Electric Fuel Pump Diagnosis

In the old days, the fuel pump was a mechanical device. It was a simple design, utilizing a lever that moved a diaphragm to pump gasoline to the engine. The fuel pump was mounted on the engine in a location that allowed the lever to be actuated by a cam on the camshaft. Diagnosing a problem with the pump, as well as removing and installing it, were pretty simple. In most cases, the only place you saw an electric fuel pump was in a race car.

The mechanical fuel pump has been left behind in history, replaced by an electric one that can meet the demands of the modern fuel injected engine. The operation of this new style pump is controlled by the Powertrain Control Module (PCM), which receives input signals from the ignition voltage, crankshaft position sensor and camshaft position sensor. Based on this information, the PCM provides battery voltage to the fuel pump through the fuel pump relay.

The electric fuel pump is located in, and is a part of, the fuel pump module (Figure 1). It is a positive displacement, immersible design with a permanent magnet electric motor. The fuel pump, suspended in the fuel in the fuel tank, draws fuel through a strainer and pushes it through the motor to the outlet. 
A check valve, in the pump outlet, maintains pump pressure during engine-off conditions. The pressure regulator adjusts fuel system pressure to a specified value.

The rear-wheel drive Chrysler 300, Dodge Charger and Dodge Magnum models (LX Body) use a saddle-type fuel tank. This tank has a reservoir on both sides of the rear driveshaft. Because of this design, the tank has two fuel modules, one in the left side reservoir and one in the right side reservoir. The fuel pump is located in the left side, while the pressure regulator, along with the fuel outlet, is in the right side. Two hoses connect the two reservoirs. One is the fuel supply line and the other is the return, or siphon, hose.

It should be noted that the fuel pump is not a serviceable item. If it requires replacement, the complete fuel pump module must be replaced. And as you might have guessed, diagnosing problems with the electric fuel pump is more complex, compared to the mechanical pump. Also, servicing the fuel pump is a more challenging task than in days gone by.

DIAGNOSING FUEL PUMP PROBLEMS

Onboard diagnostics can often help the technician diagnose a wide range of problems. While there are a number of fault codes that indicate problems with fuel ratios, the fuel pump relay circuit and other related issues, there is no fault code for a physical problem with the fuel pump (this includes an inoperable pump, restriction in the fuel line, plugged inlet strainer, etc.). All is not lost, however. Fuel pump problems are often suspected when the vehicle cranks, but does not start. The experienced technician will realize that there are tests that can be run to diagnose this problem and pinpoint its cause.

Let’s review two basic tests that are detailed in the appropriate diagnostic manual. The first is Checking Fuel Delivery and the second is Checking Fuel Pressure Leak Down. Essentially, after performing these two tests, you should have a pretty good idea if the fuel delivery system is operating properly.

CHECKING FUEL DELIVERY

Test #1 determines if the fuel pump is operating. Using the DRBIII,® or aftermarket diagnostic tool equivalent, actuate the ASD Fuel System test and listen for the sound of the fuel pump operating at the fuel tank (a mechanics stethoscope might be helpful). If the fuel pump is operating, go to Test #2. 
If the pump is not operating, go to Test #5. Test #2 checks the fuel pressure. Before performing any fuel system test, you must release the fuel system pressure to avoid possible personal injury. After the fuel pressure has been released, install a fuel pressure gauge. Using the DRBIII, actuate the ASD Fuel System test and observe the fuel pressure gauge. There are three outcomes: 1. If the fuel pressure is within specifications, the fuel delivery system is operating normally 2. If the fuel pressure is below specifications, go to Test #3 3. If the fuel pressure is above specifications, replace the fuel filter/fuel pressure regulator

Test #3 determines if there is a problem with the fuel supply line. Turn the ignition off. Release the fuel system pressure. Raise the vehicle on a hoist and disconnect the fuel pressure line at the fuel pump module. Install the special 5/16 fuel line adapter tool between the disconnected fuel line and the fuel pump module. Using the DRBIII, actuate the ASD Fuel System test and observe the fuel pressure gauge. If the fuel pressure is within specifications, repair/replace the fuel supply line. If the fuel pressure is 
not within specifications, go to Test #4 (the supply line is okay).

Test #4 determines if the fuel pump strainer is plugged. Release the fuel system pressure. Remove the fuel pump module and inspect the inlet strainer. If the strainer is plugged, replace it. If the strainer is not plugged, replace the complete fuel pump module.

At this point, we have found the cause of our fuel pump problem if the pump was operational in Test #1. If the pump did not operate in that test, we jump ahead to Test #5. This test will check the fuel pump module harness connector.

Turn the ignition off. Disconnect the fuel pump harness connector. It is critical that the connector is free from any signs of corrosion and/or damage. Clean/repair the connector, as required. Turn the ignition on. Using the DRBIII, actuate the ASD Fuel System test. With a 12-volt test light connected to ground, probe the fuel pump relay output circuit at the harness connector. Does the test light illuminate brightly? If yes, go to Test #6 to check the ground circuit. If no, go to Test #8 to check the fuel pump relay fused B+ circuit.

For Test #6, turn the ignition off. Using a 12-volt test light connected to battery voltage, probe the fuel pump ground circuit at the harness connector. Does the test light illuminate brightly? If yes, go to Test #7. If no, repair the open/high resistance in the fuel pump ground circuit.

If you arrive at Test #7, there are no possible causes remaining for a fuel delivery problem. At this point, simply replace the fuel pump module.

Test #8 will check the fused B+ circuit. Turn the ignition off. Remove the fuel pump relay from the Power Distribution Center (PDC). With a 12-volt test light connected to ground, probe the fuel pump relay fused B+ circuit at the PDC. Does the light illuminate brightly? If no, this circuit is bad. Repair it and also look for an open fuse in the PDC. If yes, go to Test #9.

Test #9 checks the resistance of the output circuit. Turn the ignition off. Disconnect the fuel pump module harness connector. Measure the resistance of the fuel pump relay output circuit from the relay connector in the PDC to the fuel pump module connector. If the resistance is below 5.0 ohms, replace the fuel pump relay. If the resistance is at or above 5.0 ohms, repair the fuel pump relay output circuit.

CHECKING FUEL PRESSURE LEAK DOWN

The previous tests determine if the fuel system was providing fuel. This test will check pressure. First, release the fuel system pressure. Install a fuel pressure gauge. Start the engine and observe the fuel pressure reading. Turn the engine off. Monitor the fuel pressure gauge for a minimum of 5 minutes. If the pressure does not fall below the specified point, the fuel system is operating normally. If the pressure drop exceeds the specified amount, continue to the next step.

Visually inspect the fuel delivery system for external leaks. Repair any leaks that are found. Turn the ignition off. Install the special 5/16 fuel line adapter tool. Attach a fuel pressure test gauge to the T fitting on the special tool. Start the engine and allow the fuel system to reach maximum pressure. Turn the ignition off.

Using the special hose clamp pliers, slowly clamp off the rubber hose on the fuel pressure adapter between the fuel pressure gauge and the engine. Monitor the fuel pressure gauge for 5 minutes. Does the fuel pressure fall below the specified amount?

If yes, check the fuel delivery system between the fuel gauge and the fuel pump module. Check for any fuel leaks. If no leaks are found, the check valve in the fuel pump is not operating properly. Replace the fuel pump module.

If no, check the fuel delivery system between the hose clamp pliers and the engine. Check for any fuel leaks (don’t forget to check the fuel rail). If no leaks are found, check the leak down of each fuel injector until the faulty injector is located.