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How To Troubleshoot A Fuel Injection System

How To Troubleshoot A Fuel Injection System.Troubleshooting a fuel injection system is not always easy in the absence of definitive fault data, and the vast array of hoses, tubes, pipes, wiring, and other components that make up a modern fuel injection system can be confusing, if not intimidating, even to some professional mechanics. However, as with all other car problems, the process of troubleshooting a fuel injection system follows a logical sequence of checks, tests, and readings that almost anybody can perform with basic tools and equipment. However, how do you know you have fuel injection problems, and not another, unrelated problem that mimics some of the symptoms of a defective injection system?

This is of course entirely possible, so to help you identify fuel injection system problems, and how to go about troubleshooting a fuel injection system, let us take a brief look at how the system works, and how to recognize the symptoms of a defective system.

How does it work?

All fuel injection systems, whether gasoline or diesel, work by supplying pressurised fuel to the cylinders through small orifices in the injectors. In all cases, the fuel is pressurized by a pump which in the case of gasoline systems, is usually located inside the fuel tank. In diesel systems, the high injection pressures required makes the use of an electrical pump impractical, which is why diesel injection systems use engine-driven pumps.All systems also make use of fuel filters, and in troubleshooting fuel injection systems, due care must be taken that the fuel flow rates through the filters is checked to see if the flow rate is not impeded by dirt that may have clogged the filter element. Nonetheless, once the fuel system is pressurized, the moment and duration of injection is regulated by a microprocessor that opens and closes the injector via a small solenoid valve, although on some older diesel engines that are not fitted with common rail technology, injection is controlled by the pressurized fuel overcoming the tension of a compression spring inside the body of the injector.When troubleshooting a fuel injection system, which is pressure-sensitive, many mechanics leave the fuel pressure regulator till last. However, the pressure regulator is a critically important component since it regulates and maintains the system pressure during cycles of acceleration and deceleration when the demand for fuel decreases, but system pressure remains constant, since the pump delivers fuel at a constant rate and pressure.The above is the short, and vastly over-simplified version of how fuel injection systems work, and while troubleshooting a fuel injection system is relatively easy, fixing it is often another matter entirely, so in the next section, we will look at some typical fuel injection system problems, and how to tell them apart from problems that have similar symptoms.

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Common symptoms of fuel injection system problems.

  • Non-starting .
  • Hard starting.
  • Hesitation on accelerating.
  • Poor engine performance.
  • Engine speed is slow to increase. (Fails to rev up.)
  • Black, blue, or white smoke issuing from the tail pipe.
  • Increased fuel consumption.
  • Frequent sparkplug failures.
  • Misfiring at high engine speeds.
  • Engine shuts off unexpectedly.
  • Erratic, or rough idling.
  • Increase in oil level due to unburnt fuel collecting in the oil pan. (Oil dilution.)

The root causes of all of these symptoms can of course also involve defects and malfunctions in the electrical, vacuum, or emission control systems, and many of these causes/issues/problems are common to most cars- which is why it is important to always first check these systems before beginning to troubleshoot a fuel injection system:

  • Check if the spark plugs work, but remember that this can be dangerous because of the high voltages involved. The safest way to check spark plugs is to connect an induction timing light to the #1 plug lead, and then to check the light pulse against the timing marks on the crankshaft pulley while an assistant cranks the engine. Timing marks, and their location differ from car to car, so consult your repair manual for the relevant information and procedures.

If the ignition system delivers a spark at the correct moment, the timing light will flash at the moment the timing marks align, which when it happens, means that the ignition system is OK, and that the problem must be sought elsewhere.

  • Check the vacuum system, since it can also mimic some fuel injection system problems, but unless a major, and therefore obvious vacuum leak exists, such as a rupture in a large diameter vacuum hose or pipe, the engine should still start, even though it may not run at idle or if it does run at idle, it might do so erratically, or the idle speed may fluctuate. So before you begin troubleshooting a fuel injection system, check the vacuum system for broken, perished, or damaged vacuum lines.
  • Some of the symptoms mentioned above can be caused by defects in the emission control system, such as a clogged catalytic converter or a defective oxygen sensor; however, such failures are unlikely to be the problem in the absence of an illuminated warning light on the dashboard.

