Honda Prelude 1990 Online Reference Owner's Manual page 131

Evaporative Emission Control System
The Evaporative Emission Control System is designed to prevent
fuel vapors from escaping into the atmosphere.
(2.0 Si,Si)
Fuel vapors from the fuel tank are directed into the charcoal
canister where they are adsorbed and stored while the engine is
stopped or idling. When the coolant temperature rises to a certain
value, the vapors are drawn into the engine through the throttle
body and the intake manifold during normal engine operation.
(2.0 S)
Fuel vapors from the fuel tank and carburetor are directed into the
charcoal canister where they are adsorbed and stored while the
engine is stopped.
When the engine is running and engine coolant temperature is above
the set temperature of the thermo sensor, the purge control
diaphragm valve is open so that fuel vapors in the charcoal canister
may be drawn into the engine through the carburetor and the intake
manifold.
Engine Exhaust Controls
The engine exhaust emission control systems are designed to
control combustion during idle, acceleration, cruise, and deceleration.
These systems are entirely separate from the crankcase and
evaporative emission control systems described previously.
HONDA PGM-FI System (2.0 Si,Si)
The PGM-FI system consists of three independent sub-systems;
Air intake, Electronic Control and Fuel Control, thus allowing more
accurate control of air/fuel ratios under all operating conditions.
The Electronic Control Unit (ECU) detects the amount of air
drawn into the cylinders and determines the amount of fuel to be
injected to provide the optimum air/fuel ratio for all engine needs.
(cont'd)