Recent posts have addressed the problem of fuel pump failure caused by fuel filter restriction. Replacement of the fuel filter at scheduled service intervals is a step in the right direction. A certain Chrysler Corp. FSM recommends R&R at 52,000 mile intervals. It would seem annual changes at 12,000 miles more reasonable. The problem with fixed intervals, either by time or mileage, is the deposit accumulation in the filter is highly variable. Conceivable it could be clogged by a single tank of particularly dirty fuel. There is an economically feasible engineering solution for the prevention of the very inconvienient, expensive, and potentially hazardous event associated with sudden, catastrophic fuel pump failure caused by a flow restricted filter. The solution is outlined as follows: 1. The fuel filter should be of a spin-on design for rapid and leakproof R & R servicing. 2. The filter element should be oversized to extend its service life. 3. The filter should be mounted to a die cast fixture to which the inlet and outlet hoses and/or steel fuel lines are connected. Clamp on hoses should not be specified as such connections are unsatisfactory from the standpoint of difficulty in prying or cutting off used hoses, distortion and deterioration of the hose ends and possible leakage as clamping force diminishes with age. The typical EFI system operates at 55 psi., unsafe for clamped configurations. Banjo fittings, Airquipt screw-on swedged hoses or flare tube fittings are to be specified. 4. Mounted to the fuel filter fixture as a screw-in, a damped pressure differential switch is installed. Activated by the pressure differential across the inlet-outlet, it closes an electrical contact when the pressure rises above a certain value. This sends an electrical signal to the EFI computer which then activates a coded "check engine" light on the dash. The code will be "excessive fuel filter backpressure". This warning light then puts the driver on notification that something is wrong and service is required. The shop can read the code and service the filter before damage is done to the fuel pump or performance problems appear from fuel starvation at high speeds and loads. In essence, the system monitors the fuel filter. What happens is the pressure drop gradually increases across the filter with use. This pressure can be measured with a manometer. The pressure drop at maximum engine BMEP and fuel flow is probably about 0.25 psi with a new filter. When it exceeds ten times that value, or 2.5 psi, for five seconds or more, the damped pressure valve will close a switch and send an impulse to the computer to trigger the warning light. Without this warning system, the failure mode is insidious. There are no symptoms because the supply pump pressure rises to maintain the regulated 55 psi pressure at the fuel return. The pump pressure is equal to 55 psi plus the fuel filter pressure drop. It can rise considerably, overloading the pump. Eventually the pump will limit output to its internal relief valve setting or fail. In the former situation, the engine will malfunction but continue to run; in the latter case, the engine will quit suddenly, stranding the vehicle. The cost for this improved fuel filter design is nominal, perhaps as little as ten dollars at the manufacturing level. Certainly it can be considered for incorporation into the range of higher priced models, none of which enjoy engineering sophistication of this component rising above the level of the lowest priced cars sold.
