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Subaru Pulse-Width Modulated Fuel Pumps

Many technicians aren’t familiar with PWM fuel pumps because they’re generally very reliable. But now that many import vehicles equipped with PWM fuel pumps are rolling up the miles, it’s time to turn the page from diagnosing two-line and single-line mechanically pressure-regulated fuel pumps to diagnosing electronically controlled PWM fuel pumps.

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Since Subaru's pulse-width modulated (PWM) fuel system is as simple as it gets, we're featuring a 2008 Forester equipped with the turbocharged 2.5 L engine to illustrate this month's Diagnostic Solutions.

Since Subaru’s pulse-width modulated (PWM) fuel system is as simple as it gets, we’re featuring a 2008 Forester equipped with the turbocharged 2.5 L engine to illustrate this month’s Diagnostic Solutions.

I think it might have been a 2003 Subaru WRX, but it was parked in front of my local Subaru specialist’s shop after going on its fourth fuel pump replacement from other shops in less than six months. Both the Subaru specialist and I agreed that fuel pump failures on Subarus were relatively rare, so the previous repair shops had obviously overlooked some very important details. The last shop had actually installed a high-performance racing pump, thinking that the WRX’s turbocharged engine was “running out of fuel.”

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Not trusting the “high-performance” replacement, the Subaru specialist had installed a new original equipment (OE) fuel pump. I mentioned that the WRX probably had a pulse-width modulated (PWM) fuel pump and should therefore have a fuel pump control module (FPCM) located somewhere on the chassis. We located the FPCM in the engine compartment, checked for power and ground and decided that the FPCM was intermittently defective.  The specialist replaced the FPCM and, at last report, the WRX was still running great.

OLD-SCHOOL DIAGNOSTICS

Many technicians aren’t familiar with PWM fuel pumps because they’re generally very reliable. But now that many import vehicles equipped with PWM fuel pumps are rolling up the miles, it’s time to turn the page from diagnosing two-line and single-line mechanically pressure-regulated fuel pumps to diagnosing electronically controlled PWM fuel pumps. While pressure and volume testing are mandatory for pressure-regulated fuel systems, pulse-modulated systems instead require scan tool diagnostics.

If fuel pressure were to be measured on a PWM system, we would discover that fuel pressure varies according to operating conditions. Unlike mechanically regulated systems, PWM systems operate as a feedback, closed-loop system with a fuel pressure sensor providing feedback to the FPCM. Since pressure-regulated fuel pumps operate at full capacity 100% of the time, and PWM fuel pumps operate at far less than maximum capacity most of the time, the PWM fuel pump usually lasts much longer.

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PULSE-WIDTH MODULATION

Thanks to modern electronics, the speed of any direct-current electric motor can be efficiently controlled by rapidly switching the supply current “on” and “off.” The ratio of positive to negative “duty cycles” determines motor speed. To illustrate, a 30% positive duty cycle indicates that the current to the motor is being switched on 30% of the time and off 70% of the time. A 50% duty cycle indicates the current to the motor is being switched on and off in equal time. A 100% duty cycle indicates that the motor is operating at full voltage and amperage.

The inputs required to operate a pulse-width modulated fuel pump system are generally produced by the engine control module (ECM), the FPCM and a fuel system pressure sensor. The ECM commands the FPCM to increase or decrease fuel pump on-time to meet the required fuel demands. If, for any reason, the fuel pump can’t meet the commanded values, a diagnostic trouble code is usually stored in the ECM.

FUEL PUMP DIAGNOSTICS

Before we get into diagnosing PWM fuel pump systems, let’s review some basic fuel pump testing procedures. Any electronic fuel delivery system needs power and ground circuits, so check all fuses for voltage or continuity at both fuse pins. In most cases, it’s a simple task to eliminate the fuel pump relay as a possible cause by switching the relay with a less-used horn or accessory relay. The relay should generate a perceptible “click” as the ignition is turned on. Adding a gallon or two of gasoline from a safety-style fuel will help ensure that the problem is a faulty fuel pump rather than a faulty fuel level gauge.

