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How to Test Mitsubishi's Fuel SystemPosted 2/17/2003By Todd Plourde
Fuel economy, emissions standards and performance ... these are the key points facing today's automotive engineers. In an effort to make vehicles smaller and lighter, engineers use electrical devices whenever possible instead of mechanical, and also cut the amount of unnecessary amperage used by these devices. A mechanical fuel pump takes more horsepower from the engine than an electrical pump that draws about 6 amps. When an engine is at idle, most of the fuel pumped through the fuel rail is directed back to the fuel tank, unused, but more volatile than when it was at rest. So why run the pump at full power if it's not needed? Because it boosts fuel economy by cutting that little extra drag on the motor from the charging system when not needed. Engineers can also make the fuel pump smaller, since most of the time it won't be running at full power. Again, smaller parts are lighter and better for fuel economy and overall performance. Another advantage of not running the fuel pump at full power all the time is the ability to cut down on evaporative emissions. Mitsubishi has implemented a variable speed fuel pump technology on some of its vehicles, using a two-speed or three-speed pump system. Both systems lower the amount of unused fuel that travels through the engine compartment, without having to turn the fuel system into a returnless fuel delivery system (returnless fuel systems are often plagued by misfires due to air trapped in the fuel rail). Limiting the amount of fuel sent through the engine compartment helps prevent the fuel from vaporizing. Even though a canister is still needed to capture the hydrocarbons, the fuel does not vaporize as much as with a fuel system that is constantly circulating its contents at a rate of about one pint every 20 seconds. Therefore, the need for a larger canister to keep up with the vapors becomes unnecessary and a smaller, lighter charcoal canister can be used. Two-speed pump systems use two relays together to attain the desired pump speed: the multi-port injection relay and the fuel pump relay. The multi-port injection relay is energized when the power control module (PCM) acquires a signal from the crankshaft position sensor of 50 rpms or higher. This supplies full power to the pump through the second relay. Depending on calculated load determined by the Mass Air Flow sensor (MAF) and engine rpm, the PCM will either decide to energize or de-energize the fuel pump relay. The relay limits the power supply by sending it to the pump through a resistor only when it is energized by the PCM. To test the system, you can use the scan tool to take control of the relays while checking the pump test connector for voltage. You will see the varying voltages unless the pump grounds or if the feed to the pump is open. In that case, all you will see is open circuit voltage (full battery voltage). In a case where the voltage doesn't change and is constantly full battery voltage, check to see if the pump's powers and grounds are actually there. If they are, check the fuel pump relay voltages manually right at the relay. Starting in 1995, some California-model Mitsubishis were manufactured with the three-speed pump system. The components used to control this three-speed system are a fuel pump relay, the pump relay module and the PCM. Just like the two-speed system, the PCM is looking at the rpm and the MAF signals. After the PCM calculates how much fuel is needed, it sends pulsed signals to the pump module relay over a single control wire. Once the pump module receives the signal, it will be able to control pump speed by varying the resistance on the ground control circuit. This causes the pump supply voltage to equal 10 to 12 volts due to the voltage drop on the ground side. Since the pump is controlled on the ground side by the module, you cannot just go to the fuel pump test connector and supply power to it. There is no ground unless the pump relay module is supplying ground. You could try it with the key turned on. But if the module is bad and not supplying a ground, it will not work or at least, it won't work correctly. To test the actual pump itself, try going right to this relay module. Disconnect it and then supply power and ground to the pump from there. You can also run the pump with a scanner to make sure there are three different voltages across the pump's power and ground circuits. If it does vary its pressure, you know it's an input problem. Otherwise, put the PCM into open loop fuel control by unplugging an important sensor, such as the MAF, and check for fuel system pressure. If you have questions on one of these systems, just give us a call!
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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