Fuel Pump Diagnostics Using Scan Tools

Most common fuel pump diagnostic procedures in the past could be performed with a pressFuel Pump Cross Sectionure gauge and a volt meter. Today, the most important tool is the scan tool. On early import vehicles, the fuel pump was energized when the key was turned on and a vacuum-operated diaphragm regulated fuel pressure. Today, input from two modules and various sensors are required to operate the fuel pump. While this may sound complicated, it actually simplifies diagnostics. With a scan tool, it’s possible to verify if the modules controlling the fuel pump are receiving the correct data like oil pressure, crank position and key ­position. Some late-model imports have even turned the fuel pump into its own module or node on the high-speed serial data bus. The module may share data like the fuel level and tank pressure with the ­instrument cluster module and the ECM. This means that this information can be monitored with a scan tool. If the serial data bus cannot communicate with other modules, the fuel pump can shut down. Most late-model vehicles have return-less fuel systems. Instead of using engine vacuum to a pressure regulator under the hood, the system uses engine data and varies the speed of the pump to meet fuel requirements. The pump is energized with pulse-width modulated voltage. This means that if you connect your voltmeter to the fuel pump circuit, the readings will bounce around instead of being a constant voltage. These systems have different modes for start, acceleration, deceleration and fuel cut off. On some vehicles, these modes can be observed on an enhanced or factory scan tool as part of the Mode 6 Data.

Initial Diagnostics

The most common customer complaint for fuel pumps are a no-start condition, intermittent no-start condition, or hard starting. First, perform a visual inspection.Fuel Pump Flow Chart Then verify the customer’s claim. If they say it doesn’t run, make sure it doesn’t run. Forget your “noid” lights on most modern vehicles. There is no way to install a noid light on a gasoline direct injection vehicle. If you do feel compelled to prove the injectors are pulsing, try using a scope. Some port fuel injection systems and all GDI systems have pressure sensors that can be observed with a scan tool.

Scanning the Fuel Pump

After the visual inspection and verifying the customer’s complaint, it’s time to connect the scan tool. First, pull the codes and make sure the modules are communicating on their communication buses. Some low-end generic tools may not be able to talk to all the modules. Many enhanced or factory scan tools can perform a “health check” that can pull codes and parameters from the modules on the vehicle with just one press or click. Some scan tools have automated tests that can bi-directionally control components to automatically confirm operation. If you receive any interrupted communication codes from the scan resolve those first because even if they don’t seem connected to the fuel pump, chances are that they might be causing the no-start condition. After you’ve performed the checks with your scan tool and have confirmed with the service information that it could be the fuel pump causing the no-start condition, you can carry out the physical tests to confirm the condition of the fuel pump.

GDI Strategies and Scan Tools

Diagnostics are fundamentally the same for GDI and convention systems. These systems are very efficient and overtime, become easier to work on due to their tighter fuel trim parameters. GDI makes more power for a given engine size making V8s perform like V10s and V12s while using less fuel. The diagnostic strategies are similar to port fuel systems, but most of these systems have an additional fuel pump, pressure sensors and a different style of injector. Factory and enhanced scan tools can monitor pressure transducers on the high and low sides of the system. This information can be used to diagnose the health of the low-side and high-side pumps. These tools will have the PID parameters for these components as part of the Mode 6 data. These parameters can tell you what the pressures should be during the different modes of operation. Also, if this data is used in conjunction with the waveforms of the injector pulses, it’s possible to ­perform cylinder balance and other diagnostic tests. The pressure transducers can also be used to monitor system pressures to diagnose hard-start ­problems. Maintenance and the reduced frequency of ­engine oil changes have been known to take their toll on some GDI high-pressure pumps. For example, some VWs and Audis are experiencing wear on the follower on the pump due to poor lubrication and oil that has broken down. With late-model imports, so much of the diagnostic process for fuel pumps can be performed from the driver’s seat of the vehicle with a scan tool. This makes you a more productive technician and the ­diagnosis more accurate, and this translates into a more profitable shop.