Every car has a fuel filter, not just diesel cars or trucks!

The reason we tend to forget about it is because, it is often hidden away under the car or even in the fuel tank. It may not be top of the list when it comes to car servicing, but a bad fuel filter can lead to a whole host of problems. In fact, the symptoms of a clogged up fuel filter are often overlooked when trying to diagnose problems with a car. 

All of the fuel used by the engine goes through the fuel filter. It's main purpose is to protect the fuel injection system from dirt that may have found its way into the fuel.

Over time, the filter will become less porous, and will need to be changed. Sometimes it will get clogged up so badly it will start to affect the running of the engine. A dirty fuel filter can even stop your car from starting.

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.

bad_fuel_filter

A new diesel fuel filter on the left beside an old and dirty filter on the right

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. In reality, these filters are usually much larger than the serviceable kind, so they may last around 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 it's 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. But 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.

2. 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.

3. 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 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.

Fuel Filter Service Tools

There are lots of different types of connectors used in fuel systems. Many newer vehicles use snap-on connectors that can easily be removed by squeezing the tabs on the side of the connector plug. 

Another popular connector utilises the garter spring design. To remove it, you'll need a special tool that pushes back an inner spring to release the connector. 

A master disconnect set contains everything you'll need to tackle any fuel line connector on any vehicle. They often include tools for disconnecting air con, transmission and coolant lines too. Here's a few worth considering:

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How and When to Change a Fuel Filter

Most serviceable fuel filters are good for around 30,000 Miles. If a fuel filter change is recommended by a manufacturer, they may do it every 2-3 oil changes, especially if it's a diesel engine.

Fuel filters are fairly cheap, and can be changed easily by a competent DIY mechanic. It's impossible to describe the process for all cars, but here's a few things to remember before you start.

  • check
    Fuel is under pressure in the fuel lines. Take care when removing the fuel lines from the filter. 
  • check
    The fuel filter may be under the car, connected to the fuel lines coming from the tank. It can also be located in the engine bay. In many petrol cars, the fuel filter is not a serviceable item, and is located in the fuel tank. If it needs to be changed, then it may involve removing the tank and might not be a DIY job.
  • check
    There are lots of different connector types when it comes to connecting the fuel lines to the filter. Most modern direct injection vehicles operate under high fuel pressure. These cars often use special snap on connectors that are easily disconnected from the filter. Older cars will use a hose clamp or nut and bolt. These connectors are not suitable for high pressure engines.
  • check
    After the fuel filter is changed, you may need to bleed the system and prime the engine. This process will vary depending on the car. Some cars will have an automated sequence that will start the fuel pump to bleed the system. Other cars will have a hand pump along the fuel line, and it will be necessary to manually pump the air out of the system before the engine will start.
  • check
    It's often a good idea to cut open the old fuel filter after it is removed. This can reveal the condition of the filter and can help diagnose reoccurring problems.

What happens if you don't change your fuel filter?

Not changing the fuel filter regularly can lead to engine performance issues, and check engine error lights showing up on the dash. These issues can usually be remedied by fitting a new filter.

Sometimes, an old filter can fail to trap debris and may start to break down internally. If this happens it can lead to serious and costly engine damage. Fuel injectors can become clogged very easily, and this will cause significant damage to an engine. 

So it's important that the fuel filter is not overlooked when it comes time to service your car. If you want to be extra vigilant then change it once a year, especially if you are racking up high mileage or driving under extreme conditions.

A bad fuel filter usually isn't the end of the world, but it could keep you off the road until it has been replaced.