Auto Service Professional

OCT 2017

Magazine for the auto service professional

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Page 17 of 77

16 | ASP October 2017 Technical not all fuel pumps are able to compress the fuel vapor to a liquid again. In this case, the engine won't start until the vapor cools and condenses on its own. Fuel pressure testing Low fuel pressure or delivery problems can be caused by a worn or damaged fuel pump, a faulty pressure regulator, a clogged fuel filter or by high resistance in the fuel pump circuit. at means there are two ways to test a fuel pump: mechanically and electrically. Both are needed for an accurate diagnosis, but check- ing fuel system pressure with a mechanical gauge is the most logical place to start. Fuel system pressure is measured with the engine warmed up and running at idle, using a test gauge connected directly to the fuel injector rail or Tee'd into the fuel supply hose. Sometimes the fuel pump will generate full pressure when the starter is cranking but stop running after start-up, and the engine will stall quickly. is indicates a problem with an engine sensor or with the powertrain control module (PCM). It is not a fuel pump failure, but it is often misdiagnosed as a bad fuel pump. If fuel system pressure meets specifications at idle, many techs will assume everything is OK and look no further. at's a mistake, because it's possible to have correct pressure at idle when demand is low but not enough pressure or flow volume at higher speeds and loads. e pump should be tested under load. On older vehicles with a return-type fuel system, an easy way to load-test the pump is to connect the gauge directly to the fuel supply line and run the pump to measure its maximum or "deadhead" pressure. Most pumps should be able to generate at least twice the vehicle's specified fuel pressure this way. If so, the fuel pump and its voltage supply circuit are probably in good condition. With the engine running at idle, the pressure regu- lator can be checked by simply disconnecting the vacuum hose to see if pressure increases. On vehicles with a mechanical returnless fuel supply system, you can't really check deadhead pressure because the pressure regulator is inside the fuel tank. A common technique for load-testing the pump is to drive the vehicle with a pressure gauge connected. If pressure is within spec during cruise but falls during acceleration, that means either the fuel filter is clogged or the pump can't keep up with demand. When fuel pressure is low, a scan tool will show long-term fuel trim is significantly posi- tive all the time and short-term fuel trim will reach maximum on acceleration (fuel trims max out at 25%). However, a vacuum leak can produce similar fuel trim readings. If fuel system pressure is low only under load, that doesn't automatically mean there's something wrong with the fuel pump. Yes, a clogged filter can cause low pressure and low volume under load, but low voltage at the fuel pump connector will do the same thing. Now it's time to test the electrical system. But first, let's look at newer models with variable speed fuel pumps. Fuel pump driver module Most vehicles now have an electronic returnless fuel supply system that has no fuel pressure regulator. Pressure is controlled by a fuel pump driver module (FPDM) or a fuel pump control module (FPCM) in response to commands from the PCM. On these systems, fuel pressure is managed by controlling the speed of the fuel pump. e PCM decides how much fuel pressure is needed based on demand, from idle to full load, and also based on signals from the fuel rail pressure (FRP) sensor and a fuel rail temperature (FRT) sensor. ese sensors are often in the same housing, and their data will show on a scan tool. When the ignition switch is on, full battery voltage is supplied to the control module and to the pump, either through the control module or through a separate fuel pump circuit. e control module operates the fuel pump by controlling the motor's ground circuit on a pulse width modulated duty cycle. e scan tool will display pump speed com- mands as percent duty cycle. Depending on the scan tool, you may need to look up those commands in a service information system

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