Ensure the health of fuel injection components

Jan. 1, 2020
Every technician learns early on that an engine needs three things to run: sufficient compression from the engine itself, a strong spark from the ignition system and the proper fuel charge from the fuel system.

Here are a few tips to offer your tech customers on isolating fuel system-related complaints.

Every technician learns early on that an engine needs three things to run: sufficient compression from the engine itself, a strong spark from the ignition system and the proper fuel charge from the fuel system.

And it all has to happen at the right time. A discrepancy in any of these areas will lead to driveability complaints, higher emissions levels and lower fuel economy.

The fuel system's job

To diagnose any system, the tech must understand what that system is supposed to do. At its most basic level, the fuel system delivers a clean, constant supply of fuel in the correct amount needed by the engine under all conditions. We can break this down into two separate functions: fuel delivery and fuel control.

Fuel delivery is the job of the fuel pump and its related components. Basic fuel delivery tests should be a part of any driveability diagnosis, so let's start there.

The most common test of a fuel delivery system is the fuel pressure test. The fuel pressure gauge should be connected at the supplied test port. Most specifications for fuel pressure are taken with the key on and engine off, and most systems will shut the fuel pump circuit off after a few seconds if no crankshaft position (CKP) signal is received. Techs will have to command the fuel pump on with their scan tools, or cycle the key on and off until a maximum reading is reached. They may have to bleed the air out of their tool, so be sure to have them consult the tool's instructions as well.

A reading similar to the one shown in Figure 1 should result. Compare this reading to specifications. If it is low, the vehicle may have a weak fuel pump, a restriction to flow on the intake side of the pump or a fuel pressure regulator that is stuck open.

If the gauge reaches specifications, but quickly bleeds off pressure, the system may have a leaking injector or fuel pump module check valve. Consult the model-specific troubleshooting information for specifics on isolating a low pressure condition.

The next test

With the gauge still connected, have the tech start the engine. It is best to have the engine at normal operating temperature for this test. With the engine running, the system should drop pressure 3 to 5 psi. This is the additional pressure drop caused by the injectors opening and closing. A gauge that fluctuates rapidly in pressure can indicate air in the fuel supply, typically from flow restrictions in the pump's pick-up strainer.

On a returnable system, techs should next disconnect and plug the vacuum line going to the regulator. Here pressure in the rail should increase 8 to 10 psi. In a returnable system, the regulator is in charge of adjusting fuel rail pressure to the injectors.

When manifold absolute pressure is highest, maximum pressure is needed in order for the correct amount of fuel to pass into the combustion chamber (differential pressure). When manifold absolute pressure is low (high intake vacuum), fuel rail pressure required is lower in order to maintain the same differential pressure. The measured fuel pressure difference is generally half of the measured intake manifold vacuum.

Fuel pump volume test

Whether pressure is within specification or not, another basic test is the fuel pump volume test. This test measures the flow rate of the pump and can help isolate fuel system restrictions or weak pumps.

The tech must release fuel system pressure as specified in the service information and disconnect the return line on returnable systems or the supply line on returnless systems. After feeding the line into a clean measured container, the tech can energize the fuel pump via a scan tool or by using a fused jumper wire at the relay.

Measure the quantity of fuel that collects in 30 seconds; a good rule of thumb is approximately one pint. While collecting a sample, have the tech look for smooth, continuous flow with no air present that may indicate a cavitating pump and remind him or her to look for any signs of dirt, debris or contaminants.

If all these tests pass but the tech still suspects a fuel delivery issue, he or she can check the health of the fuel pump by measuring the current it uses with a digital storage oscilloscope (DSO). This pattern will indicate if the pump is working too hard or not hard enough, as well as the health of the motor.

Here, experience plays a hand in comparing the pattern with known good ones. Resources like the International Automotive Technicians Network (iATN) allow access to hundreds of known-good and -bad pump patterns that may help in isolating intermittent fuel related issues. Figure 3 shows a pattern indicating an old pump with some wear. Notice the irregular "hump" occurring every eighth peak.

