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Few drivers ever give much thought to a device on their vehicles which they use again and again - the gas cap. To most, it's thought of as little more than a plug which must be removed to allow the tank to be filled, then replaced. But, there's a lot more to gas caps than meets the eye. Current designs must meet a set of complex performance specifications which are much stricter than anything ever imagined just a few years ago.

This guide provides a brief look at the history of gas cap design as well as several key developments which affect gas cap performance today. The following outline highlights the key points.

Historical Developments

From the beginning of automobile production until 1970, gas caps were generally considered simply plugs to keep rain and other contaminants out of the gas tank. In most cases, they were simple unsophisticated metal caps although some were designed with vent holes to equalize the pressure between the outside atmosphere and the inside of the tank as fuel was drawn out. But beginning in the early 1970's two significant forces have driven the evolution of gas cap design: safety and environmental concerns. Safety concerns centered on keeping the fuel in the tank to reduce fire danger. The environmental concerns centered on the growing problem of contamination from fuel vapors which were escaping from the fuel systems and contributing to ground level ozone.

Key points:
In the early 1970's, Stant responded to fuel loss stemming from the positioning of filler necks lower on the fenders and to rear filler locations with new cap designs.

Around the same time, more attention was focused on preventing fuel loss from impact and rollover situations, protecting occupants from possible burn injuries or death.

During the next two decades, fuel cap design continued to progress and caps were developed which maintain the seal even if the crown is completely broken away.

Continual testing of cap design and construction is done at the Stant Technology Center using a variety of testing methods, including the pendulum and ball-drop to simulate crash situations.

In 1971, attention was drawn to the problem presented by the escape of hydrocarbon vapors and the first Federal requirements concerning evaporative loss were put in place.

Stant responded to the requirements with new designs which incorporated advanced pressure/vacuum valve technology.

By the mid-90's, the restrictions on evaporative loss were 50 times stricter than they were in 1971.

New designs and technology have been developed at Stant to meet the new requirements including:

  • More positive attachment of the cap to the filler neck.      
  • Caps that are easier to attach correctly.      
  • New, more accurate venting systems.      
  • Improved materials which resist deterioration from more aggressive fuels which incorporate ether, alcohol and other components.      
  • Stant also developed the Prevent cap which allows pressure built up in the fuel tank on hot days to be released gradually when the cap is opened. This prevents fuel from erupting and spraying out on the motorist.

Onboard Diagnostics Systems (OBD)

As automobile makers came under increasing pressure to reduce engine emissions, they made significant advances in the ability of technology built into vehicles themselves to monitor correct operation of various engine and other related systems. These information processors, called Onboard Diagnostics (OBD) systems were first generation devices which could detect conditions which were outside of design parameters resulting in, among other things, higher than allowable emission levels.

A new generation, called OBD II, periodically checks the entire fuel system as the vehicle is operating, to ensure that inadvertent fuel vapor losses will not go undetected. That's important since poorly designed or missing caps can result in the loss of up to 30 gallons of gasoline per year. Even when a cap is mis-installed, it can cause the automobile's OBD II to trigger a "check engine" indicator light on the instrument panel notifying the driver that the vehicle needs service.

Key points:
In 1988, the state of California required OBD systems to monitor emission controls.

By 1990, the Clean Air Act amendments called for a new generation of OBD systems called OBD II.

OBD II systems test the fuel system by either pressurizing the system or applying a vacuum and measuring even slight changes which might indicate a leak and a possible loss of fuel vapors.

Gas caps are among the most critical components of the fuel system and if not correctly designed, calibrated and installed can cause the OBD II system to display a warning message to the motorist.

Stant caps have patented valve designs including special high quality, finely calibrated springs which allow the valves to open and close precisely, and meet OBD II requirements.

The "Quick-On" or 1/8th turn cap design assists the user in achieving a positive seal when installing the cap which helps avoid false readings and check engine lights during the OBD II system checks.

Specifications are so critical that non-original equipment caps are likely to fail to meet the OBD II test standard even though they may appear to be an exact replacement.

I/M 240

Another key clean air initiative has been testing vehicles to monitor their exhaust emissions levels. Those whose emission levels are over the allowable limit must be serviced and retested.

In some areas of the country, where ground level ozone levels are above allowable levels, emission testing may not be enough to lower the levels to meet Federal requirements. In those areas, a new, more stringent test procedure referred to as I/M 240 not only requires exhaust emission testing but testing of the fuel system for leaks as well. Since worn or damaged gas caps are likely sources of vapor emissions, the I/M 240 test process includes pressure testing the gas cap for seal integrity.

Stant offers three gas cap test devices which allow fast, accurate testing of gas caps to meet the requirements of I/M 240.

Key points: 
The Stant gas cap testers electronically compares the pressure integrity level of the cap being tested against a pre-set standard. If it meets or exceeds the standard, it is passed. If it does not, the cap must be replaced and the vehicle retested.

Fuel caps, especially those from suppliers who deal only in aftermarket caps, may appear to be direct replacements but may not meet I/M 240 test standards. That causes the vehicle to fail, in some cases even when the cap was purchased as a replacement when the original cap failed.