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Diagnose Fuel Pump (Part 3): Fuel System Scan Tool

Read Diagnose Fuel Pump (Part 2) : Fuel Pressure Test

Fuel System Scan Tool Checks

Look on a fuel pressure PID, if your scan tool can display OBD system data. Compare the specifications against the fuel pressure reading if it’s within range or not. To see if the result is the same, also compare the actual reading on the vehicle with a gauge with the scan tool pressure reading. Fuel pressure sensor is bad if the gauge reads higher. The system can’t develop and hold normal fuel pressure when the fuel pump is weak, you have a fuel pressure regulator leaks or low voltage to the fuel pump because of low fuel pressure.

To see the engine running lean, check the values of STFT (Short Trim Fuel Trim) and LTDT (Long Term Fuel Trim). Plus or minus 8 or less is the normal range values. Engine is running lean if you see Long Term Fuel Trim value greater than 10 or Short Trim Fuel Trim value greater than 8. A weak fuel pump or low fuel pressure possibly cause the lean readings issue. Also, engine vacuum leaks and other conditions can caused lean reading occurs.

Professional level scan tool that has bidirectional capabilities can be used to test the fuel pump. You can choose to run fuel pump self-test option from the scan tool diagnostics menu. To test variable speed pulse modulator operation, and to check the pump is runs, you can energize the pump when performing this test.

Fuel-related fault codes can be pulled from the PCM using any code reader or scan tool.

Fuel System Scope Tests

On a digital storage oscilloscope (DSO) you can view the pump’s current waveform, it can be done by connecting fuel pump’s voltage supply wire to a Low Amps probe. Internal wear on brushes and commutators that may not appear in volume tests or traditional pressure, it can be revealed by scope patterns.

To find out whether the normality of pump amp draw for application or high or low can be observed from the waveform, and when pump is operating at normal speed or running slow. All can be seen clearly in waveforms, if problems such short or open in the armature or bad spot on the commutator.

Generally the minimal variation and the relative consistency between high and low and seesaw back is a “good” electric fuel pump waveform. The presence of large or irregular drops on the pattern is a “bad” waveform, and between high and low have large different.

Engine Won’t Start Even The Fuel Pump Check Result Is OK.

The problem is the fuel injectors are probably not opening when your engine won’t start, but the fuel pump delivering normal pressure to the engine and there a compression and engine has a spark. To pulse the injector, PCM uses a trigger signal from the crankshaft position sensor, and/or camshaft position sensor. When key is turned on, the battery voltage should be received by the injectors, to energize the injector, the ground connection is provided by the PCM (Powertrain Control Module) driver circuit.

When the key is turned on, the voltage at the injectors is the first thing to check. If there no voltage, probably that caused the problem is a fuel injector power relay is bad, the wiring harness to the injectors faulty, or a blown injector circuit fuse. In power distribution center in the engine compartment or in the relay box, usually where the relay and injector fuse are located. If no problem with the fuse, in order for the engine can start, try switching the relay with the other to see if there is a voltage in the injectors. If there is no difference, probably the injectors don’t get a pulse from PCM, or there another injectors that does not open because of a shorted injector that robs power from the circuit.

TIP: Using an ohmmeter the resistance of each injector can be measured. Measure the resistance between the two terminals by unplugging the injector. Replace the injector, if resistance is outside specifications (high or low).
TIP: Crank the engine and try unplugging the injector one by one, and see if the engine can starts. When you unplug the particular injector and engine start, it’s mean shorted injector should be replaced.

Checking The Injector Pulses

To check injector pulses there a tool used by professional technicians called a NOID light. Flashes an LED light and digital signals in a circuit can be detected by a NOID light. It’s mean PCM is not pulsing the injectors when you are cranking the engine and there no flashes from the injector circuit. NOID light can’t help (is useless) when on newer systems of GDI (gasoline direct injection).

Check the spark plug and see if it’s wet, if so, maybe too much fuel flooded the engine, it’s can cause the engine won’t starting even the injectors working normally. Try again after the engine sits for a while, or set PCM into the “Clear Flood by hold the gas pedal all the way down while cranking the engine.

NOTE: Sometimes an improper starting technique on the part of the driver that cause a hard starting problem occurs. When trying to start the engine, on a fuel injected engine don’t pump or push the gas pedal. If you do that, the engine will puts on Clear Flood mode, and will make engine harder to start because it will cut off fuel to the engine temporarily. Don’t touch the gas pedal while try to crank the engine.

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