What pressure difference between the inner and outer portions of a fuel filter indicates the filter must be changed?

• A. 15psi
• B. 25psi
• C. 5psi
• D. 40psi

Answer: (A)

Fuel filters work by allowing fuel to flow through a porous element. As the element traps dirt, flow is increasingly restricted and the pressure on the two sides of the filter begins to diverge. When that differential pressure reaches about 15 psi, the blockage is significant enough to noticeably reduce fuel flow, potentially causing starting or performance problems. Replacing the filter restores proper flow and prevents fuel starvation.

A much smaller differential, like 5 psi, would suggest the filter is still relatively clean. A much larger differential, such as 25 or 40 psi, would indicate a severe blockage or another fault and would typically prompt immediate action beyond a simple change.