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Fuel Delivery Testing & Diagnostics The Powertrain Control Module (PCM) calculates injector pulse-width based on engine speed and load. Pulse-width is further modified to compensate for changes from multiple inputs, including engine temperature, throttle angle, fuel trim and battery voltage. The PCM relies on the fuel delivery system to provide enough gasoline for all possible engine operating conditions. Fuel pressure and volume are considered “known good” values from the perspective of the PCM and are not directly monitored. Typically, fuel delivery problems will not directly set Diagnostic Trouble Codes (DTCs). However, low-pressure or low-volume conditions can cause changes in the operation and performance of the powertrain that may set codes as a symptom of improper fuel delivery. Low fuel pump pressure or volume can cause a lean mixture and fuel starvation at high speeds that may induce lean-related DTCs. Excessive pressure can result in poor fuel economy, rich mixtures and can cause rich-related DTCs to be stored in memory. Fuel delivery testing will ensure that the proper fuel pressure and volume are available under all possible operating conditions. Testing can be categorized into two areas: • No-start diagnostics • Performance-related diagnostics All fuel delivery test procedures are basically the same. They measure both the pressure and volume of fuel being delivered through the system under standardized testing conditions. In addition to testing for pressure and volume, measuring the amperage of the fuel pump circuit can help determine the cause of a malfunction. CAUTION: Releasing gasoline under pressure can cause fire and injury. Fuel Delivery Test Procedure – Pressure

Install a fuel pressure gauge to determine that fuel pressure is sufficient per manufacturer specifications. After it has been determined that the fuel pump is operating and supplying sufficient pressure, a volume test should be performed to determine if the proper amount of fuel is being delivered to the injector.

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Fuel Delivery Testing & Diagnostics (continued) Fuel Delivery Test Procedure – Volume A fuel pump volume test is performed by opening the fuel pressure line, energizing the fuel pump and collecting a sample in an approved measuring container. The fuel pump should deliver a specific quantity of fuel within a specified period of time. The volume of the gasoline measured should be compared with manufacturer’s specification. An example of a volume specification would read as one pint of fuel in a 15-second period (or the equivalent). Restricted fuel filters or clogged/damaged fuel lines can cause excessive resistance in the fuel pump circuit and adversely affect fuel volume. In cases when these items prove to be in good working order, additional diagnostics of the fuel pump and fuel pump circuit will be necessary. Initial fuel pump pressure and volume tests are usually performed statically (key on without the engine running). Static tests allow the technician to quickly determine if the fuel pump is capable of operating and if it is capable of generating pressure. Some vehicle systems allow the technician to enable fuel pump operation by controlling the fuel pump relay with a scan tool. Other systems may incorporate a test terminal where voltage can be applied to the pump feed circuit using a fused jumper wire.

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Fuel Delivery Testing & Diagnostics (continued) Fuel Delivery Test Procedure – Current Draw A Fuel Pump Current Draw test should be performed any time a performance complaint causes the technician to test fuel delivery. The fuel pump current can be measured by inserting an ammeter in series with the fuel pump load circuit at the fuse block and measuring current while the fuel pump is running. After verifying the integrity of the electrical circuit(s) a fuel pump that does not meet specifications may require replacement.

(See fuel pump amperage specifications on page _______ of this catalog).

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Fuel Delivery Testing & Diagnostics (continued) Fuel Delivery Test Procedure – Current Draw (continued) As a rule of thumb on a PFI-equipped vehicle, fuel pump current draw increases by 1 amp for each 10 pounds of fuel pressure. An amperage level above or below the specification indicates a malfunction is present. If pressure is low and amperage is above specification, check: • Fuel filter • Fuel line restrictions • Possible defective fuel pump If pressure is low and amperage is below specification, check: • High circuit resistance, voltage side • High circuit resistance, ground side • Possible defective fuel pump If pressure is high and amperage is above specification, check: • Possible restricted regulator and/or fuel return line Dropping Resistors Some fuel delivery systems include a dropping resistor connected to the fuel pump to reduce the voltage required to drive the fuel pump and a controller for determining an optimum transition point between relatively high- and low-speed fuel pump operations so that a controlled amount of fuel is delivered to the engine. The controller calculates the fuel pressure increase across the fuel pump, determines the minimum fuel pump voltage necessary to drive the fuel pump (such that a predetermined amount of fuel is supplied to the engine) and determines the current draw of the fuel pump. The controller then calculates the voltage drop across the dropping resistor and compares the resulting voltage with the minimum fuel pump voltage. The controller then selectively bypasses the dropping resistor based on this comparison. NOTE: Dropping resistor circuits can affect a fuel pump current draw test, voltage drop tests and available voltage tests.

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