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Explained: Holley EFI Sensors and What They Do

By: Holley EFI | 11/16/2020 < Back to Motor Life Home
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The concept that electronic fuel injection (EFI) is complex has scared off many hot-rodders over the years. However, when you boil down the control strategies used within any EFI system, it comes down to the basics: an engine needs fuel and spark to operate correctly. This article will detail the significant sensors an EFI system uses to control the fuel and spark regardless of horsepower potential.

It is important to note that there are three general types of sensors: temperature, pressure, and position that provide key data to the engine control unit (ECU) to allow it to control the engine.


These sensors can be divided further into specific configurations and calibrations to ensure the signal they send to the engine control unit (ECU) is consistent and repeatable. The ECU controls the fuel mixture, idle speed, ignition timing, and in some cases, valve timing, and drive-by-wire throttle-body if so equipped. It uses the sensors to monitor specific engine parameters and make changes to those based on the information it is receiving about the engine’s operating conditions. When you consider everything the ECU needs to observe and adjust to achieve maximum efficiency — especially in serious performance engines turning north of 9,000 revolutions per minute, or 150 times per second — it truly puts into perspective what a miraculous job the ECU is performing.

Pressure sensors, or pressure transducers as they are technically called, are ingenious devices but at the same time, quite dumb. They don't know what they are measuring, they only know to convert the pressure acting upon them into a measurable electrical signal the ECU can read. For example, a pressure sensor has no clue if it's measuring air pressure, oil pressure, or water pressure. Below are the most common sensor usages in a Holley EFI system, however pressure transducers can be applied to any form of pressure in an automotive system, from oil pressure, to exhaust back pressure, and the data gathering usages in racing can get quite creative.


MAP Sensors


Holley EFI requires several pressure inputs in order to run an engine. The first and most important is the manifold absolute pressure sensor (MAP). In a Speed Density–style fuel injection system, this sensor helps the ECU determine how much fuel to inject by measuring the vacuum, or in the case of forced induction, boost, in the engine's intake manifold. That measurement is combined with air temperature, RPM, and a few other variables to determine fueling. The MAP reading is also a direct indicator of engine load.


For those who have ever seen an EFI fuel table, the MAP sensor is what creates the entire Y access of that table, with the X axis being engine RPM. MAP sensors are created in several different bar ratings, a unit of pressure, which is a key distinction when selecting a sensor. One bar is the atmospheric pressure we experience at sea level (14.7psi), so when a naturally aspirated engine is at peak load and wide open throttle, in theory it would never see more than one bar of pressure in the intake manifold. When an engine utilizes forced induction through a turbo or supercharger, the amount of pressure in the intake manifold can be drastically higher. An engine with 14.7psi of boost would require a 2 bar MAP sensor in order for the ECU to accurately measure manifold pressure. This is because 1 bar (atmospheric pressure) is already being applied to the engine naturally and a second bar of pressure (from the forced induction) takes the manifold pressure to 14.7 psi. For this reason Holley offers several different styles of MAP sensors in different bar ratings.


“We call it a manifold absolute pressure sensor, but it’s really just a pressure sensor," says Holley EFI engineer, Ryan. "An absolute pressure sensor uses absolute vacuum as its 0 reading, whereas a relative pressure sensor, your typical zero-to-100 psi sensors, use atmospheric pressure as their 0 point. They will show a reading that is 14.7 psi, or 1 bar, offset from each other. All a sensor does at the end of the day is put out a voltage — It’s scaled to read what it needs to read, and it puts out a voltage that the EFI system can then convert back into pressure.”


Fuel Pressure Sensor

Another pressure sensor in an EFI system is the fuel pressure sensor which signals what pressure in the fuel rails. Some ECUs log this pressure and factor it into fueling calculations while others do not. Holley EFI does not actively incorporate fuel pressure into it's fueling strategy.


Oxygen Sensors


The oxygen sensor is an ingenious little device and one of the most critical sensors in any EFI system.


