Delivering the Goods

Jan. 1, 2020
Every tech knows you have to have three things for an engine to run: compression, spark and fuel. But that is a little simplistic. An engine can have good compression, but if it can't breathe, it won't run right. An ignition system can have good spar

Proper fuel delivery is essential to engine performance, so here's how to make sure the fuel pump circuit does its job.

Fuel Injectors fuel pumps fuel pump diagnosis fuel pump diagnostics fuel systems repair shop repair shops automotive aftermarket
Every tech knows you have to have three things for an engine to run: compression, spark and fuel. But that is a little simplistic. An engine can have good compression, but if it can't breathe, it won't run right. An ignition system can have good spark, but if delivered at the wrong time, it means nothing.
The fuel system is a little more involved. Split between fuel delivery and fuel control, there are a lot of things that can go wrong and lead to everything from "no start" complaints to reduced fuel economy. This month, let's focus on the delivery side of the fuel system, and its main component, the fuel pump.
There are three basic fuel delivery designs in use on modern automobiles. They are the return type system, the mechanical-returnless system and the electronic- returnless system. Even though there are different system designs, they all share the same responsibility. All three are charged with delivering sufficient fuel quantity at a specified pressure.

As a basic check, we all are used to measuring fuel pressure. Proper pressure is needed in order to make sure the proper flow is achieved through the injectors. We all know the air/fuel ratio, right? For every 14.7 pounds of air, we need to add 1 pound of fuel. Have you ever seen a fuel mass (weight) PID (Parameter Identification) on your scan tool? Of course not. Instead, the ECM (Engine Control Module) relies on what it's been told the injectors will flow at a specified pressure.

This volume can easily be translated into mass, and the ECM then calculates injector opening time to allow for the proper amount of fuel to be added. This pressure requirement must also take into account the pressure on the pintle side of the injector. Higher throttle openings create more pressure in the intake manifold and at the pintle. This "differential pressure" between the pintle side and the rail side affects fuel flow, and has to be corrected for. In a return type system, this function falls on the vacuum pressure regulator. In a mechanical-returnless system, a set pressure regulator is used and corrections are programmed into the ECM. In Ford's electronic-returnless system, a Fuel Rail Pressure sensor provides the feedback to the ECM, which then varies the "on time" of the fuel pump via the Fuel Pump Driver Module to maintain a consistent pressure differential across the injectors.

Pressure is only half of the job. Sufficient flow is also needed to meet the engine's demands. At idle, it takes very little volume to keep the engine running, with flow as little as 0.03 gpm (gallons per minute). However, the system must meet all operating conditions, and full throttle pulling a trailer up a mountain will require a lot more fuel volume than that. Testing the system for fuel volume is dependent on the type of system you are servicing.

On a return type system, disconnecting the return line and running a line from the rail to a clean, graduated container may be all that's needed to verify volume. There are some manufacturers now including specifications and test procedures for volume testing, but most do not. According to one pump manufacturer, a useable rule of thumb is a flow rate of at least 1 quart over 30 seconds. You can be more accurate. There are software calculators available that will calculate a given engine's maximum fuel flow requirement based on displacement. A flow gauge can then be used to tell if the pump, and the system itself, are capable of meeting the demand.

According to the Fuel Pump Manufacturer's Council, one major cause of pump failure is fuel tank contamination. Fuel is drawn in through the bottom of the pump and passes through the pump housing and out of the outlet side of the pump. The fuel is used to both cool and lubricate the pump motor. Dirt and other debris in the system can restrict fuel flow into the fuel pump module. Even worse, it can pass into the pump itself and accelerate wear.

Rust is another popular "pump killer." Metal tanks that are kept low on fuel can rust internally, and these rust flakes can clog the pump module strainer and work its way through the system. Debris accumulation downstream of the pump will certainly reduce fuel flow as well, but it also puts a higher load on the pump motor, leading to overheating and premature wear of the pump. Cleaning the tank, and replacing any accessible filters, should be part of any fuel pump replacement.

Did he say filters? There is more than one. Besides the inline filter and pump strainer we are all familiar with, there are typically filters or screens at the pump inlet itself, in the return type pressure regulator and in each injector. Some primary filters are now in the tank, often incorporated in the outer diameter of the pump module housing.

You wouldn't rely on just a compression test to tell you if the engine is healthy. Don't rely on just a pressure test to tell you if the fuel delivery system is healthy. Verify both fuel system pressure and flow volume as part of your basic diagnostic routine to make sure fuel delivery is as it should be.

