Would you put the brains of your car under its seats?

Yoshikazu Tsuno/AFP

Parts of the EFI System

Understanding fuel mapping technol­ogy is much easier if you know the territory. The center ­of the whole EFI system, which controls the fuel map, is the engine control unit (ECU). Think of this component as the car's brain. Sensors located in the engine and throughout the rest of the vehicle send information to the ECU. The ECU interprets this information and uses it to keep the car working at its best.

The ECU looks like a black plastic box with the electronic brains inside. Its location varies wildly by manufacturer. Some put the ECU in the engine compartment near the battery, some put it near the glove box or steering column in the passenger compartment. Some even put it under one of the seats.

The ECU, though, is useless without its sensors, just like our brains wouldn't be much good at interpreting the world around us without our senses. While there are dozens of sensors in a car that feed information to the ECU like the one that triggers that annoying "Check Engine" light, we'll just list the ones that create the fuel map.

  • Mass Air Flow (MAF) Sensor: This sensor measures the amount of air coming into the engine. Less air is drawn into the engine when it's idling, so less fuel is needed. More air is drawn into the engine once the car's in motion, so more fuel is needed from the injectors.
  • Oxygen (O2) Sensors: Located in the exhaust system, these sensors detect the amount of unburned oxygen and fuel coming from the engine. The ECU can adjust the amount of fuel injected into the engine to increase efficiency.
  • Throttle Position Sensor (TPS): This sensor tells the computer how hard and how quickly the driver pushes on the gas pedal. The farther and faster the pedal is pushed, the wider open the throttle moves, increasing the amount of fuel that needs to be added to the engine for speed.
  • Manifold Absolute Pressure (MAP) Sensor: This sensor measures changes in the engine's manifold pressure, which tells the ECU how much load the engine needs to bear (towing or going uphill) and how fast it needs to happen (speeding up or slowing down). If the sensor reads high pressure, the ECU will lower the engine vacuum and add more fuel. If there is low pressure, the ECU will raise the vacuum and dial down the fuel injection.
  • Vehicle Speed Sensor (VSS): This tells the ECU how fast the car is moving and adjusts the fuel accordingly. This sensor also sends signals to the speedometer and the cruise control computer.

These are the parts, but where is the map? That's what we're going to talk about next, so get out your graph paper.