Problems and symptoms of a bad or failing fuel pump and how to test it

Internal combustion engines rely on fuel to start the engine. It is sent from the tank to the engine using a pump. The fuel pump is immersed inside the tank and is part of an assembly, which allows gas to enter only at the end of the pump. Its role is to absorb gasoline from the tank and send it to the injector ramp.

What is a fuel pump?

The fuel pump is very important in the operation of the car and unfortunately, it can fail quite easily. After starting the engine, the fuel pump is actuated by the electric current from the battery and controlled by the engine control module. Depending on the installation method, they can be “in-line” pumps or external (mounted outside the fuel tank) and “in-tank” type pumps (mounted inside the fuel tank).

External fuel pump

The external fuel pump mounted in line outside the fuel tank, draws fuel with the centrifugal force generated by the moving rotor of a ferrite motor and sends it further into the fuel supply circuit.

The external fuel pump consists of:

  • rotor plate, motor-driven;
  • the pump housing assembled eccentrically to the rotor plate;
  • rollers, mounted between the pump housing and the rotor plate, as a spacer

How does an external fuel pump work?

The centrifugal force generated by the rotor pushes the outer part of the spacer, which will move along the wall creating a vacuum space between the rollers and this spacer of the pump, at the suction part, the void will be filled with gasoline.

The rollers rotate, increasing the space and sending gasoline to the exit. During this movement, space decreases at the exit side. It causes an increase in the pressure released by the gasoline, which will pass through the area of ​​the engine armature opening the control valve.

Then the gasoline passes through the damper in the gasoline supply circuit. During the operation of the pump, the suction-compression-release cycle is performed completely at a single rotation of the rotor.

Internal fuel pump

Fuel pumps that are in the tank are the favorites of car manufacturers and are used in most cases, due to the advantages of limiting vapor blockage and noise reduction.

The pump consists of a direct current motor, a check valve, and an overpressure/safety release valve that works with a current of relatively high intensity, controlled by a control relay.

Electric fuel pumps (Ex: Bosch pump) are optimized for each type of engine and are produced by the most demanding quality standards:

  • Extremely reliable operation
  • Particularly demanding requirements for a new car are met
  • Long life
  • Minimum noise level
  • Optimal fuel supply
  • Optimal reliability for fuel supply
  • High interference reduction

The electric fuel pump must supply the engine with a sufficient amount of fuel at the pressure required for injection for all operating conditions. It is also increasingly required to function as a pre-supply pump for modern direct injection systems, both for spark ignition and diesel engines. The main requirements are:

  • Supply flow between 60 and 200 liters/hour at rated voltage
  • Pressure in the fuel system between 300 and 450 kPa (3 – 4.5 bar)
  • 60% increase of the system pressure – 70% at the nominal voltage; for this, the cold start operation is definite.

What are the most common symptoms of a bad fuel pump?

  • The engine does not start
  • Heavy engine start
  • If the engine starts hard after the key is turned in the ignition, the pump may have a problem pulling enough gasoline for the engine.
  • The car does not accelerate as before, the acceleration seems forced
  • The engine can still run, although the fuel pump no longer runs in the parameters, the acceleration feels much weaker than usual, and the acceleration takes longer.
  • Suspicious noises. If a noise (humming or rumbling) is heard from the tank, there may be a problem with the gas pump.
  • Forcing or locking the engine. A bad fuel pump that provides inconsistent fuel causes the engine to run in shocks, first with too little fuel, then immediately forced when the normal amount enters the cylinders with the regulator open. This symptom is very dangerous, as increasing efficiency can cause driver problems.
  • You can easily remain without fuel, although the dashboard shows that you still have fuel. The deposits in the tank clogged the fuel monitoring sensor, which does not go down at all, as the gasoline level drops, falsely indicating that there is still gasoline. The pump will run without liquid and the car will suddenly stop.

What are the most common causes by which a car fuel pump can fail?

To find out more easily if your car’s problem is a faulty fuel pump, pay close attention to the symptoms present and use a car tester or car scanner suitable for the make of the vehicle you want to test.

  • bad fuel pump (could be the pump, pump safety, pump relay, or a fault in the pump connection circuit).
  • inefficient pump (pulsating supply voltage from an imperfect contact, the 125-micron sieve of the pump’s internal filter clogged, the main pump shaft, dynamically unbalanced pump, etc.)
  • external fuel filter disconnected.
  • low fuel pressure (inefficient pump, partially blocked fuel line, low voltage at the pump, or faulty fuel pressure regulator).
  • clogged bypass hose (for older models), faulty bypass valve or relay (for modern cars).

Tips for avoiding a fuel pump failure.

