How to diagnose a faulty fuel pump

Fuel supply system fault accounts for a large proportion in the injection engine, one of fuel pump fault is relatively common in fuel supply system. Based on the analysis, elimination and summary of many cases of fuel pump faults. There are mainly two kinds of fuel pump faults: the fuel pump motor is open circuit and the fuel supply is insufficient. In this article we will discuss how to diagnose a faulty fuel pumpFuel pump fault, diagnosis, fuel supply system, injection engine, fuel pump, fuel supply, pump motor, all of these are the primarily elements and will be discussed.

The fuel pump motor is open circuit

Open circuit faults of fuel pump motor is the brush of fuel pump motor in bad contact with the commutator. In order to tell if the fuel pump motor has an open circuit fault, connect a lamp with an output power about 20W to the circuit of the fuel pump motor, and then observe whether the test lamp is on ( it should be on) when starting the motor. Use a multimeter  to test whether the fuel pump motor is under the supply voltage, this method is not reliable, because the normal voltage can often be measured when the low-voltage circuit is in bad contact. The resistance between two leads of fuel pump motor is measured with a multimeter (should not be less than 1Ω, along with the brush and commutator of the contact status becomes poor, the resistance is get higher), which can only determine the current of fuel pump motor is on” or “off”, but you can't tell if it can work normally. The working current of the fuel pump motor (3A - 5A) is the standard to determine whether it can work normally.

Generally, the fuel pump will not stop working, due to the partial open circuit of the motor brush and commutator when driving, because the inertia of the armature of the fuel pump motor can make the brush pass over the open circuit part of the commutator, and make the fuel pump motor not stop rotating, because of the instantaneous open circuit. However, if the brush and commutator are located in the open circuit or the brush is stuck, it  will come in bad contact with the commutator when the fuel pump is stopped. The fuel pump will not work when the engine is started again. At this time, if there is high voltage for ignition, also can hear the sound of the fuel injector action. But there is no fuel pressure, the engine can not start normally, and the fault diagnostic instrument can not detect the fault code. If the car encountered this situation on the road, a tool can be used to hit the fuel pump position at the bottom of the fuel tank. Maybe the vibration make the brush and commutator to restore conduction by hitting the fuel pump position at the bottom of the fuel tank. This method has proved to be very effective, as long as the fuel pump is turned on, it can continue to work.

If open circuit of fuel pump motor occurs only during engine operation, while everything is normal after the engine starting, it indicates the engine ECU can't store fault code. Because the moment when the engine ECU does not carry on air-fuel mixture ratio control, the oxygen sensor signal will work after the failure last for a certain time when engine ECU confirmed the existence of the fault.

The fuel supply is insufficient

The fault of fuel pump supply is insufficient. An insufficient fuel pump supply refers the fuel pump can operate normally, but the fuel supply is insufficient. When a fault diagnosis instrument is used to search for faults, maybe there are two occasional fault codes, which are excessively thin mixture and mixture adjustment exceeding the upper limit respectively. The reasons for these two fault codes are as follows: the fuel supply of the fuel pump is insufficient, the fuel pressure is lower, the fuel injection quantity of the fuel injector is reduced, and the oxygen sensor outputs the signal of thin mixture. Therefore, the engine ECU increases the fuel injection quantity (extends the fuel injection time), attempting to make the air-fuel ratio of the mixture reach the specified value. However, due to excessive fuel pressure drop, the engine ECU adjusts the fuel injection volume to reach the limit (more than the standard value 25%), while the air-fuel ratio of the mixture still fail to reach the specified value. Therefore, the engine ECU records the above two fault codes.

If there is a serious insufficient fuel supply in the fuel pump, it can be seen that the fuel pressure decreases as the throttle opens wide. When the engine is speeding up in situ after the oil pressure gauge is connected on the fuel supply line. If the fault is minor, it is difficult to observe the pressure drop (both engine idling and acceleration in-situ are normal), it is necessary to use the chassis dynamometer for loading test, or plug the return line of fuel pressure regulator by measuring the maximum fuel pressure, then determine the fuel supply capacity of fuel pump. At the moment, if the fault code shows the mixture is too thin, it may be caused by another cause. 

Note: It is necessary to ensure the filter element of fuel filter normal, then determine the fault of insufficient fuel supply in the fuel pump. The clogged filter element will make the fuel pump unable to supply fuel normally. If the fuel pump wear out, the fuel pressure and fuel supply will decrease. 

James Smith        August 13, 2020

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