Mild Clogging

• Mild clogging will typically manifest as hesitation, surging and bucking under sudden, high-load conditions where the engine needs a lot more fuel in a hurry. This hesitation can occur under anything from mild to extreme acceleration, but will be most evident while towing or going up steep grades.
  
  

Severe Clogging

• As the fuel filter accumulates junk, it'll lose its ability to transfer fuel volume and pressure to the carburetor or injectors. Ultimately, the clogged filter will result in a rough idle, reduced engine power, stalling and a dead-in-the-water no-start condition. Fuel-injected vehicles are typically a bit more sensitive to fuel filter clogging, as they require a steady flow of high-pressure fuel for accurate metering.