Auto Service Professional

OCT 2017

Magazine for the auto service professional

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Page 21 of 77

20 | ASP October 2017 Technical calls for no less than 20 psi after 20 minutes, but most will engines will start if pressure remains positive while the engine is still warm. Hold pressure can leak down due to a leaking injector, but this is a relatively slow leak. If hold pressure leaks down immediately after the engine is switched off, that's a bad check valve. A fuel system can have good pressure but not enough fuel flow to run the engine properly at higher speeds. Flow rate can be tested by disconnecting the fuel supply line at any convenient point and attaching a hose that leads to a measuring container. Activate the fuel pump for 15 seconds and measure the amount of fuel that flows into the container. Some manufacturers publish a fuel flow speci- fication, but some do not. Either way, expect a healthy fuel pump to flow a minimum of one pint in 15 seconds. Electrical tests As noted earlier, high resistance in an elec- trical circuit reduces the amount of voltage downstream of the point of high resistance. Less voltage to the motor means the motor makes less power. Simply checking for voltage with the fuel pump disconnected does not tell you if there is any resistance in the circuit because the resistance only causes the voltage drop while the circuit is working. So before condemning a fuel pump, it's important to make sure full battery voltage is available at the fuel pump connector while the pump is running. is is the voltage drop test. When measuring measure voltage drop, it helps to remember that a voltmeter measures the difference in voltage reaching the meter's positive and negative probes. In this test, you're not measur- ing the difference between battery positive (B+) and ground. Instead, you're measuring the difference between battery positive and the rest of the positive circuit. With the voltme- ter's positive probe connected to the battery through a long jumper wire, use the negative probe to back-probe the fuel pump connector while the pump is running. If there is absolutely no resistance to the flow of current between the battery and that connector, there will be no difference in electrical pressure, no loss of voltage in that circuit, and the voltmeter will show zero volts. In reality, there is no perfect circuit, so the total voltage drop in a good fuel pump circuit might be as high as 0.5 volts. However, some vehicles will suffer driveability problems with a voltage drop as low as 0.165 volts (165 millivolts). If voltage drop is high, start looking for a corroded or overheated connector, a faulty relay or even a chaffed wire. Some replace- ment fuel pumps come with a heavy-duty When checking voltage drop across a connector, anything greater than zero volts indicates resistance in the connection. Total voltage drop in the whole circuit from battery to fuel pump should be less than 0.5 volts. Photo courtesy of AGCO Automotive Repair Service

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