Carburetor Parts Shopping Cart

Fuel Injector - Everything You Need to Know

Posted by Mike on 11/9/2019 to Fuel Injector
Fuel Injector - Everything You Need to Know
Everything You Need To Know About Fuel Injectors

The purpose of this article is explaining everything you need to know about fuel injectors in an easy-to-understand language, regardless of your current knowledge on the subject.

Ready to start your journey into the fascinating world of fuel injectors?

It's all about emissions and fuel economy

It's All About Emissions

Electronic fuel injection (EFI) systems date back to the early 20th century. However, it was around the 1970s that car manufacturers around the world started adopting EFI systems. Why?

If the history of fuel injectors has taught us anything, it is that their development has been determined by two key factors:

  1. Increasingly rigorous environmental regulations

  2. An enormous pressure to lower engine fuel consumption

Before the decade of the 70 environmental regulations were somewhat unknown. That situation changed dramatically with the promulgation of the U.S. government Clean Air Act (CAA) in 1970.

The CAA gave the federal Environmental Protection Agency (EPA) the legal framework to create air pollution regulations. The new emission regulations were not only aimed at large industries but to any pollution source. Was the dawn of a new era for the automotive industry.

Do you remember the American muscle cars of the 50s and 60s? Now you know who to blame for their extinction. As the EPA started enforcing the first generation of "Regulations for Emissions from Vehicles and Engines" automakers were forced to reduce engine emissions by any mean possible. The industry was not prepared for such regulations. Carburetors were not good enough, vehicle engines were too big, cars were too heavy.

Add to the already convoluted picture the 1973 and 1979 energy crisis triggering soaring oil prices. Combine those ingredients and you will have a clear idea of 'why' a solution was desperately needed.

Fuel injection systems were not invented in the 70s, but overnight they became a priority. Fifty years later, the same factors continue driving huge advances in fuel injectors technology.

What is a fuel injector?

A fuel injector is a special type of solenoid valve whose activation is controlled by the Engine Control Unit (ECU). Fuel injectors are in charge of delivering the necessary amount of fuel to the engine in addition to ensuring homogeneous atomization of the air-fuel mixture.

Although that definition is technically correct, it leaves some gaps that should be clarified. What is a solenoid? ECU, what is that? Let's analyze the fuel injector in more detail.

What is a solenoid valve?

In order to avoid discussing electromagnetic theory, think of solenoid valves as parts capable of regulating the passage of a fluid through an electrically operated valve. In our case, the solenoid valve is the fuel injector and the liquid it regulates is gasoline.

Each time the Electronic Control Unit (ECU) sends what is known as a "pulse", the fuel injector is opened. In the absence of that signal, an internal spring closes the fuel injector valve preventing the passage of gasoline.

ECU, the mastermind behind scenes

In many ways, the Electronic Control Unit (ECU) act as EFI systems’ brain. It receives signals from dozens of sensors installed on the vehicle and then, based on that information. it decides the best fuel delivery strategy. Moreover, as mentioned, the ECU is also responsible for controlling fuel supply through fuel injectors.

Back to fuel injectors’ definition, you can think of the relation between sensors, ECU, and fuel injectors in a similar way than voice-activated lighting. When you arrive home and use the corresponding voice command, a microphone sends that command to an intelligent device that fulfills your order by turning on the lights.

The microphone is the sensor, the smart device is the ECU, and smart bulbs are the fuel injectors.

How does fuel injectors work?

Formally speaking, fuel injectors are considered 'actuators'. As such, they don't send information to the ECU, their role is just blindly obeying ECU orders.

That's why the technology behind the fuel injector focuses on improving their manufacturing process, their mechanical and electrical efficiency, and their size.

In this section, you will learn a little more about the two main aspects surrounding the fuel injector: its mechanical side and its electrical side.

The electromechanical nature of solenoids

As you know by now, fuel injectors are a special type of solenoids designed to deliver gasoline to the engine. You will find literature referring to solenoids as “electromagnetic” or “electromechanical” parts. What does that mean?

In simple words, electromechanical is a technical way of saying that solenoids consist of electrical and mechanical parts.

The figure above helps to visualize such internal parts:

  • The terminal, wiring, and coil constitute the electric portion of the fuel injector.

  • On the other hand, the armature, valve body, needle, spring, and housing make up the mechanical portion.

Each time the ECU sends an electric “pulse” to the injector, the coil energizes, beats the spring force and opens the valve. Once the pulse ends, the mechanical spring moves the needle downward until it seats and closes the valve. There is a close relationship between the mechanical and the electrical part, one cannot work without the other.

Understanding the injectors impedance

Arguably, the impedance of the fuel injectors constitutes their most important electrical characteristic.