The oxygen sensor is a critical component of any electronic engine management system, and relays real-time air fuel mixture data to the computer for minute adjustments to fuel mixture. It is also a huge contributor to the self-learning capabilities of Holley EFI systems. If the oxygen sensor is designed for fuel correction, why does it measure oxygen? Well, when air and fuel enter into an engine's cylinder and are ignited by the spark plug, the resulting exhaust gasses contain trace amounts of several elements such as nitrogen, oxygen, etc. The more complete the burn, the less oxygen is present in the exhaust gas. So, through interacting with the unburned oxygen in the exhaust, the sensor produces an electrical signal which the computer interprets as a rich, lean, or stoichiometric condition (the ideal 14.7:1 air-fuel ratio at which gasoline and air burn optimally). The ECU can then take that information and use it to influence the amount of fuel it injects. This is called closed loop operation, where the O2 sensor actively influences the ECUs fueling strategy. There are however many points of engine operation where the computer operates on a fixed fuel table. This is known as open loop operation and can occur for a variety of reasons such as at cold start when the oxygen sensor warms up.


Oxygen sensors measure in lambda, which represents all fuel type's stoichiometric value as 1.00. So an engine that is consuming fuel at its stoichiometric ratio would show a lamda reading of 1. If it were running lean, that reading would be higher than 1 and rich would be lower than 1. The common air-fuel ratios we are familiar with are actually a calculation based on the observed lambda reading multiplied by the stoichiometric ratio of the fuel. So a .87 lamda reading with gasoline, which has a stoichiometric ratio of 14.7:1 is equivalent to a 12.78:1 air fuel ratio .87 x 14.71=12.78:1.


There are two types of oxygen sensors installed on fuel injected vehicles and they are wideband and narrowband sensors. Wideband sensors, like their name suggests, have a very wide scale and can read lamba numbers that are both extremely rich and lean. (Narrowband sensors can only measure lambda right around the stoichiometric ratio which means at certain conditions, such as wide open throttle where mixtures are considerably richer than stoichiometric, they are unable to broadcast data. These narrowband sensors typically come equipped on factory vehicles that operate in hyper-defined ranges as intended for emissions. They are more cost effective for mass-produced vehicles but many manufacturers are moving toward wideband sensors in the unending quest for improved fuel economy.



Temperature plays a huge role in how an engine operates. Anyone who's driven a performance car on a cold evening can attest to the extra kick the cool air has to offer. When it comes to EFI, the two temperatures that impact operation the most, or at least the only two your ECU cares about, are intake air temperature (IAT) and coolant temperature (CTS).


Intake Air Temperature Sensor


The IAT sensor is typically mounted in the intake tract very close to the throttle blades and it provides a 0-5V reference signal to the ECU that is interpreted as the relative temperature of the air the motor is consuming. This is important for two reasons: Cold air is denser than hot air and packs more oxygen molecules per unit of volume. With more oxygen, more fuel is needed to preserve the desired air-fuel ratio and prevent the engine from running lean. The side affect hear is the completely welcome addition of horsepower. The opposite is also true in that warmer air is less dense and requires less fuel to maintain the desired air-fuel ratio. However, warmer air is also less resistant to detonation so when the ECU looks at the signal from the IAT, it not only adds or subtracts fuel, it can also add or subtract timing to improve power output when air is cold or prevent detonation when air is warm.


Coolant temperature Sensor


The coolant temperature sensor directly communicates the heat in the engine to the ECU. This is useful for a plethora of tuning reasons, such as cold start where the ECU can detect that the engine is cold and utilize specific maps to help it start easily, warm up quickly, and even be instantly driveable. The ECU can also use temperature data to adjust fuel and timing. For example, a cold engine cannot effectively burn fuel as well as an engine at operating temperature so the ECU needs to add fuel in order to help a cold engine idle and run. Once the engine reaches operating temperature this enrichment stops. Much like with air temperature, if the engine is very warm or potentially overheating, the ECU can adjust timing to prevent detonation.

