Auto Service Professional

APR 2018

Magazine for the auto service professional

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20 | ASP April 2018 Technical check the oil level on these systems. Before breaking any fuel lines on these systems, either remove the fuel pump fuse or remove the fuel pump relay and crank the engine over until the engine stalls and high fuel pressure is relieved. Before Ford came out with their GDI systems, they were using electronic returnless systems as early as 1998. On the injector rail is a three-wire fuel rail pressure sensor. It also had a vacuum line hooked to a manifold vacuum source. e engineering idea here is that the PCM can see the exact fuel pressure drop across the injectors. e rule is that for every 2 in. of vacuum, the tips of the injec- tors that are exposed to it will increase the pressure drop across the injectors 1 psi. For example, at 18 in. of vacuum, the pressure drop across the injectors will increase by 9 psi. If the rail pressure reads 35 psi, the scan tool will read 44 psi. e fuel pressure gauge readings and the scan tool fuel pressure readings will never be the same. One problem we have seen on these systems is that the sensors are known to leak fuel pressure into its vacuum source, causing rich conditions and negative fuel trim corrections. In this case, by unplugging the vacuum source and noting fuel trim values corrected verifies a leaking sensor. On the GDI systems, the injectors cannot operate with 12 volts as in conventional PFI systems. e injectors are charged by capacitors inside the PCM with 65 volts. Two injectors are charged at a time but only one injector is supplied with a ground by the PCM via the sequential firing order of the injectors. When diagnosing a no-start, where you have ignition and fuel pressure, the need to check the injector drive circuit would be necessary. Noid lights and test lights would not work. In addition, access to the injectors would require a top engine tear-down. e easy access would be at the PCM or at the wiring harness. When viewing a voltage trace, a two-channel scope would be needed. One channel would go to one wire to the injector while the other channel lead would go to the other wire. During cranking you would see 65 volts on both channels. When the PCM wants the injector to be energized, the PCM will again supply the ground (see Figure 4). A low inductive current probe was clamped around the injector control wire. e amperage values on these GDI injectors vary between 8 and 12 amps. On this GM GDI system peak current flow reached 12 amps before the PCM modulates the ground to limit the current flow. e voltage trace on the right shows that the PCM capacitively charged the injector with 65 volts. e resistance values of the GDI injectors are very low in the range of 1.5 ohms. Removing the injectors will require a small slide hammer tool. In addition, a bore brush would be needed to clean the injector bore. Remember, the injector is inserted into the Figure 4: When the PCM wants the injector to be energized, the PCM will again supply ground. A low inductive current probe was clamped around the injector control wire. The amperage values on these GDI injectors vary between 8 and 12 amps. On this GM GDI system, peak current flow reached 12 amps before the PCM modu- lates ground to limit current flow. The voltage trace on the right shows that the PCM capacitively charged the injector with 65 volts.

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