Side bar #1

Pump Current Testing

For years, techs have looked at a fuel pump's electrical health by monitoring the current passing through the systemCurrent is affected by resistance, but not just the static resistance you measure with an ohmmeter. As the magnetic field builds in the pump's armature, a Counter Electromotive Force is created that impacts the current trace you see on your scope.
Additionally, the force required for the pump to turn against the fuel mass —and any upstream restrictions — will draw even more current to keep the pump moving. For example, a pump on the bench takes only about 1 amp to spin it at 7000 rpm...but with the load placed on it by the fuel being forced through the system, that same pump takes almost 6 amps to spin at 5,600 rpm.

Let's say you have little to no fuel pressure. Is it the pump? Connect your low amp clamp to a power supply point that includes the pump motor circuit as well as the relay control circuit. Now try and start the car. If you read "0" amp, then nothing is turning on, check the relay control circuit for problems first. If you read anywhere from 0.3 to 0.8 amp, then the relay control circuit is operating, but the pump side of the circuit is not flowing current. This could mean an open in the circuit, maybe the relay itself. If you read anywhere from 3.0 to 6.5 amps, the pump is working and probably working normally. There is some mechanical fault preventing fuel from reaching the rail, or bleeding it off before it's getting to your gauge.

By adjusting the settings on your scope, you can magnify the pattern to look at the actual current flow across each commutator strip. Worn brushes and armatures that add resistance, lower current flow and result in a weak pump are easily seen. Look for a repetition in the pattern to determine how many commutators there are (typically eight or 10) and use the time cursors on your scope to measure the time for one revolution of the pump. Calculate its rpm by dividing that number into 60,000 (number of milliseconds in one minute). Typical running rpms are in the range of 3,000 to 6,000 rpm. Test known good pumps on cars you normally work on to get a feel for what is normal.

Low current and normal to high rpm? Pattern showing no sign of abnormal wear? The fuel isn't getting to the pump. Look for restrictions on the inlet side of the pump itself. High current with normal to low rpms can indicate restrictions on the outlet side of the pump (inline filter) or a pump that is mechanically binding. Low to normal current coupled with low rpm can indicate that there is a voltage drop issue — not enough voltage provided to the pump from a corroded connection or bad ground.

While it's true that a scope pattern is insufficient as a testing method in and of itself, measuring current and rpm and taking a closer look at the electrical wear in the pump can help in determining the exact cause of the failure. Sure beats pulling the tank down to measure voltage drop right at the connector.

Sidebar #2

The FuelZapp

For catching fuel delivery issues that have an impact on a variety of driveability problems, the FuelZapp provides all the information at a glance. It's all about the differential pressure across the injectors, and the ability of the system to provide sufficient volume. This tool measures fuel pressure, fuel volume, intake manifold vacuum and exhaust backpressure.

Connect the tool inline on a return type system and slowly "deadhead" the line for maximum flow. Returnless systems require a little more effort. It includes a PC calculator program for your desktop that allows you to determine maximum volume requirements for any given displacement, making your volume measurements more precise. Variances from normal in either manifold vacuum or exhaust backpressure can impact the actual differential pressure at the point it matters most — the injector tip.

For more information, visit www.thompsonautolabs.com.

Sidebar #3

Ford's Electronic Returnless System

Ford's electronic-returnless system is a little different than most returnless systems. The first feature you can't see — there is no mechanical pressure regulator. That's because Ford uses a feedback sensor to keep the ECM informed of fuel rail pressure adjusted for manifold pressure. It's called the Fuel Rail Pressure sensor, and it is located in the fuel rail, along with a fuel damper to keep the noise from fuel pulsation minimized.

It all starts with the ECM commanding the fuel pump to a specified duty cycle, somewhere between 5 percent and 51 percent. This command is sent to the Fuel Pump Driver Module, which doubles that value and applies it to the pump. By varying the duty cycle, the pump's volume is adjusted to maintain the desired fuel rail pressure. Feedback from the Fuel Rail Pressure sensor keeps the ECM informed of changing conditions, to which the ECM can quickly respond.

The pressure being reported to the ECM can be found on your scan tool. However, this is not the same pressure you will read simultaneously with your fuel pressure gauge. Remember, the value is corrected for intake manifold vacuum and its impact on the differential pressure needed to maintain accurate flow through the injector pintle.

About the Author

Pete Meier | Creative Director, Technical | Vehicle Repair Group

Pete Meier is the former creative director, technical, for the Vehicle Repair Group with Endeavor Business Media. He is an ASE certified Master Technician with over 35 years of practical experience as a technician and educator, covering a wide variety of makes and models. He began writing for Motor Age as a contributor in 2006 and joined the magazine full-time as technical editor in 2010. Pete grew the Motor Age YouTube channel to more than 100,000 subscribers by delivering essential training videos for technicians at all levels. 

Connect with Pete on LinkedIn.

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