Troubleshooting a fuel injection system.
For the purposes of this exercise, we will assume that the fuel injection system is suspected to be causing a non-starting condition. Once you have eliminated, or at least reduced the chances of something else being the problem, you can focus your attention to troubleshooting the fuel injection system in a logical, and step-by-step manner, and the first thing to do is to purchase a good repair manual for your car, as well as a dedicated fuel pressure gauge and a digital multimeter with which to check the voltages and amperages passing through various parts of the system, and upon which the system depends to function correctly. Details on pressures and electrical currents are available in your manual, without which it is very difficult to locate fuses, relays, and other components, and almost impossible to trace any problem on a modern car. So let us begin troubleshooting a fuel injection system in a logical sequence of tests, each of which will include information on diesel systems as well. Below is an image that is representative of all fuel injection systems, and which is useful for reference purposes when we discuss the diagnostic procedure below

What the letters mean.Before we begin troubleshooting a fuel injection system, we need to be able to identify the various components, which look pretty much the same on all cars, except for the pressure regulator, which in this case, is a custom item. On standard models, the fuel pressure regulator would be where the letter “E” now indicates a custom fuel pressure gauge. So starting at the bottom, the letter:

  • “A” indicates the main feed line from the fuel pump.
  • “B”indicates the common rail from which all the injectors are supplied with pressurised fuel.
  • “C”indicates the electrical connector that feeds power to the injector.
  • “D”indicates fuel injectors.
  • “E”indicates a custom pressure gauge, which is a nice-to-have, but they are not fitted to stock systems.
  • “F”indicates custom fuel pressure regulator and its connecting high-pressure feed line. On stock systems, the pressure regulator would be where “E” indicates a custom pressure gauge.
  • “G”indicates the manifold pressure sensor.
  • “H”indicates the throttle body.
  • The green arrows indicate vacuum hoses that can influence the operation of the fuel injection system, which is why all vacuum hoses should be the first items to be checked when troubleshooting a fuel injection system.

Both“G”and“H”can also influence the functioning of the fuel injection system if they fail or malfunction, but even if they stop working, the engine will still start although it may perform poorly. Proper diagnosis of either of these items is best accomplished with suitable diagnostic computers.

Check fuel pressure.

To do this, you need to follow the instructions in your manual to locate the correct attachment point. Almost all engines need to crank before the pump will start, so with all connections secure to prevent fuel leaks, and thus pressure losses, have an assistant crank the engine while you monitor the pressure gauge. The reading must be within a few percent of the value in the manual, so if the pressure checks out OK, the pump is fine, but just as importantly, the fuel pressure regulator is fine as well. If it were not, the pressure gauge might give a low reading or no reading at all. However, if there is no pressure, do the following:

  • Locate, and replace the fuse that protects the pump. It is entirely possible that a fuse that appears to be fine, is in fact defective.
  • Locate, and replace the fuel pump relay. The contact points in relays sometimes burn away, thus preventing current from reaching the pump.
  • Locate the fuel pressure regulator, and isolate it from the system by inserting a piece of impermeable, but fuel resistant material between it and its attachment point. This will prevent fuel from passing through it, and if the engine starts, the problem will have been the defective fuel pressure regulator. If the engine does not start after having isolated the regulator, the problem must be sought elsewhere.

If the pump still does not work after replacing the fuse and relay, do the following:

  • Consult the manual to locate the fuel pump wiring, and using the multimeter, check the wiring that at least 12 volts are reaching the pump when the engine is cranked. If the current checks out OK, the pump is likely defective, but before you remove the pump from the tank, check and replace the fuel filter if it is located outside of the tank. Clogged fuel filters can easily prevent the flow of fuel, so replace the filter and check the fuel pressure again to see if the problem is resolved.

However, some systems use in-tank filters that can become clogged, so if sufficient current reaches the pump, the problem could be due to the filter being clogged. With sufficient current, and a clean fuel filter, the next step is to remove, and replace the fuel pump. However, this can be tricky, so consult the manual as to the correct procedure.Diesel injection systems:Diesel injection pump failures are rare, and the most common causes of pressure losses and poor engine performance are the following:

  • Air in the system:

Almost all diesel system use a manually operated lift pump that is incorporated into the filter assembly with which to purge air from the system after filter replacements. However, over time, the lift pump may develop vacuum leaks where its plunger passes through the filter housing, and if this happens, the pressure pump draws out all of the diesel in the filter, leaving it empty.Because of the vacuum leak, fuel is prevented from being drawn into the filter at the same rate the pump sucks it out, which means that the engine shuts off because it is starved of fuel. One way to confirm a vacuum leak in the lift pump is to feel if there is stiff resistance when you depress it. If a resistance builds up after a few operations of the pump but disappears again after a while, the pump is leaking air, and the only remedy is replacement of the complete filter assembly.

  • Clogged pump strainers.

Diesel injection pumps are fitted with very fine strainers where the main fed line enters the pump, and despite regular filter replacements, these strainers can become clogged, which has the effect of preventing fuel from entering the pump. If your diesel engine refuses to start, or will not accelerate, consult the manual on the correct procedure to remove, clean, and re-install the strainer.

  • Defective fuel cut-off solenoids:

Many diesel engines still employ a small electrically operated solenoid valve to shut off the flow of fuel to the pump when the engine is switched off. While it can happen, the failure of these solenoid valves are rare, and it is more likely that there is an interruption in its power supply which keeps it in the closed position, thus starving the engine of fuel.Consult the manual to locate the fuse that protects this circuit. Nine times out of ten, a simple fuse replacement will get the solenoid working again.

