Why You Should Leave the Gas Cap Off on Your Car If There Is Too Much Pressure in the Gas Tank

Written by ian brown
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Why You Should Leave the Gas Cap Off on Your Car If There Is Too Much Pressure in the Gas Tank
A typical car gas cap protected by a bodywork flap (Jupiterimages/Polka Dot/Getty Images)

Gas caps seal car fuel tanks, prevent fuel spillage, reduce fuel evaporation and prevent water, dust and debris from entering the fuel system. Some caps also maintain uniform fuel tank pressure.

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Reason for Too Much Pressure in the Gas Tank

Fuel pumps feed fuel to engine carburettors by pumping it from fuel tanks. Removing fuel causes a vacuum that, if not released or maintained at the correct pressure, becomes too great, causes pump failure and starves the carburettor of fuel. Until this vacuum is released, the car will not run.

Actions to Take to Cure the Problem

Most gas caps have valves fitted that automatically release the vacuum at optimum pressure. Removing the cap enables the car to run, but this solution should not be a long-term fix. If the cap has a faulty valve, replace the cap. Alternatively, some cars have a breather pipe fitted. If this pipe becomes blocked, clear or replace it. As a temporary fix, remove and immediately refit the cap to release the vacuum.

Conclusion

Removing the cap prevents a tank vacuum, stops the car from stalling and stops pump malfunction. But it increases the risk of fuel vapour contamination, pump blockages, increased consumption, poorer performance and fire hazard. These risks are increased greatly if the cap has no bodywork cover.

Why You Should Leave the Gas Cap Off on Your Car If There Is Too Much Pressure in the Gas Tank
An older-type gas cap with no bodywork cover (Jupiterimages/Photos.com/Getty Images)

Warning

Do not insert a rag in the open fuel tank filler pipe to prevent spillage and evaporation. This greatly increases fire danger.

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