The Fuel pump is considered the heart of the engine. Nothing in your vehicle will be more annoying than a malfunctioning fuel pump.
Often people notice issues related to the fuel pump since it is the part that feeds the engine.
Problems related to the fuel pump will never go unnoticed since it directly manifests into vehicle driving issues.
So, the fuel pump glitches shall be diagnosed as early as possible to keep the wheels moving. Otherwise, it would result in makes blunders during usage.
Often people experience that the fuel pump runs continuously with key ON and it is due to issues like bad ECU, incorrect wiring, faulty/stuck fuel pump relay, malfunctioning crank position sensor or cam sensor, bad oil pressure switch, ignition switch short, defective airflow metering sensor, or wiring connection mix-up issue.
If you are experiencing simillar issue, then you are not alone. This is one of the most frequently reported problems that people often face.
Here in this article, we will get to know the details for the cause of this constantly priming fuel pump and the solution to fix it.
Table Of Contents
How Fuel Pump Priming Works?
If you are observing that the vehicle battery is frequently draining off and can hear fuel pump operating sound from the back of your vehicle even if the key is in ignition on condition, then probably it is associated with the constantly priming fuel pump which endlessly draws current from the battery until it dies.
Before we talk about the reasons for the subject problem, we shall first understand what is fuel pump priming and how it functions?
Last time, i wrote an article on fuel pump primes but no fuel pressure on pump outlet. You may go through that article if you want to understand fuel pump priming function in detail.
The fuel pump priming is a function that is incorporated in most of the in-tank electric fuel pumps which ensures that the pressurized fuel is available at the engine injector inlet at any given time.
Before we start cranking the engine, when the key is in ignition on position, the vehicle ECM (Electronic Control Module) grounds the fuel pump relay to fclose the power supply circuit to the fuel pump to run the fuel pump for 3 seconds.
After 3 seconds, the ECM cuts off the power supply to the fuel pump by switching off the ground to the fuel pump relay, until the engine motion is detected by the crank sensor.
The crank sensor sends signals to the ECM to activate the fuel pump when the engine starts running. This sequence is pre-defined and programmed in the ECM.
This ensures that the fuel pump runs and maintains the fuel supply hose under desired pressure and ready for fuel injection before the start of the engine crank.
This is required for the atomization of fuel upon injection into the engine manifold for ensuring proper combustion.
But many times the fuel pump keeps on constantly priming and the fuel pump won’t shut off with the key in ignition on position after the preset timing.
If the fuel pump stays on with key on, the first thing you would do before it kills the battery overnight is to remove one of the battery terminals to shut off the fuel pump.
But it is frustrating to do this temporary fix on daily basis and you may accidentally end up screwing some other electrical parts of your vehicle.
Fuel Pump Runs Continuously With Key On? (9 Causes And Solutions To Fix It!)
Below are the probable causes for the fuel pump to run continuously with key on. You may try to figure out what is causing this problem after analyzing each root cause in your vehicle for taking further course of action.
Bad ECM
If the fuel pump is constantly priming, then it means that the ECM that is supposed to shut off the power supply to the fuel pump after 3 seconds of initial priming of pump; is not doing so.
This means either the ECM is bad or the driver in the ECM could be shorted, otherwise, it would have ungrounded the fuel pump relay to trigger the fuel pump shut off.
When the fuel pump priming is completed, the voltage to the fuel pump shall be stopped but due to bad ECM, the fuel pump relay is continuously grounded. Hence the fuel pump keeps on continuously running when car is off.
You may notice traces of moisture and burn marks on ECM. These are the confirmatory signs that the issue is due to the bad ECM and it’s time to replace it with a new one.
If any of your friends have the same vehicle, then you may try swapping the ECM to confirm that it is the culprit before you buy a new one.
Faulty/stuck fuel pump relay
The Fuel pump relay is the part that controls the power supply to the fuel pump and if the fuel pump relay is stuck closed or faulty then it would send continuous power to the fuel pump, even when the key is kept in ignition ON position (not engine cranking)
So the fuel pump would be constantly priming regardless of the key position if the fuel pump relay contacts get stuck closed.
Auto mechanics sometimes short the fuel pump relay by putting a fuse or jumper wire between the load side relay blades to fix the problem with the relay.
This keeps the fuel pump running continuously without the ground input from the ECM, after 3 seconds of fuel pump priming.
With the key in ignition ON condition, the contacts of the relay should be open after the completion of fuel pump priming. If not, it means that the fuel pump relay is faulty.
Fix for the faulty fuel pump relay
Generally, the ampere rating of the fuel pump relay matches with that of some of the other relays in the TIPM plastic box (Total integrated Power Module houses all vehicle relays).
