Here are a few tips on isolating fuel system-related complaints.
Every tech learns early on that an engine needs three things to run: It has to have 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, you must understand what that system is supposed to do. At its most basic level, the role of the fuel system is to deliver a clean, constant supply of fuel in the correct amount needed by the engine under all conditions. We can break this down further 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. Connect your fuel pressure gauge at the supplied test port or using the appropriate adaptors in your test kit. Most specifications for fuel pressure are taken with the key on and engine not running, and most systems will shut the fuel pump circuit off after a few seconds if no crankshaft position (CKP) signal is received. Therefore, you will have to command the fuel pump on with your scan tool, or cycle the key on and off until you have reached a maximum reading. You may have to bleed the air out of your tool, so consult the tool's instructions as well.
You should get a reading similar to the one shown in Figure 1. Compare this reading to specifications. If it is low, you 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, you may be looking at 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 your gauge still connected, 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. See Figure 2. This is the additional pressure drop caused by the injectors now opening and closing. A gauge that fluctuates rapidly in pressure can indicate air in the fuel supply, typically from flow restrictions in the pick-up strainer on the pump itself.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 measured intake manifold vacuum.
FUEL PUMP VOLUME TEST
Whether pressure is within specification or not, another test that should be considered basic 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.
To perform this test, 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 – we want to include any losses caused by as much of the fuel system as possible. Feed the line into a clean measured container, and energize the fuel pump via your scan tool or using a fused jumper wire at the relay.Measure the quantity of fuel that collects in 30 seconds. Very few manufacturers list a specification for this, but a good rule of thumb is approximately one pint. While you are collecting your sample, look for smooth, continuous flow with no air present that may indicate a cavitating pump and look for any signs of dirt, debris or contaminants that could indicate a problem in the delivery side of the system or lead to restrictions in the control side.
If all these tests pass but you still suspect a fuel delivery issue, there is one more step you can take. Check the health of the fuel pump by measuring the current it uses using a digital storage oscilloscope (DSO). This pattern can tell you 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 your 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 5 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 are charged with carrying 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. After all, they are only responding to the PCM's directions, and if the PCM's calculations are off because of misinformation, the injectors are simply the "bearer of bad news."
Fuel injectors on today's vehicles are most commonly 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 – both short term and long term – under different conditions. Check and record 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 of us do not have the luxury of in-house injector flow benches or test equipment. There are, however, a few methods you can use to check injector flow rates.
The first is the "injector balance" test. Some manufacturers provide for this test using your scan tool's "enhanced" mode. If not, you will need an injector actuator. See Figure 6.To perform this test, 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. Repeat this procedure 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. See Figure 7. This will give you an average. 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, 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 you an idea of what to look for.
The first is a voltage pattern, shown in Figure 8. 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, you can verify if the injector is opening and gain an idea of the electrical health of the coils themselves.A current waveform of the same injector is shown in Figure 9. Here you can measure the current required, and using Ohm's Law (Voltage = Current x Resistance), determine the resistance of the coil windings that you can compare to specifications.
Short of owning your own flow bench, these techniques can help you nail down fuel injection-related failures. I hope these techniques I have shared help you the next time you have a customer complaint of, "It just doesn't run right!"