The fuel pressure is OK- but it still does not start.

If the previous steps in troubleshooting a fuel injection system did not resolve the problem, do the following:

  • Check the injector power supply:

Consult the manual to locate the electrical connectors that supply power to the injectors. Each connector will only have two wires, so detach the connectors, and with an assistant cranking the engine, check the current in each, but also check that the current reaching all the connectors are the same.This procedure applies to diesel engines that are fitted with common rail injection technology as well, since the injectors are electronically controlled.

NOTE: On some diesel engines, each injector is connected to its own dedicated pump incorporated into the main pump body via a long steel pipe, and in these cases, it is best to remove the entire injector from the engine in order to check its spray pattern, and the volume of fuel that each injector delivers against specified amounts and values that are available in a repair manual. Nonetheless, if there is no power to the injectors, but the fuel pressure conforms to specification, it is entirely possible that:

  • the microprocessor that controls the injectors has failed. There are no reliable DIY tests to confirm or eliminate a defective ECU (EngineControlUnit), and the best option would be replacement of the ECU. Bear in mind however that an ECU can be defective even if it still delivers current to the ignition system, so before you replace it, obtain professional advice.
  • The engine wiring harness has suffered significant damage, thus causing a break in the power supply to all injectors simultaneously. However, it is very rare for this type of damage to occur without signs of it being visible or obvious, so if you suspect a major wiring related issue, it is best obtain professional advice.
  • One or more injectors have failed simultaneously, although this is rare. Even if 50% of the injectors on any given engine have failed due to say, dirty, or contaminated fuel, the engine should still show signs of life when you attempt to start it.

If it turns out that all injectors have power fed to them, but the engine will still not start, do the following:

  • Have the injectors checked:

Many repair shops will check fuel injectors for correct operation, so consult the manual on the correct procedure to remove them from the engine. Doing this yourself can save you a bundle, but we strongly advise that you have this done professionally due to the complexity of the procedure.The repair shop will measure the fuel flow through the injector, as well as the way it sprays the fuel, which is critical for complete, or correct combustion to take place. However, it is almost impossible to clear blockages in the almost microscopically small injection orifices, which means that simply replacing injectors is always the better option, instead of wasting time and money on trying to clean them out.

Rough running issues.

As stated before, troubleshooting a fuel injection system in the absence of definitive fault data can be confusing, which is why it is just as well to remember that it is possible for an engine not to start or if and when it does start, to run rough, even if the fuel pressure is OK, there is a spark at the right time, and compression appears to be within specifications. This applies equally to gasoline and diesel engines, and the cause is more often than not worn, dirty, or heat-damaged injectors, although not always.The condition of fuel injectors have a direct bearing on how well an engine performs, but unfortunately, there are many causes of poor performance besides defective injectors.

So to either confirm or eliminate defective injectors as the cause, do the following:

  • Disconnect the injectors in turn:

If the engine idles, no matter how roughly or erratically, disconnect the electrical connector on top of each injector in turn. In cases where an injector leaks when it is closed, there will be an immediate change in how the engine idles when the injector is disconnected since no more fuel will be allowed to pass through it. Leaking injectors cause incomplete combustion because they enrich the air/fuel mixture to the point where it cannot combust fully.Any change in the way the engine idles when an injector is disconnected points either to that particular injection being defective, or perfectly normal, and it can be difficult to distinguish between which is which, even for professional mechanics if there is no fault data available. However, if there is smoke visible from the tail pipe, and it disappears when an injector is disconnected, it is possible that the injector had been causing the problem.However, smoke that is the result of excessive internal mechanical wear can also disappear if an injector is disconnected, since there will be no combustion to ignite excess oil in the cylinder. Nonetheless, if the vehicle’s oil consumption has not increased and the sparkplugs appear normal, (free of black, oily deposits), it is safe to say that the cause of the rough running is due to one, or more defective injectors.

  • Perform a compression test.

When troubleshooting a fuel injection system, the most reliable way to distinguish between excessive mechanical wear and defects in the injection system, is to perform a compression test to eliminate low compression as the cause of many symptoms that resemble those of a defective injection system.Although gasoline engines are not quite as sensitive to compression losses as diesel engines, which depend on compression for combustion, it is nevertheless important to be sure that the compression in all cylinders conform to specifications. Low compression on one, some, or all cylinders can mimic many of the symptoms of defective fuel injection systems, which makes it important to know what you are troubleshooting- a defective fuel injection system, or compression losses due to any number of reasons.

One more thing…You do not need luck to diagnose your own car problems, nor do you need to be a mechanical genius- all you need is a good repair manual, some mechanical skills, and a small collection of basic, but high quality tools.

However, when troubleshooting a fuel injection system, or any other car problem for that matter, it is important to know as much about that particular system as possible. Once you have a basic understanding of how something works, it becomes easier to devise a diagnostic strategy that is logical, to the point, but most importantly, effective.



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Updated on 04/13/2022

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