Today’s fuel delivery systems have many different variations including conventional, pulse-modulated, and direct injection. Each variation has a specific set of components and testing issues. Here are a few ways to test fuel systems, and how to avoid common mistakes in diagnosing fuel delivery systems.

Conventional Fuel Delivery Systems

The PCM commands the fuel pump tho apply 35-65 psi of fuel pressure to the injectors by grounding the primary fuel pump relay circuit. On two-line fuel delivery systems fuel pressure is controlled by an external, vacuum-modulated fuel pressure regulator. On a single-line fuel delivery system fuel pressure is controlled by an internal, non-modulated fuel pressure regulator in the fuel pump module.

The two-line fuel delivery system returns excess fuel to the tank using a return line, while a single-line fuel delivery system uses the same line. Most conventional fuel delivery systems include a Schrader valve at the fuel injector rail to allow for mechanical pressure testing.

When the ignition switch is turned on, the fuel pump is activated for several seconds to prime the fuel injectors. The fuel pump activates again during cranking, but deactivates when the ignition key is released. When the crankshaft position (CKP) sensor indicates to the PCM that the engine is running, the PCM continues to activate the fuel pump relay without input from the ignition switch.

Pulse-Modulated Fuel Delivery Systems

Pulse-modulated fuel delivery systems, unlike conventional fuel delivery systems, control fuel pressure by changing fuel pump speed using a pressure sensor in the fuel injector rail. In most cases, the fuel control module changes the pulse width of the current being delivered to the fuel pump. These systems are diagnosed with a scan tool.

Direct Injection Fuel Delivery Systems

Direct injection fuel delivery systems are the most fuel efficient. Direct gasoline injection uses a conventional single-line, low-pressure fuel pump and integral pressure regulation system to supply fuel to a high-pressure mechanical fuel pump mounted on the engine, and uses separate sensors to monitor the low and high pressure portions. Since high pressure fuel can seriously injure a technician, direct injection fuel delivery systems repairs have to follow specific procedures such as replacing fuel lines rather than reinstalling, and using a scan tool to relieve the pressure before disassembly. Last, engine oil must meet OEM specifications to prevent premature wear on the engine camshaft and high-pressure fuel pump follower. Since the various direct gasoline injection-operating modes are too voluminous to address in this brief space, suffice to say that direct fuel injection requires scan tool-based ­diagnostics and a specific understanding of the system’s various operating modes and failure points.

Check the Basics First

Symptom-based fuel pump ­diagnostics aren’t always accurate because a hard-starting complaint can be caused by something as simple as stale gasoline. Since stale gasoline doesn’t vaporize properly, it causes hard starting and poor cold-engine performance. Using E-85 in a non-flex fuel engine can also cause problems.
Rusty gasoline can completely clog a filter, even if it looks new. Also, a defective sensor can cause driveability issues that mimic a defective fuel pump. This is still another reason to use your scan tool to evaluate sensor data streams and search for pending trouble codes before condemning the fuel pump.
The fuel level sensor can get stuck, so always add a few gallons of gasoline before testing to ensure the fuel pump is completely submerged. In addition, visually inspect the fuel tank for dents or other damage that might interfere with pump operation.

Cranking, No-Start Testing

The first step should be to hook up a scan tool to help identify what kind of fuel delivery system you’re dealing with by looking at the data returns. The second step would be to poll the various modules for error codes, and access bi-directional fuel pump controls because some PCMs may not activate the fuel pump if a module is malfunctioning. Finding a code for the problem saves many steps in diagnosing. In addition, many scan tools include bi-directional controls that allow the technician to test the fuel pump relay and pump circuit by electronically activating the fuel pump. Here again, a simple activation test saves many pinpoint electrical tests. If the fuel pump activates, complete the testing by adding a gallon of fuel to verify fuel level sensor operation and attach a mechanical gauge to measure fuel pressure on conventional systems. If the fuel pump doesn’t activate by scan tool command on a conventional system, remember that some fuel pumps turn off after a collision or air bag deployment are set off from off road driving. Some PCMs might also use data from the airflow or CKP sensors to deactivate the fuel pump if the engine has stalled, so a bad sensor or connection could prevent the fuel pump from activating. As a last step, check for available voltage and for a clean ground at the fuel pump tank connection. Fuel-related loss of power complaints can be addressed by recording short- and long-term fuel trim numbers with a scan tool while road-testing the vehicle. If the short-term fuel trims begin to exceed about 20% at wide-open throttle (WOT) and stores a P0171 on in-line engines and a P0174 on V-type engines, the engine has a fuel delivery problem. But remember that a faulty MAF sensor can cause high positive fuel trims. This can be checked against the calculated engine load. If it’s less than 80% the MAF is bad, if it’s higher, suspect a fuel delivery problem in the fuel pump, filter or fuel lines.