Understanding Engine Computers

The "computer" in your car doesn't have a screen or a keyboard - but it's got lots of connections.
Your car's on-board computer doesn't look anything like the kind that's used on a desktop. The "computer" is a small box called an ECM (Electronic Control Module), often mounted behind the glove box or in the engine compartment.
The ECM controls all the essentials of engine operation: spark timing, fuel delivery, emissions, and in some cases even the air conditioning or the automatic transmission. Keeping all those systems in tip-top shape is no simple job - unless you have the right tools.
The computer continuously receives electrical signals from sensors and input devices, located in and around the engine. It analyzes this information and sends a stream of control signals to valves, controllers and other output devices, to balance the requirements of power, fuel economy and emission control. Some of the most common devices are explained later in this Tech Tip.
Every engine, every car and every computer system is different - but all the sensors and all the output devices must be in perfect sync for the car to run properly.
No system is foolproof. When problems develop, the malfunctioning parts must be replaced or adjusted. Otherwise, the car won't run right, the gas mileage will be poor and the car won't pass an emissions inspection. Only with the right diagnostic equipment and technical data, can you find out what's wrong and restore your car to original performance.

Watch for these warning signs that tell you your vehicle needs service:

The Check Engine or Service Engine Soon light is on

"Rotten egg" smell from the exhaust

Hard starting

Rough running or stalling

Engine knocking or pinging

Unstable, surging or high idle

Lack of power

Poor gas mileage

Engine "runs on" after turning key off

What Can You Do?
The old-fashioned tune-up used to be done "by ear", but this isn't possible with computer-controlled engines. Using affordable tools that are readily available to the DIYer, you can diagnose and repair many engine problems including:

Test idle speed, throttle response, engine temperature, fuel system pressure and manifold vacuum.

Tap in and read the signals being sent between the computer, its sensors and the output devices.

Retrieve and investigate any trouble codes stored in the engine's computer, indicating that something is wrong or out of spec. Restore your car to original performance specifications, reliability and emission control requirements.