So try swapping the fuel relay with the same rating and confirm whether the fuel pump stops constant priming.
If yes, then replace the fuel pump relay with a new one. If not, you are not done. The issue lies somewhere else.
Normally, when the fuel pump is reaching its end of life, it would draw more current which may cause the fuel pump relay contacts to fuse.
You may bypass the fuel pump relay, if you want a temporary fix to deal with a faulty fuel pump relay.
Or if you keep the relay powered up long enough then also this problem may happen. So you may also look for signs of a failing fuel pump.
Incorrect wiring
Check whether the wire from the fuel pump relay to the ECM has become corroded or damaged and is exposed by touching the frame of the vehicle thereby grounding the fuel pump relay permanently.
If so, then it would keep the fuel pump relay constantly ON that would give the fuel pump a constant 12 V with the key on. Hence the fuel pump won’t shut off in this case.
If this non-stop fuel pump running thing occurs once in a while, then it also indicates that there is a false ground in the wire between the fuel pump relay and the ECM due to intermittent contact with the vehicle frame.
It is also possible that the fuel pump wiring is accidentally plugged into the 12 V supply directly during diagnostic testing by the mechanic and has bypassed the fuel pump relay altogether.
All of the above incorrect wiring conditions would lead to the fuel pump constantly priming problem.
Work around for incorrect wiring
Ensure that the electrical wiring from the fuel pump relay to the ECM is routed away from any sharp objects to prevent unintended fouling.
This would cause wear and tear of wiring due to the vehicle vibrations and thus leads to possible permanent earthing of wire.
If you notice that the wiring harness is compromised, then fix these areas by covering short pieces of electrical tape over the exposed area and thereby prevent unintentional ground connection to the fuel pump relay.
While replacing or repairing the electrical connectors and wiring, you need to confirm the fuel pump wires color codes to avoid possible wrong wiring. Otherwise, the vehicle won’t start as the fuel pump won’t get power.
Refit fuel pump relay and fuse
Even though it may sound strange as it looks, many times simply pulling out the fuel pump relay and the fuses and plugging it back, helps in shutting off the fuel pump with key on.
This is because the electrical parts are always sensitive to dirt and dust particles. Unclean and dirty contacts often lead to this kind of erratic behavior.
Faulty crank position sensor or cam sensor
The ECM sometimes receives a crank RPM signal even when the engine is not running, in case of a faulty crank position sensor.
Once the ECM senses crank rotation signals, then the fuel pump would continuously run on irrespective of the key position (ON or start).
A malfunctioning crank or cam sensor may fake the fuel pump and it won’t shut off. So it is advisable to inspect the crank position sensor and replace it with a new one.
Bad oil pressure switch
This is provided as a backup in case the fuel pump relay fails. Either one can activate the fuel pump.
Usually, the oil pressure sending unit has 3 wires. Orange, Grey, and Tan. If it does, then pull off the wiring connection to the oil pressure switch, when you experience the issue of the fuel pump continuously running with key on without cranking the engine.
If the pump shuts off, then it’s the oil pressure switch that keeps the fuel pump constantly priming.
Your vehicle may either have residual oil pressure which supplies to the fuel pump or the oil pressure switch is stuck closed or it is the bad oil pressure switch that keeps the fuel pump constantly priming.
So you would need to replace the oil pressure switch with a new one to fix the issue. The oil pressure switch is usually located above the engine oil filter.
Ignition switch short
If the fuel pump stays on with key on, then probably the ignition switch is shorted and is feeding the power supply to the fuel pump even the car is off.
Put the key in the ignition switch and shake it a bit and see if the fuel pump shuts off. Often if there is an internal short and the fuel pump would constantly prime and won’t shut off. But if we wiggle the ignition switch, it may help in breaking the unintended contact.
If you measure 12 V at the ignition fuse with the ignition switched off, then the Ignition switch is probably faulty and it is time to replace it with a new one.
Defective airflow metering sensor
In old vehicles, we have this vane airflow metering sensors (VAF) to measure the volume of airflow through the engine. This airflow sensor works by sensing the movement of the spring-loaded flap which is connected to a potentiometer by a lever arm.
This arm converts the flap movement into voltage signals for sending it to ECM to control the air-fuel ratio along with other vehicle sensors input.
The airflow sensor has a safety switch for the electric fuel pump relay. The air flow through the sensor activates the fuel pump.
This safety switch shuts off the fuel pump in case of an accident scenario (when the engine is not operating, so no air flow) to cut off the fuel supply.
