

Cabin filter

 Some dismantling of the interior trim might be necessary, to gain access to the housing, but cabin filters need installing the correct way up, to ensure optimum airflow.