What are fuel mapping computers?

­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.