What are fuel mapping computers?

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.

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.

­While the car's ECU doesn't need to visualize the­ fuel map, it's helpful for us humans to picture how this computer comes to its conclusions. You don't need a physical piece of graph paper, but imagining one will make it easier. The fuel map looks a lot like something you'd learn in a junior high school math class.

Imagine that piece of graph paper and draw a simple X-Y axis on it: one line going across (the X) and one line going up and down (the Y). The numbers along the X axis represent the engine's revolutions per minute (rpm). That's how fast the engine's internal components are turning to do whatever the driver needs -- speeding up, slowing down, waiting at a red light or even towing a boat. The Y axis represents the load on the engine, or the energy required by the engine to do the task at hand.

Now imagine points scattered all along that graph paper that represent different driving situations. That's the fuel map. At each point -- and there are hundreds of possible combinations -- the ECU decides what to tell the fuel injectors to do.

Pulling a fifth-wheel camper up the Rocky Mountains at highway speed, for example, puts the engine under a huge load and requires a lot of energy. The ECU gets input from all of the sensors on vehicle speed, air intake, pressure, and temperature and plots a specific point on the imaginary graph. The computer is programmed to tell the fuel injectors what to do at that very point on the fuel map, and it sends out the appropriate message -- without any more input from the driver.

Once the ECU has received the information from the sensors and figured out what to do based on the fuel map, it can change three basic things to make the engine run at its best -- the fuel flow rate, spark timing, and idle speed.

But what can you, the car owner, do to change these things? Change the ECU and sensors, of course. We'll look at improvements and troubleshooting for the fuel mapping system on the next page.

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There are basically two types of people who'd want to make changes to the fuel map: Performance hounds and fuel misers.

Performance-minded people who are interested in eking every last hundredth of a second from their cars can reprogram the ECU's fuel map to deliver more fuel. There may be some fuel wasted and a lot more unburned fuel in the exhaust, but that's the price you pay for a win at the Saturday night drag races.

On the opposite end of the scale are those drivers who are willing to sacrifice performance for fuel economy­. A leaner fuel/air mixture is going to sap some power from the engine, but burning less fuel means higher miles per gallon and less fuel wasted in the process. There are even adjustable MAP sensors on the market specifically aimed at the fuel-saving crowd.

Which brings us to one last point: You must remember that the information in the vehicle's brain is only as good as what the sensors are telling it. If a sensor is bad and sending faulty information, the ECU will not adjust the fuel injectors correctly. Failure usually occurs due to a dusty sensor or a corroded or loose electrical connection. Occasionally, the sensor itself will just go bad and send incorrect signals.

To stick with our earlier example of an engine under very heavy load -- the camper on the highway in the Rockies -- let's think about what would happen if one of those sensors were bad. If the Mass Air Flow (MAF) sensor, for example, wasn't working correctly, it might tell the ECU that there wasn't a lot of air moving into the engine. This would move the point on the fuel map, and the ECU wouldn't know to send a signal to the fuel injectors to increase the amount of fuel in the mix to make up for all that air. The engine would seem sluggish, since it wasn't getting the energy it needed to do such a difficult job.

The fuel map inside the ECU is able to adjust the air/fuel mix for maximum efficiency and performance in any condition - that is, as long as the sensors are giving it the right information.