You will find a lot of articles talking about low impedance injectors and high impedance injectors. Let’s briefly review their differences:

  • Low impedance fuel injectors. Although they were widely used in the 80s and 90s nowadays few vehicles still use this type of injector. Low impedance injectors basically work with a two-step signal. The ECU initially sends a high current signal to beat the injector spring inertia, followed by a low current signal to keep the injector open as required. This approach was necessary to prevent ECU from overheating.

  • High impedance fuel injectors. Modern fuel injectors operate with a continuous low-intensity current for the entire duration of the pulse. This current provided by the ECU is called “saturation signal”. High impedance injectors benefits include lower operating temperatures, smaller and lighter designs, lower reciprocating mass, and the ability to handle extremely high pressures.

For the purpose of this article, what you should remember is that low impedance injectors and high impedance injectors are not interchangeable.

The obvious question is how you can differentiate low impedance injectors from high impedance injectors. Well, you have two options. Reading their specs sheet or measuring their impedance using a multitester.

  • Low impedance (peak and hold) injectors. This type of injector usually have a resistance ranging from 2 to 4 ohms

  • High impedance (saturated signal) injectors. This type of injector usually have a resistance ranging from 10 to 14 ohms

Fuel injector electrical connector

Similar to other actuators, fuel injectors work with a voltage of 12V. The power terminal is fed by the battery voltage while the ground terminal is used by the ECU for controlling injector aperture.

Fuel injectors use different versions of the electrical connector depending on their make and year. The most common types are USCAR, Bosch EV1 (aka Minitimer/Jetronic), and the most recent EV6.

Understanding the injector mechanical specifications

The main components of the fuel injector ‘mechanical portion’ are:

  • Injector body (aka housing): depending on the make and year it can be plastic, metal or both.

  • Fuel injector hydraulics: constituted by the microfilter, internal galleries, and other hydraulic components.

  • Injector seals: fuel injectors count with two o-ring type seals, the bottom o-ring usually made of Viton to withstand intake manifold high temperatures and the top o-ring made of Buna. Both seals prevent fuel and/or vacuum leaks.

  • Valve spring: in charge of quickly close the fuel injector when no electric pulse is present.

  • Nozzle: used to deliver fuel and create the appropriate ‘spray’ pattern. There are 3 common nozzle designs, pintle, disc, and ball/seat. Pintle design usually requires frequent maintenance since the valve seat area is small and thus is prone to get clogged. Disc and ball/seat designs benefit from larger seat areas and thus are less prone to carbon buildup.

Factors affecting fuel injector flow rate

Throughout the article, it has been mentioned several times that the ECU controls fuel delivery by means of an electric pulse sent to the fuel injector. While that is true, to fully understand how fuel injectors work you should know that there are other factors that also come into play.

  • Fuel pressure: engineers take into account fuel pump pressure during Electronic Control Unit software development. Modern vehicles even have a fuel pressure sensor to inform the ECU of real-time pressure. However, when the fuel pump starts to fail or when fuel filters get clogged the system pressure may affect fuel injectors’ performance.

  • Fuel injector ‘calibration’: petrol fuel injectors don’t count with calibration as their diesel counterparts. Yet, they do have a maximum flow rate calibration (or limit) set during their design and construction. It’s for this reason that manufacturers publish the flow specification of each injector expressed in lb/hr or cc/min at a preset pressure. When replacing the injectors you must respect the nominal flow to avoid problems.

Different Types of Fuel Injectors

Now that you know a little more about what is a fuel injector and how it works it’s time to talk about its ever-evolving design. The highly efficient fuel injector that we enjoy today was possible thanks to several evolutive iterations. In this section, we will briefly review the most important stages of the fuel injector journey.

  • Continuous Injection System (CIS)

  • Throttle Body Fuel Injection (TBI)

  • Multiport Fuel Injection (MPI)

  • Gasoline Direct Injection (GDI)

Continuous Injection System (CIS)

It is beyond the scope of this article to explain in detail CIS injectors technology since it has been ruled out by car manufacturers a long time ago due to its inability to comply with emission regulations. However, as a precursor to modern fuel injection systems, it deserves at least a brief review of its principles.

Continuous Injection Systems (aka CIS) use mechanical injectors to continuously and simultaneously deliver fuel to the engine. These systems use a mechanical analog of the modern Mass Air Flow sensor (MAF) attached to a mechanical fuel distributor to regulate the fuel supply. Petrol mechanical injectors are physically similar to their diesel counterparts of that time. They have no solenoid or any other electrical part, thus its name ‘mechanical’.

