To maintain the air-fuel ratio within an optimum range, the control unit must take account of coolant temperature and air temperature. Extra fuel is needed when the engine is cold, and when the air is colder, and therefore denser.
The coolant temperature sensor is immersed in coolant in the cylinder head. It consists of a hollow threaded pin which has a resistor sealed inside it. This resistor is made of a semiconductor material whose electrical resistance falls as temperature rises.
The signal from the coolant temperature sensor is used to control the mixture enrichment when the engine is cold and is processed in the control unit.
Enrichment occurs during engine cranking, then slowly reduces as the engine warms up. This ensures a steady engine response immediately after releasing the starter. The control unit continually monitors coolant temperature during engine operation.
If the air temperature sensor is installed in the airflow sensor, it’s positioned in the airstream, and it’s called an intake air temperature sensor, or IAT. When it’s installed in the intake manifold it’s located in one of the intake runners, and it’s called a manifold air temperature sensor, or MAT.
In both cases it relays information on temperature, and, therefore, the density of the air. The control unit can then vary the setting accordingly.