Diagnosing Bad Fuel Filter Symptoms

Bad Fuel Filter Signs

One of the worst problems that can happen to a motor vehicle is when the fuel filter goes bad.  When the filter goes bad, it can limit the amount of fuel that enters the engine.  With that in mind, most of the symptoms of a bad fuel filter have to do with the car stalling out.

Fuel filters have been making a rearward progression for the last 50 years.  For a long time, you’d find it under the hood.  It looked like a little pill, and it would be quick and easy to replace.  This was in the era of carburetted vehicles.

Things began to change in the 80’s.  Fuel injection requires higher fuel pressure.  Higher fuel pressure meant that a more robust fuel filter would be required.  This meant the fuel filter was no longer left under the hood.  It was put in the steel fuel line, between the gas tank and engine bay

These fuel filters were more reliable, but they also were tougher to change.  It meant getting under the car and disconnecting steel fittings.  It really still isn’t that difficult to deal with, as long as you give the vehicle a few minutes for the fuel pressure to go down.

The trend for the last 20 or so years to move the fuel filter into the gas tank with the fuel pump.  This means that you will either have to drop the fuel tank (most vehicles) to change the filter.  Some vehicles have an access door built into the body.

If this is the case, you’ll have to either remove the rear seat or access it from the trunk.  It is a huge timesaver over dropping your tank, but it is still way less preferable to an inline filter.

The good news is, no matter how old your vehicle is, the symptoms of a bad fuel filter are universal.  It doesn’t matter if it’s in-tank or in-line.  And, fuel filters are cheap.


Signs of a Bad Fuel Filter

There are physical signs that you can feel when the filter is getting really clogged.   Before taking anything apart, check to see if your engines computer has any trouble codes saved in it.

The engine may throw a P087 trouble code.  This trouble code detects that the fuel rail/system pressure is too low.  Any trouble codes that exist could be a clue.  Sometimes when there is low fuel pressure, you may also get an oxygen sensor related trouble code as well.  The real issue with diagnosing a bad fuel filter is that it resembles many other vehicular issues in terms of symptoms.

There are also a few ways that the vehicle will tell you that the filter may be going out.  Here they are:


  1.  Sputtering Under Load–  The only way that you are going to know that your fuel filter may be going out is that it is no longer letting enough fuel through to keep the engine running under a heavy load, or at high-speed.Typically, if you are heading up a hill (or accelerating) the engine may start to feel like it is just running out of steam.  That’s what it feels like with a bad fuel filter.Now, a bad fuel filter can also feel a heck of a lot like a bad catalytic converter.  Both conditions will make the motor feel like it is running out of wind when you hit the gas.A bad catalytic converter will not feel as jarring.  It’s a smooth lack of power.  If you are unsure, check out this article on troubleshooting catalytic converters.
  2. Hard to Start Engine–  One of the best indicators of a clogged fuel filter is going to be that the vehicle is difficult to start.  This because there’s not enough fuel pressure to properly atomize the fuel as it enters the combustion chamber.If your car is difficult to start, a clogged fuel filter would probably not be the place to start looking, especially if you are getting no trouble code for fuel pressure or the oxygen sensors.Instead, we’d recommend taking a look at the ignition system first.  Often it’ll be bad spark plugs, or a bad coil pack.
  3. Engine Not Starting–  If the fuel filter has become so contaminated that the minimum amount of fuel has not been let through it, the engine won’t start at all.At the most basic of levels an engine needs three things to start:  Air, fuel, and spark.  If any of these things aren’t present, the engine won’t turn over at all.If there’s been zero warning that the fuel filter could be going bad (hesitation, difficulty starting, etc) than a clogged fuel filter might not be the place to start.  It could certainly cause the problem though.

Bad Fuel Pump vs Bad Fuel Filter

A fuel pump that is going out is going to provide the exact same set of conditions as a bad fuel filter.  This makes telling if you have a bad fuel pump vs bad fuel filter very challenging.

How do you tell them apart?  A clogged fuel filter will have high pressure on its input side and low pressure on its output side.  They are both going to cause the same hesitation at high speed and difficulty starting.  The only way to truly tell is to place a fuel pressure gauge on both sides of the line and get a reading from both.

If your car isn’t starting at all, and you hear the fuel pump prime that is usually a good indication that the pump is working to some capacity.

A priming fuel pump sounds like a high-pitched whine for a few seconds and can be heard with a sharp engine right after the ignition key is turned, but before the starter is engaged.