The most common diagnostic procedures for fuel pumps in the past were analog and hands on. Most fuel pump-related problems could be solved with a pressure gauge and voltmeter. Today, the scan tool is the most important tool when diagnosing a fuel supply problem.
On early vehicles, the fuel pump was energized when the key was turned on and a vacuum-operated diaphragm regulated fuel pressure. Today, input from at least two modules and various sensors that are networked on a high-speed serial data bus is required for a fuel pump to operate. While this may sound like it would complicate the diagnostic process, it actually simplifies diagnostics and can save you from unnecessarily dropping a fuel tank.
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.
What this also means is that this data can be monitored with a scan tool. If the serial data bus is unable to communicate with certain modules like the theft deterrent system or even the Body Control Module (BCM), it could cause the fuel pump to 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.
A scope is required to graph the amperage and 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.
The most common customer complaints when it comes to fuel pumps are a no-start condition, intermittent no-start condition or even hard starting. The first step in any diagnostic process is to perform a visual inspection of the vehicle.
Next, verify the customer’s complaint. Many diagnoses go wrong because the technician fails to verify the customer’s concern. If the customer says it does not run, make sure it will not start and run.
Forget your “noid” lights on most modern vehicles. This low-cost tool worked well on simple vehicles, but with modern vehicles it can lead you down a diagnostic black hole. If the vehicle has Gasoline Direct Injection (GDI), there is no way you could even access the injectors to install a noid light. If you do feel compelled to prove the injectors are pulsing, try using a scope.
Forget the fuel pressure gauge at this point in the diagnostic process. Even if there is pressure at the fuel rail, this information is of little use on newer vehicles without having access to the parameters. Some port fuel injection systems and all GDI systems have pressure sensors that can be observed with a scan tool. Also, GDI-equipped Asian and European models do not have ports to attach the gauge.
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. This is where an enhanced or factory scan tool comes into its own.
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.
With the codes pulled, you can come up with a diagnostic strategies and further tests to resolve the no-start condition. Service information is just as critical of a tool as a pressure gauge.
Every fuel system has a set of parameters that must be set in order for the pump to be energized. For some systems, this may include a crank sensor signal, oil pressure and maybe a check with the vehicle theft deterrent module.
If the vehicle has any “loss of communication” codes like U1000, resolve those problems first before diagnosing or replacing the fuel pump. While these codes may seem like they have nothing to do with the fuel pump, often a dead module or short in the serial bus can result in a 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 fundamentals for GDI are not that much different than conventional fuel injection systems. These systems inject the right amount of fuel directly into the cylinder. These systems are very efficient and are able to get the right amount of fuel into the cylinder so no fuel is wasted by not having to spray on the back of the intake valve.
In fact, after working on a few GDI systems, you may find that they get easier to work on due to the tighter long-term and short-term fuel trim parameters.
GDI makes more horsepower for a given engine size. This is why Mercedes-Benz and BMW have been able to get away from V10 and V12 engines. Utilizing GDI systems, their new V8s are able to make more power 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.
With the injector in the combustion chamber, the pintle and seat of the fuel injector are under extreme pressures. To overcome the cylinder pressures, the fuel pressure supplier to the injector may be as high as 2,000 psi.
The in-tank pump in GDI systems is more responsible for volume than pressure. Fuel on this side of the system is called the low-pressure side. A fuel pump on the engine pressurizes the fuel for high-pressure injectors. This pump is driven off a lobe on the camshaft. This part of the fuel system is called the high-pressure side.
The pressure from the high-pressure fuel pump is monitored by the Powertrain Control Module (PCM) through a sensor and can be modulated by changing the volume of fuel entering the pump inlet. While specific pressures vary among different vehicle applications, most high-pressure pumps are capable of producing at least 2,000 psi of fuel pressure. These extremely high fuel pressure levels are required to overcome compression and combustion pressures inside the cylinder and to inject a relatively large volume of fuel directly into the cylinder in a very short amount of time.
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. The follower that rides on the camshaft can wear and lose metal at the base.
The pump is very sensitive to the changes in dimensions of the follower and it can result in lower fuel pressures. This condition is initially diagnosed with a scan tool and not feeler gauges.
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.
Article courtesy Underhood Service.