Symptoms of a failing fuel pump
Fuel pump symptoms — Car won’t start right away and requires extended cranking time after sitting overnight
Fuel pump symptom: You turn the key and the engine cranks for more than 5-secs. before the engine starts.
Extended cranking on cold starts is often a sign of a failed check valve. The check valve prevents fuel pressure from draining back into the tank after engine shut down. It also maintains close to full fuel
pressure in the fuel rail rapid starts. A drop in fuel pressure in excess of 5-psi. overnight is often a sign that the check valve has failed. When pressure falls, the fuel pump must refill the fuel rail and build adequate fuel pressure to work the fuel injectors. That requires the fuel pump to run for longer periods and results in extended cranking. The check valve is located in the fuel pump and cannot be replaced separately.
The most reliable test for a check valve is to do a leak down test. The technician attaches fuel pressure gauge to the fuel rail and starts the engine. Then the engine is shut down and left overnight. The technician check fuel pressure the next morning and records the pressure reading. Generally, a drop of more than 5-psi. means the check valve has failed. But often the technician will see a total loss of fuel pressure due to a leaking check valve.
If you don’t own a fuel pressure gauge and want to do a DIY test, perform this on a cold start first thing in the morning:
Turn the key to RUN, not start. Listen for the fuel pump to start and run for approximately 2-secs.
Then turn the key to OFF and then back to RUN. The fuel pump should start and run a second time for approximately 2-secs.
Repeat the procedure one more time. Each time the pump runs for 2-secs., it’s priming the fuel rail and building pressure.
Then turn the key to START. If the engine fires right up without extended cranking, the check valve is most likely the culprit and the fuel pump must be replaced.
Car won’t start right away and requires extended cranking time after driving for a while
Fuel pump symptom: Extended cranking after engine has run for long periods
Extended cranking after running for long periods is often due to an overheated fuel pump motor, worn armature, or dead spot on armature. Fuel pump motors can become overheated and damaged by extended periods of driving on a low tank. Electric fuel pumps rely on fuel to cool the motor. See the image above and follow the fuel flow arrow to see how fuel flows directly through the pump to cool it.
The pump actually heats the fuel while running, and if you drive the vehicle with the tank low on fuel, you’re literally trying to cool the pump with warm fuel. That causes fuel pump failure.
To test for a worn or overheated motor, the technician should perform a current draw test, scope the voltage drop while cranking the engine to detect a dead spot on the armature, perform a voltage drop test across the fuel pump relay contacts, and check for voltage drop at the ground connection.
Fuel pump symptoms — the fuel pump makes a loud whine when key is turned to RUN position
A good fuel pump makes a slight humming sound while it’s priming the fuel rail at startup. However, a loud whining sound from the fuel pump indicates a more serious
problem. The loud whine can be caused by a clogged in-tank filter, clogged post pump fuel filter, or a bad fuel pressure regulator or pulsator. A pulsator is a noise reduction device that dampens the vibrations created while pumping.
However, since the fuel pump, fuel pressure regulator and pulsator are often situated inside the fuel pump module, replace the entire fuel pump module instead of trying to replace the individual components. The bulk of the labor in removing the fuel pump module is in the labor to drain the tank and lower it.
Fuel pump symptoms — The vehicle lacks power, hesitates or stalls while driving
The fuel pump must deliver the proper pressure and fuel volume to match the requested engine load and speed. A failing fuel pump might deliver the proper fuel pressure but not the correct volume and vice versa.
To test fuel pump operation, a technician would install a fuel pressure gauge and note the pressure readings while attempting to recreate the driving conditions when the problem occurs. If the fuel pump maintains pressure under those circumstances, the technician would perform a fuel pump volume test which involves measuring how much fuel the pump delivers in a specified time period.
Depending on the year, make and model, the technician should also test the fuel pump driver module for proper operation. Newer cars use a driver module to regulate the speed of the fuel pump motor to conserve power. The fuel pump driver increases fuel pump speed to match input from the vehicle driver.
Fuel pump symptoms — Intermittent no start
This is the toughest symptom to diagnose since it happens intermittently. In this case the shop may request that you leave the vehicle for several days so the technician can attach test equipment and drive it until they duplicate the problem.
©, 2017 Rick Muscoplat
Posted on by Rick Muscoplat
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