As in any field, it is easier to prevent than to correct, so any driver should:

  • Clean the electrical panel. This consists of cleaning the electrical contacts with a contact spray, all the fuses, and terminals of the electrical relays.
  • All pumps draw magnetic wear from the bottom of the tank (powders resulting from the gas station pumps, whose gears produce fine powders in operation and non-magnetic deposits (organic suspensions of any kind from the gasoline distillation process), therefore periodically, at approx. 70,000 km (43.500 miles), or at least when changing the timing, it is recommended to wash the tank and also the float flap blades.
  • To avoid the bad situation of remaining out of gas, many drivers refuel when the dashboard indicator shows the minimum fuel level in the tank (red), usually with the same amount, with which they know they will cover a certain number of miles. It is preferable to refuel once and then refuel with the same amount of gasoline they are used to, after the scheduled number of miles
    improperly, the fuel pump must be checked first.
  • When the engine is moving, the fuel pump should make a small noise. If no noise is heard, it means that the pump is not rotating.
    In most vehicles, the pump is powered by the MCP (Motor Contact Point) via a relay. When we turn on the ignition, the MCP energizes the gas pump relay for a few seconds and the pump will run to generate pressure. After starting, the pump circuit can also be connected via an oil pressure switch and/or an inertial safety switch which closes the pump in the event of a fault.
  • If the engine has not started, the MCP stops the fuel pump relay (which stops the pump), if it does not receive an rpm signal (start line confirmation reception) from the engine (for car models after 2008).
  • Cars manufactured before 2008 were not equipped with this protection element, so the pump burns. In this situation, the key from the ignition should be removed immediately.
  • A faulty fuel pump relay will prevent it from operating. Remove the relay and shake it. If you hear noise from inside, the relay is faulty and needs to be replaced.

How to test the fuel pump

To test the fuel pump, suspend the relay to direct the supply directly to the pump. If it works when the relay is bypassed, it means that the relay is faulty or there is a fault in the wiring circuit that supplies the relay. Bypassing a relay can be difficult because you need to know which terminals to jump. It is necessary to consult the electrical diagram of the fuel pump (from a manual or online).

The forward and return flow are connected by a bypass. Old models used a plastic hose for this purpose, which, if clogged, emptied the tank in a very short time, leading to the burning of the gas pump. Modern cars are equipped with a directional valve and a hole (bypass valve), with the same role, or a bypass relay. Their failure leads to the same result: the pump burns out.

A gas pump relay bypass tool allows you to energize and test the fuel pump. Measure the fuel pump pressure (30 to 80 psi). Fuel injection engines are very sensitive to fuel pressure. If the engine’s fuel ramp pressure is only a few milligrams lower than the specifications, the engine will not start or run incorrectly.

The pressure specifications differ depending on the type of fuel injection system, performance, fuel economy and emission requirements for that vehicle, and model specific to the year. Every application is different, so always look for pressure specifications when troubleshooting fuel-related performance issues.

Diagnosing your car yourself to avoid damaging the fuel pump

With the engine is idling, the fuel pressure must be within the limits specified by the car’s manufacturer. If it is smaller, the possible causes are: an inefficient pump (pulsating supply voltage from an imperfect contact, a 125-micron sieve of the internal filter of the pump clogged, dynamically unbalanced pump, etc.), low voltage at the pump, the fuel filter is clogged, a bad fuel pump assembly float inside the fuel tank, a bad fuel pressure regulator or the fuel tank is almost empty.

Fuel static pressure test

With the ignition on and the engine off (so with the fuel pump running), the fuel pressure should appear quickly and remain stable at the same value. Compare pressure reading with specifications.

If you get a pressure reading, but the value is lower than the one specified by the manufacturer, the cause may be: an inefficient pump (see above), faulty pressure regulator, clogged fuel filter, fuel line, inlet nozzle, or clogged tank.

Also, the low voltage at the pump prevents it from rotating fast enough which would generate a normal pressure (it could just be an insufficiently charged battery).

If the voltage value conforms to the manufacturer’s specifications, check the fuel filter and obstacle lines; the pump could be good.

If you do not get a read value, check the voltage at the pump. If there is a voltage but the pump still does not work, then the fuel pump is faulty.

Residual fuel pressure test

If you stop the pump, or it stops running, the system must still maintain the residual pressure for a few minutes (by consulting the specifications you can find out exactly what pressure must be maintained for a certain period).

If the pressure drops rapidly, the car may have a cracked fuel line, a one-way valve on the defective flow line, or the fuel pressure regulator may leak.

Gasoline volume test

A fuel pump that provides normal pressure can still cause maneuverability problems for the driver if it fails to provide enough fuel to meet the needs of the engine, so testing the fuel volume can be a great way to assess the condition of the pump.

The fuel volume test measures the fuel delivered within a specified time frame. This check is performed either by connecting a fuel flow indicator to the supply line or by disconnecting the return line from the fuel pressure regulator and connecting a hose from the regulator to a large container, in which all fuel in the tank can be put.

If the volume of fuel delivered in a certain predetermined time (and for which we have the value from the manufacturer’s specifications), is lower than normal, then we can have a used gas pump, a clogged fuel filter, a gas pipe clogged, a suction of the clogged gas pump, an almost empty tank, or a low voltage at the pump, which would prevent the generation of adequate fuel flow fast enough.

Fuel pump pressure regulating tests

This checks the operation of the fuel pressure regulator. Its role is to change the pressure in the pipe as a result of changes in the engine vacuum, to maintain the optimum working pressure at the injectors, and to compensate for changes in engine load.

The test is done with the engine running, and disconnecting the vacuum hose from the pressure regulator. With the line disconnected, the gasoline pressure should increase from 8 to 10 psi. If no change is recorded, then the pressure regulator is bad or a vacuum line is leaking or disconnected.

How much does it cost to replace a bad fuel pump?

If you choose to go to a workshop expect to pay between $350 and $700 for the new fuel pump and the labor but, this price can vary depending on the year, make, and model of the car.

If you choose to replace the fuel pump yourself, you will need some tools that are not that expensive and the work will take a few hours.

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