Air filter change
Rob Hawkins explains the importance of air filtration and how an air filter and induction system should be correctly serviced.
One of the most seemingly straightforward servicing jobs on a vehicle is to renew the air filter. Or is it? Blocked induction pipework, split hoses, detached electrical plugs and fiddly air filter housings are some of the issues that can slow you down.
Induction system
On most vehicles, the induction system consists of a panel filter inside a plastic housing with pipework to and from it. It is designed with several purposes in mind, not just to feed fresh air into the engine. It needs to help draw sufficient air to be able to pass it through an air filter to remove any dirt, then direct the air into the engine via the throttle body. The means of drawing or sucking the air in is the job of the engine. When a piston moves down its cylinder bore, its respective inlet valve opens, creating a vacuum that draws air in through the inlet manifold and whatever else is attached to it (plenum, throttle body, induction pipework and air filter housing). Air filtration specialist K&N measured the speed of this drawn air and calculated that it travels at around 6mph.
The drawing of air into the engine is quite noisy, so some induction systems are equipped with a resonator box to muffle or absorb the noise. This usually consists of a plastic or composite container that’s attached to the induction pipework. So if an induction system is noisy, it may be due to a problem with the resonator box (for example, if it has fractured or become detached) or a detached induction hose or pipe. In the case of turbocharged engines, a detached boost pipe can result in loss of power.
Another potential problem area concerns the pipework that draws air into the air filter housing. With most air filters located in the warm confines of the engine bay, a cold air feed is fitted to ensure cooler and denser air is drawn – the denser the air, the greater its oxygen content, which is essential for combustion. A hose or pipe between a hole in the front grille or bumper helps to direct air from outside the engine bay and into the air filter. On some vehicles, the cold air feed is located around the lower half of the front bumper, which can result in disaster should you drive through deep water. If water gets drawn through the cold air feed and into the air filter, it will also be sucked into the engine, potentially causing it to ‘hydraulic’ and fail.
The following pages provide some general guidance on renewing an air filter, examining reusable air filters and outlining the potential problems that can arise when replacing an air filter.