Let's learn how the EVAP test also known as LPFET (low pressure fuel
evaporative test) was done. The LPFET test is conducted by a BAR approved
Low Pressure Fuel Evaporative Test machine. A pressure hose is connected to
your Chevy Blazer's gas filler. This is where the gas cap goes. Your Chevy's
EVAP system and gas tank is pressurized 0.5-1.0 psi (14"-28" column of
water) and pressure drop measured for 2 minutes. The allowable drop is .40
psi. Anything greater then .40 psi shows a leaking EVAP system. The data is
entered into the smog machine, and your Chevy Blazer fails the smog test.
The EVAP system is designed to prevent the release of gasoline vapors into
the atmosphere. All EVAP systems have one similar component, which is the
Charcoal Canister. The Charcoal Canister stores gasoline vapors from the
vehicle's fuel tank and carburetor float bowl if your car is carbureted,
until the engine is ready to burn them during acceleration or cold starts.
The number one fault which causes vehicle's to fail the EVAP test is broken,
loose or disconnected EVAP fuel lines.
It is necessary to trace the entire EVAP system in order to find the
defective section and/or component. We recommend an experienced smog check
repair center conduct this repair as broken EVAP hoses may be difficult to
find for the average vehicle owner. If you have mechanical experience you
may try to follow the EVAP route yourself.