Learn how a speed density air-fuel system works

A speed density air-fuel system works differently than a mass airflow system. Learn how it works and what can go wrong

A speed-density air-fuel system is a type of electronic fuel injection system that calculates the amount of fuel to be delivered to the engine based on the engine’s speed and manifold absolute pressure (MAP). The system uses a MAP sensor to measure the air pressure in the intake manifold and the engine’s speed to calculate the engine’s airflow. The computer then uses this information to determine how much fuel to send into the cylinders.

This system is different than a mass airflow (MAF) system, which uses a mass air flow sensor to measure the air coming into the engine. A speed density system relies on air pressure and engine speed to compute fuel delivery. While MAF systems are more flexible and can compensate for external factors like ambient air temperature and altitude, speed density systems are simpler and more reliable.

What is a MAP sensor?

The MAP sensor is a critical component of the speed density system. While the engine is running, it creates manifold vacuum. A MAP sensor measures manifold vacuum and compares it to outside barometric pressure. In other words, in order to properly meter fuel, the ECM has to account for how much the engine is sucking in compared to how much the barometric pressure is pushing air into the engine. The difference between the two is the manifold’s ABSOLUTE pressure. The ECM uses this signal to calculate the engine’s airflow and determine how much fuel to send into the cylinders. The MAP sensor can be located anywhere in the intake manifold or behind the throttle body.

What goes wrong with a MAP sensor?

Carbon buildup

Since the MAP sensor sits in the intake manifold or throttle body, it can accumulate the same type of carbon buildup as the throttle plate. That would cause the sensor to provide false readings to the ECM. Unfortunately, MAP sensors can’t be cleaned like MAF sensors. If they’re clogged or have carbon buildup, they must be replaced.

Damaged sensor

A MAP sensor contains a strain gauge sensing element that can be damaged by a high-pressure event like a backfire.

Replace a MAP sensor with OE

Aftermarket MAP sensors have incredibly high failure rates. Even though an OE MAP sensor can be triple the price, they’re worth it.

©, 2024 Rick Muscoplat