In case of a defective airflow sensor or if the safety switch connection on the airflow metering sensor plate is plugged off then the safety switch will be closed, then the fuel pump won’t shut off with key on.
So ensure that the safety switch is plugged in. Generally VAF are not serviceable, but you may try removing the sensor housing and apply some electronics cleaner to clean the rheostat contacts. This may reinstate the VAF to regular operation if the contacts are not worn-out, but it would be no help if the sensor has a faulty electronic parts or a damaged flap.
Wiring connection mix-up
Ensure that the fuel-pump related wiring connections are not mixed up. For example, if the airflow sensor connectors and the cold start injector connectors are confused for one another, then the swapping would result in neither performing ok.
So you would have trouble cold starting the vehicle and at the same time, the fuel pump won’t shut off and run continuously (since the airflow sensor has a safety switch that cuts off the power to the pump)
How To Test The Fuel Pump Relay?
Often it is the fuel pump relay that attributes to the problem of continuously running fuel pump with key on, so it is worthwhile to understand whether the fuel pump relay is the actual root cause of the problem or something else by performing the below set of tests.
Fuel pump relay usually has got 4 pins (3 or 5 pins is also not uncommon). Out of these 4 pins, 2 pins are from the control side (input voltage (86) and ECM ground (85)) and 2 pins belong to the load side (fuel pump (87) and battery voltage (30)).
The pin names are usually printed on the fuel pump relay and the naming convention may vary from one manufacturer to another. But they are always self-explanatory.
Unplug the fuel pump relay which is located under the vehicle hood inside the TIPM plastic box (sometimes also called power distribution center) and connect leads to the wiring terminals sitting beneath it and plug the fuel pump relay to start testing.
Continuity test
Set the multimeter for the continuity test and connect one end of the multimeter probe to one of the load side lead (87) and connect the other probe to the other load side (30).
If the circuit is continuous, you would hear a beeping sound and would mean that the circuit is short and need to replace the fuel pump relay.
Resistance test
Set the multimeter to read the resistance across circuits. Measure the resistance across the above 2 load points (87 and 30).
If the multimeter reads any resistance like zero or in hundreds or thousands instead of showing infinity.
Then it again confirms that the fuel pump relay is shorted and needs replacement.
Voltage test
Peg the negative lead of your multimeter (or the end of your test light) to the load side battery terminal (30) and connect the positive lead of the multimeter to the load side (87) of the relay connector.
Now turn the key on (don’t crank the engine). If the multimeter read 12 V (test light come on) for 2 secs, then go out or does the light stay on?
If it comes on and stays on and the multimeter continuously reads 12 V, then it means that a default ground is established between the fuel pump relay and the ECM.
So irrespective of the key position the fuel pump won’t shut off, so look for false ground between ECM and relay and fix it off.
Beep sound Test
To isolate the fuel pump relay from other probable issue areas like ECM and wiring harness, this test can be performed to confirm a standalone issue in the fuel pump relay.
Connect the 2 probes of the multimeter to the load side leads (87 & 30). Then connect the ground lead of any 12 V power supply to the control side ground (85) of the fuel pump relay.
While you touch the 12 V trigger of the external power supply to the control side (86) you would hear a clicking sound from the fuel pump relay which ensures that the coil and the switch contacts are working perfectly. If not, you would need fuel pump relay replacement.
If you are performing this test on vehicle level, since the control side connections are already available, you need to just do the load side connections as explained above and check the fuel pump relay.
Conclusion
The ECM monitors all signals from the various sensors and the relays. It is the key component that allows the vehicles to perform various interconnected operations effectively with pre-defined logic.
Each OEM has its own way of controlling the vehicle components with different criteria to operate the ECM with varying input/output signals from the concerned components.
To determine the power supply to the fuel pump and to confirm that the engine is running, some manufacturers may use just the fuel pump relay and crank position sensor input, whereas others may monitor reference pulses from the ignition module or indications from the oil pressure switch, or airflow sensor safety switch.
Vehicle manufacturers constantly come with creative ways to revolutionize these systems.
The pre-defined fuel pump priming process time is controlled by the ECM via a grounding and un-grounding of the fuel pump relay and fuel pump constantly priming outside the boundary conditions of the ECM is the possible result of defective ECM, fuel pump relay or the wiring ground fault.
Hope the above information would have enhanced the knowledge about the troubleshooting of the fuel pump and would assist you in fixing the problem of the fuel pump running continuously with key on.
Happy Motoring!
References
https://forum.classicmotorworks.com/index.php?topic=25632.0
https://www.thirdgen.org/forums/tpi/45792-when-key-fuel-pump.html
https://www.s10forum.com/threads/fuel-pump-wont-shut-off.802689/
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