Is There Too Much Pressure In Your Fuel Tank, Do You Smell Gas Fumes? How GM Evap System Works

Does your car have too much pressure in the fuel tank? Do you have fuel smell outside the car?  It is possible that your evaporative emissions system is not working correctly. Cars in the USA have had charcoal canister vapor recovery systems since the 1970's and they have become more complex over time.  The basic intent of these systems is for the fuel tank vent to be routed to the charcoal filled canister and the engine will then draw the fuel vapors into the engine as the car is driven.  On my 3rd Gen Camaro, 1985 305 Tuned Port Injection engine, the ECM controls the system. There is a valve that controls the pressure in the fuel tank as part of this system. If the purge control valve (or fuel tank pressure control valve) malfunctions, pressure can be retained in the tank and build up until the tank overpressure valve at the rear of the car begins to vent.  If the valve is stuck open, there can be insufficient pressure in the tank for proper operation of the fuel pump.
General Motors used systems similar to this on cars in the 1970s, 1980s and '90s.  Some of the systems were not ECM controlled, and others used different valves, but the basic functions are similar.
Note: In the Camaro system, it is still possible to have the rear overpressure valve vent in some cases, even with a properly functioning system.  For example, if you're running on a hot day with a lot of closed throttle idling, the purge system may not be able to vent as much vapor as required to keep the pressure down in the tank.

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