Knowing what all the moving parts inside and outside the engine are up to is a huge part of the control strategy of a Holley ECU, as well as any fuel injection control system. You wouldn't want a fuel injector firing on the compression stroke, or your ignition hitting full advance at idle. The following, key position sensors work very differently but all help communicate to the ECU what the engine is doing, and what the driver is asking it to do.


Throttle Position Sensors


If your right foot had a direct line to the ECU, this sensor would be it. The throttle position sensor (TPS) tells the ECU where the throttle blade is at relative to its sweep and provides crucial information for things such transmission control, open vs. closed loop consideration, and also acceleration enrichment. For those coming from the carburetor world, think of it as a kick-down linkage, accelerator-pump squirter, and more all wrapped up in a tiny sensor.


Camshaft and Crankshaft Position Sensors


Camshaft and crankshaft position sensors (CPS) are often the same sensor installed in two different places on the engine. They tell the ECU both how fast the engine is spinning and where it is at in its rotation. For OEM engines, the crankshaft position sensor is mounted onto the engine block through a small bore that exposes its tip to the reluctor wheel of the crankshaft. As the reluctor wheel and crankshaft rotate, they produce an electrical signal that is relayed to the ECU to indicate RPM.


The camshaft position sensor is mounted somewhere that it tip is exposed to the camshaft's reluctor wheel (which can sometimes double as a timing gear). By cross referencing these two signals, the ECU can determine how fast the engine is spinning and where it is in its rotation to perform functions such as sequential fuel injection, variable valve timing, and more. In an early pushrod engine converted to Holley EFI where there are no block provisions for factory style cam or crank sensors, this is often accomplished via a dual sync distributor.


Other Key EFI Sensors


Idle Air Control Valve


While not technically a sensor, the idle air control valve (IAC) is an important electrical component in any EFI system and one you should understand. The IAC allows the computer to influence how much air is being let into the engine's intake manifold at idle. Again, to pull from the carburetor metaphor, it does the job of both the idle screw and the choke, only a thousand times more accurately. There are multiple types of IACs such as stepper motors and pulse width modulation motors (PWM) but all of them, under command from the ECU, manipulate air entry around the throttle blades to influence idle speed up and down. When the engine starts cold, the IAC can open enough to increase engine RPM beyond idle, allowing quicker warm up and instant drivability. It can also account for engine driven accessories, such as air conditioning, by increasing the amount of air let into the engine when they are engaged. In conjunction with idle spark control, this allows the engine to retain a steady idle in various load and temperature conditions.


Knock Sensor


The purpose of a knock sensor is to provide the ECU with an early warning when the engine has gone into detonation–commonly referred to as knock or pinging. This allows the engine to retard ignition timing and reduce cylinder pressure before internal engine damage occurs. The reason for knock could be as simple as poor quality gas, overheating of the coolant, or sustained heavy load on the engine. By being able to detect this unwanted, engine-killing condition, the computer is able to address it almost immediately. These sensors work by "listening" to the engine harmonics and sending a signal only when they detects a vibration outside of their pre-designated frequency range.


The fuel injection system of a vehicle is a network of signals, all designed to account for varying atmospheric and operational conditions. As you can see, the job of each sensor is fairly simple, but important nevertheless. And while all of the sensors mentioned in this article are integral to engine operation, they can also be used in racing for data-logging a limitless number of vehicle parameters. Holley EFI allows sensors to be custom scaled so what you can measure is limited only by your own imagination. We hope that in this installment, you have learned enough to feel more comfortable tackling an EFI installation yourself!


On EFI systems such as Holley's Sniper, all of the EFI sensors (except for the O2 and coolant temperature sender) are mounted snuggly in the throttle body. This minimizes installation time and packages really well on a diverse group of engine applications.


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