CIS became popular among European automakers during the 70s and 80s thanks to Bosch K-Jetronic technology. While in the US and Asia is was more an eccentricity that anything else. Even when CIS was not widely adopted, it set the principles of modern Multiport Fuel Injection Systems:

  • CIS swapped the carburetor and replaced it with a multi-point injection system (one injector per cylinder)

  • Continuous Injection Systems popularized the concept of ‘high-pressure electrical fuel pumps’ to feed fuel injectors

  • CIS demonstrated how efficient and reliable fuel injectors are when compared with carburetors

Unfortunately, the CIS was not free of problems. Since the whole system used mechanical components it was prone to fail over time due to fatigue, corrosion, vacuum leaks, and internal wear. On top of that, the ‘continuous’ fuel supply to the engine made the CIS technology was a big obstacle when emission controls started to show up in the 80s and 90s over Europe and the US.

Throttle Body Fuel Injection (TBI)

As their name implies, throttle body injection mimics carburetors by delivering gasoline in a single location close to the throttle plates. In fact, they are so similar to carburetors that some people mistakenly call them ‘electronic carburetors’.

TBI systems usually employ a single injector for in-line engines and two injectors for V-engines. That explains why these injectors were designed for high-flow and low-pressure. The following are the most distinctive features of throttle body fuel injectors:

  • Throttle body injectors are easily accessible

  • A typical TBI injector work with a fuel pressure of 12-15 psi

  • They are ‘big’ when compared to contemporary designs

  • Most TBI injectors use ‘side-feed’ design

  • As mentioned throttle body fuel injectors flow rate is higher than traditional EFI injectors

  • Thanks to its high caudal design they are less prone to get clogged

  • TBI injectors belong to the family of low impedance injectors (less than 2 ohms)

Although TBI systems brought a lot of benefits they also had several weaknesses inherent in their design. Among these weaknesses are:

  • As a consequence of their low impedance and huge reciprocating mass, their operating temperature was rather high

  • Throttle body fuel injection systems were prone to vacuum leaks due to TBI injectors large-diameter o-rings

  • Access to TBI injectors (either for replacement or maintenance) requires replacing all throttle body injection system o-rings and gaskets

  • TBI injectors are anything but cheap

Multiport Fuel Injection (MPI)

Without a doubt, Multiport injection offers an efficient and elegant solution when compared to CIS and TBI systems. Since the 80s, MPI systems have been the first choice of car manufacturers worldwide, this before the recent popularization of GDI systems.

What makes MPI so great?

Multiport Fuel Injectors bring to the table a lot of advantages not possible with TBI or CIS injectors:

  • They are available with low impedance or high impedance coil design

  • MPI injectors operating pressure range from 35 psi to 80 psi depending on its internal design thus they allow using a wider selection of fuel pumps to be installed

  • Automakers can choose from low, medium, or high flow rates

  • Multiport fuel injectors come in short, normal, or long body lengths

  • Available with pintle, ball/seat, or disc nozzle design

  • Their great flexibility allows carmakers to choose between using sequential fuel injection (firing one injector at a time) or non-sequential fuel injection (firing multiple injectors simultaneously)

  • They work at lower operating temperatures than TBI injectors

  • Multiport fuel injectors are more efficient than TBI or CIS injectors

  • Most MPI injectors are easily accessible

  • Depending on its manufacturer and design they can last over 300,000 miles

The list of benefits goes on and on. MPI injectors truly changed the automotive industry for good. Nowadays, you can find a Multiport fuel injector for just any purpose you can think of ranging from motorcycles, passenger cars and light trucks to marine and racing applications.

Although they are less common than before, before continuing it is worth mentioning a special type of Multiport injector often called ‘JECS’ (Japan Electrical Control Systems) in honor of the manufacturer that popularized them.

You can think of the JECS injector as a combination of TBI and MPI designs. The goal was bringing together the best of both worlds, high flow rate, and multi-point layout. The objective was achieved through the implementation of a side-feed approach instead of the conventional top-feed design used on most MPI injectors.

Without a doubt, the selling point of this injector was its incredible flow rate and small form factor. At the time, MPI injectors were larger and struggled to offer such an amazing caudal. However, as new materials were introduced, top-feed injectors progressively enhanced all their features, including size and flow rate. Once JECS injectors lost their advantage, it was only a matter of time for top-feed to replace them entirely.

As you may guess, MPI injectors also have their weaknesses. They might be less critical than other EFI implementations, nonetheless, it is good for you to be aware of them.

  • Depending on the vehicle make, model, year, and engine type, it’s suggested to perform maintenance to Multiport injectors every 15,000 - 25,000 miles since they are prone to get clogged

  • Some MPI injectors are difficult to service due to intake manifold layout

  • Depending on the manufacturer and internal features, Multiport injectors could be expensive

For more than 35 years MPI systems monopolized manufacturers' preference mainly due to the multiple benefits offered by their 'advanced' injectors. But that had to end sooner or later. The GDI technology brought a plethora of advantages that MPI injectors simply can't overcome.

