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How Does A Fuel Filter Work?

A fuel filter removes any dirt or debris from the fuel before it can get to the fuel injection system. It is usually fitted somewhere along the fuel line, although on some cars it is situated in the fuel tank.  The fuel is pumped through the filter using the pressure generated by the fuel pump. Inside of the filter is made up of a pleated, permeable material. It allows the fuel through under pressure but blocks dirt particles.  Over time, the filter material will become dirty, and less permeable. This will restrict the amount of fuel that can get through it, and to the engine.  It’s important that the fuel filter is changed regularly. Most manufacturers will recommend that it is changed every 30,000 Miles (50,000 Km). This may vary if the car has a diesel engine, or if it is driven under extreme conditions.  Many passenger cars now come with a fuel filter located in the fuel tank. These are not easily changed and are supposed to last the life-time of the car. These filters are usually much larger than the serviceable kind, so they may last up to 100,000 Miles.

Symptoms of a Bad Fuel Filter

A clogged fuel filter will often be mistaken as another more serious problem. Before you start pulling apart the engine, it’s usually a good idea to take a look at the service logs to see what the maintenance history of the car is like. If a car has been neglected, the fuel, air or oil filter could be to blame.

​​​​​​It’s also a good idea to run a diagnostic to see if there are any error codes logged. These can often point to a specific engine system and help with diagnosis.  Most vehicles don’t have specific sensors to determine the condition of its engine filters. If there is a problem caused by a clogged fuel filter, then it will show up as a fuel injection system error. Sometimes these errors can be caused by old or damaged spark plugs. More often than not, fuel pressure and fuel pump errors can commonly be attributed to a bad fuel filter. If your car won’t start, it probably isn’t caused by the fuel filter. When your engine starts it will use the fuel in the fuel lines. Unless you are pressing down hard on the accelerator, then it won’t need much fuel to sit there idling. A non-starting car will more commonly be caused by a battery not charging.

There are many other performance related symptoms of a dirty fuel filter.

  1. Lack of Engine Power

An overall lack or engine power in all gears can be due to a lack of fuel getting to the injectors. This can be caused by a dirty fuel filter. The ECU will automatically restrict the engine output to protect the engine. When this happens, the engine will go into ‘limp’ mode and a check engine light will display on the dashboard.

There are of course many other reasons for a lack of power like faulty injectors or mass air flow sensor. It’s worth checking the fuel filter condition first, as this is the cheapest and easiest to fix.

  1. Engine Stalling Under Strain

If you find that the engine is losing power under hard acceleration or going up a steep incline, then it may be down to a bad fuel filter. If the filter is nearly clogged up with debris, this will limit the amount of fuel getting through it. It may be ok under normal driving conditions, but the extra fuel needed to accelerate may not be available if a filter is blocking it.

Lack of acceleration can often be mistakenly attributed to a faulty sensor or a vacuum leak. That’s why a fuel pressure test is the best way to test if the fuel coming out of the filter is impeded by excess dirt.

  1. Random Engine Misfire

Another symptom that can occur under heavy load, is random cylinder misfire. Sometimes these will show up as error codes, or the vehicle might stutter a bit going up a hill under load.

The fuel filter may be restricting the amount of extra fuel getting to one or more of the injectors, causing the misfire. It’s worth noting that a clogged fuel filter won’t cause a car to misfire while stationary. Modern cars use very little fuel when idle. Unless the fuel filter was completely blocked, it is will probably be caused by something else.

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