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Inspecting engine filters

By autotech-nath on March 23, 2022

You may think that inspecting filters is not a productive exercise but Rob Marshall begs to differ, and explains why a few minutes spent checking them over will reap dividends.

Within the realm of automotive servicing, familiarity does not breed contempt. If a conscientious owner entrusts you with a vehicle for regular servicing, you will become acquainted with its condition and how it is used. Problems occur when you are faced with a vehicle, with which you are unfamiliar, especially if the fascia tell-tale is indicating skipped maintenance. If the owner cannot answer your questions, you could be lucky enough to discover its service history in the glovebox, or online. Otherwise, you could be in the dark, meaning that conducting a major service and replacing all the filters with OE quality substitutes is a good idea.

However, removed filters can also reveal much about how the vehicle has been used and also its condition.

Oil Filters

Valeo reports that clogged oil filters can cause a host of symptoms, including low oil pressure, increased noise, poor performance, increased emissions and even overheating. If an engine has been neglected sufficiently to cause these issues, it should be checked in greater depth for more serious damage.

As spin-on types have their filtration pleats (and valves) enclosed within an inaccessible steel case, it is impossible to check the contents, without cutting it open. While external oil leaks tend to indicate incorrect fitting, most technicians will be alerted to a rusty outer case, indicating that the filter may be well-overdue for changing. You can follow this up by assessing the oil condition, or even consider sending a sample away for more thorough analysis, as described elsewhere in this issue.

Obviously, cartridge-type oil filters allow for a more detailed look at excessive levels of swarf, or sludge. Check also that the housing is free of such contamination, prior to installing a new filter.

Air filter

First Line advises that, because air filters tend to be easy to access, the simplest method to determine whether they need replacing, or not, is to look for dust and dirt within the pleats. Valeo says that a new filter will possess either a white, or off- white, colour and that a visual inspection under bright light will show up most particles but, notably, not all of them. UFI adds that technicians should remove any larger particles from the housing, including leaves and even dust. While modern engines can adjust their fuelling to compensate for air-flow restrictions to an extent, the traditional sign of spark plug carbon fouling can still indicate air filter issues.

Consider also that the filter can bear obvious damage. Collapsed pleats, for instance, may indicate that the blocked filter has distorted. Cracks in the filter medium may also be discovered, which render the air filtration property as useless.

Poor-fitting can also result in filter damage. Additionally, the filter requires an airtight seal at the housing edges and careless installation can cause leaks that may manifest themselves as excessive intake noise, especially under load. UFI adds that attempting to make do with an incompatible filter can cause similar issues and that the selected filter must not only correspond to the vehicle but also must be of OE quality.

Diesel fuel filter

With high-pressure fuel systems being considerably less tolerant of contamination, fuel filter changes are essential, especially as a small particle of grit can damage the high- pressure pump and injectors severely. The most obvious driving symptoms that result from fuel filter issues are a loss of power and ‘kangarooing’, caused by insufficient fuel getting past the blocked filter. First Line shares that difficult starting and poor throttle responses are also typical. Valeo adds that the fuel filter can also trigger the MIL lamp that points to a fuel pressure sensor problem (P0178) and advises technicians to check the filter first, should this code be flagged.

After removing the filter, check it and the housing for foreign deposits. Sand, or rust, is likely to originate from a fuel filling station, whereas metallic particles could indicate an issue with the fuel system, such as a failing in-tank lift pump. Sludge tends to stem from bacterial growth, caused (possibly) by long periods of standing, or using diesel that has not been stored in ideal conditions. Excessive water contamination can have similar origins, which not only damages the fuel system but also the filter. Should you discover relatively large quantities of these impurities, query the owner about from where the car is fuelled.

Cabin Filter

The familiar musty smell of a damp interior is likely to indicate that the cabin filter may be overdue for a change, even if further investigation may be necessary for water ingress. It should also be replaced annually, or whenever the car is serviced. Aside from dampness, UFI adds that further signs of a blocked filter include unpleasant odours, or pollen particles being present within the cabin, plus poor visibility, due to condensation building on the glazing. It may be worth asking the owner if any allergy symptoms appear while driving and if fatigue and poor concentration are evident during long trips.

First Line states that, like the air filter, the easiest way to check if the cabin filter needs replacing is to physically remove and inspect it. Once the filter is removed, it can be checked for tears from incorrect prior fitment, as well as pollen that can manifest itself as fine dust. Should you discover mould, look into why the filter may be becoming wet. Seeking blocked drainage holes is an obvious

place to start. Large deposits, such as leaves, need to be handled carefully so that they do not fall into a HVAC system duct.

As cabin filters tend to be forgotten, First Line, through its Borg & Beck brand, is communicating to drivers why replacement is important. The resultant awareness poster is available free to workshops and one can be requested by visiting



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