Fuel control

The last leg in the fuel's journey before being consumed by combustion is through the fuel injectors. Most injectors are electrical-mechanical devices that are controlled by the powertrain control module (PCM). They carry out the PCM's orders in final delivery of the proper fuel amount needed by the engine.

Restricted flow, sticking pintles or injectors that won't close are just a few of the ways the injectors can impact the overall performance of the engine, and faults in the injectors can be among the most challenging to diagnose.

Fuel injectors on today's vehicles most commonly are mounted directly in the intake tract just ahead of the intake valves (multiport injection). Most multiport injectors can be controlled by the PCM in one of two ways.

They may fire individually (sequential) or in pairs (synchronous). On newer designs, direct injection is used, with the injectors mounted in the combustion chamber. Older vehicles still use one or two injectors mounted in the throttle body upstream of the throttle plate (throttle body injection). And let's not forget to mention General Motors' Central Sequential Port injection system, which uses one injector feeding each intake tract via poppet valves, where fuel pressure is a critical factor.

Fuel trim

A basic step in driveability diagnosis is to check the fuel trims at idle, at 2,500 rpm and at cruise speed.

While fuel trim diagnosis is a learned skill all by itself, we can note a few generalities. Fuel trims that are OK at idle and 2,500 rpm in the bay, but correcting for a lean condition under higher loads and speeds can indicate low volume or restrictions to flow. These restrictions may be before the injectors or in the injectors themselves.

Trims that are correcting slightly rich at idle, but OK at higher rpms could indicate a leaking or sticking injector(s). The key here is to make sure the data the PCM needs to make informed fuel control decisions is correct before condemning the injectors themselves.

Testing injectors

Most techs do not have the luxury of in-house injector flow benches or test equipment. But techs can use several methods to check injector flow rates.

The first is the "injector balance" test. Some manufacturers provide for this test using a scan tool's "enhanced" mode. If not, the tech will need an injector actuator (see Figure 5).

To perform this test, a tech must connect a fuel pressure gauge that is accurate within ±1 psi. Attach the actuator to an injector as outlined by its manufacturer. Operate the fuel pump until maximum pressure is reached, then turn it off and record the pressure. Actuate the injector with the tool, and record the lower pressure reading on the fuel pressure gauge. This procedure should be repeated for all the injectors, being sure that the starting pressure remains the same.

Now, total the differences in pressure, add them together and divide the total pressure loss by the total number of injectors tested for an average (see Figure 4). Any injector whose pressure drop is outside of this average by ±1.5 psi bears more attention. A larger drop in pressure indicates a leaking injector; less indicates a restricted one. If a fault is indicated on this test, techs should clean the injectors through the fuel rail and repeat before condemning the injector(s).

Another technique is performed using a DSO. Many techs can tell the whole story from viewing injector voltage and current waveforms, but I'm not one of them. I look for anomalies in my patterns. Here are a few to give techs an idea of what to look for.

The first is a voltage pattern, shown in Figure 6. Notice the little "hump" on the trailing edge of the pattern and the high spike just before it. The hump is the pintle of the injector closing, and the peak is caused by the sudden collapse of the magnetic field in the injector windings as the power is turned off. By comparing these two points among all the injectors, a tech can verify if the injector is opening and gain an idea of the electrical health of the coils themselves.

A tech can use a current waveform of the same injector to measure the current required, and using Ohm's Law (Voltage = Current x Resistance) determine the resistance of the coil windings that can be compared to specifications.

In those cases where the injectors are hidden and inaccessible without major teardown, this saves a lot of time. In this case, the measured current flow is 1.14 amps, which equates to a resistance of 11.9 ohms at 13.6 volts – within specification for this particular vehicle. Techs can also check all the injectors at the same time by increasing the time base and checking the peaks.

If your tech customer doesn't own his or her own flow bench, these techniques can help nail down fuel injection-related failures and prepare the technician for the next time he hears a customer complain: "It just doesn't run right!"

Pete Meier is an ASE, CMAT, a member of iATN and a full-time technician with CarMax in Tampa, Fla.