Tech Stuff

Electric fuel pump diagnosis: Houston, we have a problem

Part one of two

A non-delivery or weak delivery fuel problem shouldn’t automatically be blamed on the fuel pump. In far too many instances, fuel pumps are replaced needlessly.
<p>A non-delivery or weak delivery fuel problem shouldn’t automatically be blamed on the fuel pump. In far too many instances, fuel pumps are replaced needlessly.</p>

All too often when a no-start condition or hard-start is encountered, some techs may be too quick to automatically blame the electric fuel pump.

If you talk to any rebuilder/remanufacturer or new parts manufacturer’s warranty folks, they’ll tell you that in the majority of return cases, the fuel pump that was returned as faulty was in fact problem-free. Before you jump the gun and swap out a fuel pump, consider performing a simple voltage drop test to check the circuits that are responsible for pump activation.

Do we have fuel?

We realize how basic this sounds, but before performing any type of diagnosis related to a suspected fuel pump problem, verify that there’s fuel in the vehicle tank. Whether you’re dealing with a gas or diesel engine, she’s not going to fire unless the pump has available fuel to suck out of the tank and push forward to the injectors. You may laugh at this reminder, but if we didn’t mention this, I’m sure we’d get letters criticizing us for not including this caution.

Electric fuel pumps on electronically controlled fuel injection systems need to produce enough “high” pressure to allow the injectors to produce a sufficient spray of atomized fuel into the intake path (or into the combustion chamber in a direct-injection system). Fuel pressure is typically in the 35 to 45 psi range. The electric fuel pump needs to be able to produce more pressure and flow than is needed, with pressure being controlled within the engine’s requirement range by the pressure regulator. With the engine at idle (low engine speed), the regulator allows more fuel to be returned to the tank in order to keep the fuel pressure from building beyond what the engine needs at the time. At higher engine speeds, the amount of fuel returned to the tank is reduced.

What else can stop the pump?

Aside from insufficient power signal or poor ground issues that can affect pump operation, late model pumps incorporate previously separate components into the fuel pump assembly. This includes the pump, filter, fuel level sensor and in some cases the pressure regulator. In many of these configurations, the signal produced by the crankshaft position sensor and monitored by the ECM will kill the pump when/if the engine dies/stalls out.

If the vehicle was involved in a collision, impact sensors can kill the pump signal, as well as signals generated through air bag systems.

Citing various diesel applications as an example, the diesel engine may feature an external high pressure oiling system that’s responsible for injector operation. If the high pressure oil pump (or related pump components) wear or fail, insufficient (or no) high pressure oil will be available to run the injectors.

A fuel pressure drop can be caused by a clogged fuel filter, which can result in the pump drawing vacuum, which leads to fuel aeration (air bubbles in the fuel). A dirty filter will force the pump to over-work as it tries to push fuel, leading to premature pump failure.

A loss (or drastic reduction) of engine oil pressure can cause fuel pump intermittent or shut-off problems, when the oil pressure sensor signal indicates insufficient oil pressure (a safety circuit shuts off the pump as a safeguard to help save the engine).

A faulty (stuck) fuel pressure regulator that causes a reduction in fuel pressure can easily be mistaken for a pump problem.

Fuel volume should be in the range of five to seven gallons per minute (check with a fuel volume tool).

Other potential causes for fuel pump interruption:

• Inertia switch. Common to many Ford vehicles, an inertia switch shuts off the fuel pump if the switch senses an impact, which could be caused by a collision, a sharp bump at the rear of the vehicle, or even in severe bumpy off-road driving. Pressing the reset button on the switch will reactivate the pump circuit (the switch is often located in the trunk area).

• Airflow (if intake air ducting is leaking at the throttle plate or other air intake issues are present.

• Grounds. Always check for loose, missing or badly rusted tank/pump grounds before wasting a bunch of time with extensive diagnosis.

• Poorly installed or faulty theft-deterrent systems, which can kill the signal to the pump.

• Faulty ignition switch and/or ignition key.

• Faulty or improperly programmed remote-start ignition system.

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