Gasoline Direct Injection (GDI)

One thing is sure, gasoline direct injection (GDI) is here to stay. In simple words, GDI technology achieved what was considered impossible until recently, injecting fuel directly into the combustion chamber just like diesel engines do.

To accomplish such a feat, petrol engines had to mimic their diesel counterparts in several fronts:

  • The fuel pump system has to pressurize the fuel to between 1500-4500 psi which is roughly 55 times higher than the pressure of the most powerful sequential MPI system (80 psi)

  • The fuel injectors have to work at high pressures but also need to spray fuel in very short bursts of less than 0.2 milliseconds between each fire

  • Unlike diesel engines, GDI requires that both ignition system and valvetrain has to be electronically controlled to guarantee optimal air/fuel mixture

You may be wondering, Is the GDI injection worth so much effort and technology? The short answer is, yes, it is worth it.

  • GDI systems offer unsurpassed fuel efficiency.

  • Gasoline Direct Injection allow unprecedented power per liter ratios.

  • Such gains in power make possible designing smaller and lighter engines thus enhancing even further fuel economy.

  • GDI systems enable using advanced fuel delivery strategies such as stratified fuel charge that in turn allow ultra-lean burn cycles at low engine speeds.

A huge reduction in fuel consumption, as well as incredible power gains, that’s what Gasoline Direct Injection is all about. But what about GDI injectors?

GDI injectors are visually comparable to MPI, however, similarities end there. Their quality, high-end materials, and extreme manufacturing precision are second to none.

The following features are common to all GDI injectors:

  • Compared to conventional MPI injectors they have lower internal resistance

  • Their spray pattern builds extremely fast

  • They can work at pressures in excess of 4,000 psi

  • GDI injectors pulses are about ten times lower than MPI injectors

As you can see, Gasoline Direct Injectors are a technological wonder. Yet, they are far from perfect. The main drawback of the direct injection approach is the carbon buildup in the injector tip. Since conventional Multipoint injectors were located outside the combustion chamber their carbon buildup was manageable within certain limits. GDI injectors, on the other hand, are exposed to combustion chamber extreme temperatures, therefore, the problem increases exponentially.

There is no optimal solution to this issue yet. Fuel cleaners promise to get rid of carbon buildup through their decarbonizing formula. However, such a solution might be worse than the problem it intends to solve. The strong formula used by fuel cleaners can literally destroy the engine if not used carefully.

What tools do you need to replace Injectors?

When it comes to reaching fuel injectors, the level of difficulty may vary from ‘even a child can do it’ to ‘better hire a professional’. Depending on the EFI system (TBI or MPI) you may need from a couple of screwdrivers to a complete professional toolbox fully loaded with wrenches, bits, ratchets, sockets, pliers, and more.

Do you want a trick to estimate the effort required to access the injectors?

Open the hood, remove engine plastic cover and try to find fuel injectors. If you are unable to spot them chances are that they are located below the intake manifold, which means you will have to remove it along with all related accessories, sensors, and connections.

On the other hand, if you can see injectors you may or may not have to do the same work. It all depends on the fuel injector disposition and engine shape. Most in-line engines are easy to work with. You will only need to remove a few bolts using a ratchet with the corresponding socket drive. V-engines are usually more complex, needing to remove many pieces to get access to injectors.

How much does it cost to replace fuel injectors?

In simple terms, a lot of money. Labor costs alone are in the order of USD 400 while depending on the injector type parts can raise the bill over USD 1,500.

Fortunately, fuel injectors are highly reliable. If you keep a regular maintenance schedule then you should have nothing to worry about. However, there are situations in which you may have no choice but to replace your complete set of injectors:

  • If you detect a visible fuel leak in the injector body then you have to replace it

  • It’s suggested to replace the fuel injectors after major engine repairs, i.e. major engine overhaul

  • If you never performed a complete fuel injection maintenance then chances are that you might need to replace injectors after 50,000 miles of continuous usage

Keep in mind that fuel injectors can be serviced saving you thousands of dollars. Specialized ultrasonic cleaning machines and professional fuel injector testing benches exist with the sole goal of facilitating your life by recovering fuel injectors to a ‘like new’ condition.


Throughout this article, we have covered many aspects related to fuel injectors. From their history to their construction, from how they work to the different types of injectors. The goal as stated in the introduction was to provide you with a comprehensive vision of the importance of fuel injectors in an easy-to-understand language.

Hopefully, the next time you service your fuel injectors you will look at them from a different perspective. From the perspective of someone that has acquired valuable knowledge.

Add Comment

Can't find what you need? Contact Us
Idle Mixture Screw Springs
Average Rating(22)
Rochester Main Jet 120-10
Average Rating(33)
Motorcraft 2100 Manual eBook
Average Rating(5)
[availability] Free Shipping