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How Can I Tell if My Subaru Fuel Filter Needs Replacement?

If the Check Engine light's on in the dash of your Subaru, it's possible you've got a bad fuel filter. If the engine's lagging and misfiring, too, and if the car's shuddering while idling, it's highly likely. Run an inspection, and find the fuel pump is clogged, and it's practically certain.

There's one way to know for sure: plug a fuel pressure tester into the fuel rail and get a reading. First, make sure there's at least some gas in the fuel tank (to get a reading that's as accurate as possible). Then, make sure the fuel pump's working (take off the gas cap, start the engine, and listen for a whir or hum through the fuel filler area; if you don't hear it, you've probably got a bad fuel pump, and you'll need to get it fixed first).

If the pump's working, let the engine cool off completely; you're ready to test. Here's how to do it.

Test Fuel Pressure in 6 Easy Steps

Prop open the hood, find the valve cap on the fuel rail (under either the fuel rail cover or the engine cover, usually), and take the cap off.
Put the right fitting on the fuel pressure tester and thread it into the valve.
Turn the ignition on, but don't start the engine.
Check pounds per square inch (psi) on the tester's gauge. If you see a psi drop after a little while, it means you've got a leak. If the reading's the same after 5 to 10 minutes, the pressure in the fuel system's fine.
Start the engine and let it idle. Check psi; it should be steady. Now, slowly rev the engine; if rpm rises, fuel pressure should rise. If it doesn't, you might have either a clogged filter or a leak in the system.
Check your owner's manual for recommended psi. Compare it to what you see on the tester. No fuel pressure means you've likely got a failed fuel pump. Low pressure means the fuel filter's possibly clogged (in which case, you should replace it), or that the pump's faulty. Normal pressure means the fuel filter's likely fine, and you've got some other problem. High pressure means a return line problem, or an issue with the fuel pump driver module or powertrain control module (PCM).

If problems persist, or you're not sure how to go about fixing them, contact a Subaru dealer or a certified Subaru service center; the pros will be glad to help.