Welcome to our page on a weak fuel pump autopsy.

Fuel pumps can weaken over time. We wondered why... so we dissasembled a proven weak unit to see.
 Weak fuel pump disassembly The pump
 Weak fuel pump disassembly Disassembly
 Weak fuel pump disassembly Conclusions



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The pump


This pump was carefully tested to ensure it was weaker than a standard pump by several methods. It is from a 2000 Chevrolet Blazer with a 4.3L engine, which requires a certain minimum fuel pressure to start.

After only replacing the fuel pump, the vehicle started and ran fine with only a few PSI difference in line pressures during cranking. Pressures were measured before and after pump replacement at different conditions to verify this.

This is the old fuel pump after being removed from the vehicle. It is a widely used tank-mounted pump in many vehicles.

fuel pump

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Disassembly


This is a very common pump found on many vehicles. It is a standard vertically mounted in-tank pusher type electric pump with inlet on the bottom and outlet and electrical on top.

fuel pump

We started by removing the bottom end, which contains the pump rotor and body. The bottom bearing for the motor is a brass press-fit unit in the upper pump body section. The pump rotor is a plastic composite, while the body appears to be a non-ferrous pot-metal alloy. The end of the ferrous steel motor shaft where it passes through the bearing to drive the rotor is rust-corroded which is causing some slight resistance for the motor. There are also visible scratches in the pump surface directly after the output hole, indicating debris contamination.

fuel pump

The top of the pump is almost impossible to remove without damage, being a pressed-in and sealed assembly. We cut away the plastic around the top to examine the electrical and output sections. The anti-drainback valve just before the outlet tip was fine, and the brushes and motor rotor contacts were worn, but not smoothly, indicating debris contamination.

fuel pump

The vertical spring-loaded carbon brushes in this pump can be seen from inside the motor casing. They ride vertically against carbon contacts on the end of the rotor. Brush springs act not only to seat the brushes against the contacts, but to also spring-load the rotor against the bottom case bearing.

fuel pump

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Conclusions


This pump appears to have been weakened by internal rust corrosion on the motor shaft, as well as debris contamination in the fuel. The carbon brushes and motor contacts, as well as the pump body, show evidence of scratches. The motor shaft showed evidence of rust on the bottom end. Although the motor still functioned, these factors are enough to weaken the output.

These motors are not very strong to begin with. They are cooled by the continuous passage of fuel through the pump unit during operation. In more powerful or durable motors the rotor contacts are copper, which provide better wear resistance than weaker carbon. In these pumps not only the brushes wear, but the rotor contacts as well, both being made of carbon.

Obviously since all the pumped, pressurized fuel passes through the body of the unit, any debris sucked up by the pump will also pass through the motor internals. The weak carbon brushes and motor contacts are exposed to this fuel flow, and any possible contamination.

In this case the visible scoring in the pump body could have also provided an alternative escape for fuel pressure back into the tank. The pump pressure relies on a smooth seal between the pump rotor and the two pump halves. Any imperfections in these surfaces can weaken the pump.

As can be seen in the diagrams below, the pump casing is full of fuel when in operation. This continuous flow of fuel cools the motor, but also provides oportunity for debris to accumulate or get caught inside the unit. Especially ferrous particles, which will be attracted to the motor magnets.

fuel pump schematic

For better pump longevity, the motor shaft should be manufactured out of stainless steel, or something with equal non-corrosive properties. Clearly in this motor it is not. Also it can be seen how important proper filtration is, as any contamination passes directly through the internals of the motor. Another weak area is between the rotor of the motor and the pump housing, where any extra friction or debris can cause unwanted resistance due to the vertical orientation of the motor shaft. A possible failure point, although not is this particular pump we disected, is the anti-drainback valve where any debris or wear can clog or harm the valve function. We can also observe how proper fuel flow is important for motor cooling. A lack of moving fuel can overheat the motor, due to clogs, bad filters upstream, or running the tank dry.

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