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Frame rail-mounted fuel filters should be tested or replaced. If the fuel system is equipped with a Schrader test port, measure the fuel pump’s pressure and volume against manufacturer’s specifications.

pwm fuel pump subaru 2IDENTIFYING PWM FUEL PUMPS

We’re going to use a 2008 Subaru Forester equipped with the turbocharged 2.5L engine to illustrate this month’s Diagnostic Solution. In most cases, a PWM system isn’t equipped with a Schrader test port. On the other hand, some gasoline direct fuel injection (GDI) applications might include a test port for measuring supply fuel pump pressure. Since PWM is a computer-controlled feedback system, it won’t include a mechanical fuel pressure regulator at the end of the fuel rail.

But don’t be fooled by appearance alone. On some nameplates, a fuel rail pulsation dampener that closely resembles a mechanical fuel pressure regulator might be mounted on the fuel rail. The fuel pressure sensor used on PWM fuel pump systems can be located either on the fuel rail or under the vehicle near the fuel tank.

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ANALYZING WIRING SCHEMATICS

A wiring schematic for our 2008 Subaru Forester reveals that the system is equipped with a PWM fuel pump since it indicates that the FPCM is located behind the right rear quarter panel. A fuel pump relay is located behind the right side of the dash. A black wire with a yellow trace supplies power from the fuel pump relay to the FPCM. The current flow to and from the fuel pump is supplied by two wires connecting the FPCM and fuel pump. A black wire connects to the R147 ground joint connector at the right rear corner of the vehicle.

It’s easy to assign functions to some wires simply by tracing them through the schematic. For example, the black/yellow wire leading from the fuel pump relay obviously powers the FPCM. The black ground wire obviously grounds the FPCM, and both wires can be major failure points. Due to space limitations, I can’t go through all wiring functions, but now we have a starting point for diagnosing a fuel pump failure.

SCAN TOOL DIAGNOSTICS

Wiring schematics provide a lot of diagnostic information. In addition to color codes, many schematics also provide component and ground terminal locations.

Wiring schematics provide a lot of diagnostic information. In addition to color codes, many schematics also provide component and ground terminal locations.

A PWM fuel pump system can most easily be identified and diagnosed by connecting a scan tool. With the scan tool connected and the engine or fuel system data screen pulled up, look for terms like “duty cycle” or “pulse width.” (It’s entirely possible, due to the large number of configurations in aftermarket scan tools, to see either term listed on the mechanically regulated fuel system.)

With mechanically regulated fuel injection systems, the “fuel pressure” data line will probably be missing. If it’s a GDI system, between 35 psi and 75 psi primary supply fuel pressure should be what is indicated along with the roughly 500 psi fuel pressure produced by the high-pressure fuel pump at idle speed.

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Next, scroll to the bidirectional fuel pump controls. A conventional fuel pump will simply indicate “on” or “off” bidirectional controls. A PWM pump, on the other hand, will usually indicate a “fuel pressure” data line and a “duty cycle” data line. Using bidirectional, the fuel pump pulse width can be momentarily commanded to increase both duty cycle and fuel pressure.  In one of my recent cases, the FPCM would increase duty cycle as commanded, but the fuel pressure data line indicated zero fuel pressure. I concluded that the FPCM was responding correctly, but the fuel pump wasn’t. It became apparent that the root problem was an open circuit in the fuel pump motor itself.

DIAGNOSING DTC P0230

When diagnosing any pulse-width modulated fuel pump, it’s important to begin by looking at diagnostic trouble codes. A code P0230 can be set on our 2008 Forester focus vehicle to indicate an intermittent-low or high fuel pressure condition. Looking at the enable condition, a minimum of 8 system volts must be available and the fuel tank must contain at least 2.64 U.S. gallons. The condition must last at least 30 seconds with the fuel pump turned on and the engine running. The time needed for on-board diagnosis is 2.5 seconds. The malfunction indicator light (MIL) will illuminate when the malfunction occurs during two consecutive drive cycles. P0230 will clear itself after an “OK” idling cycle is completed 40 consecutive times or after an “OK” driving cycle is completed three consecutive times. Freeze-frame data will accompany code P0230. The actual diagnostic procedure for P0230 is contained in a separate document and consists of measuring the integrity of the PWM fuel pump circuit.

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Article courtesy ImportCar.

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