Before the government mandated On-Board Refueling Vapor Recover y (ORVR) to combat hydrocarbons (passenger cars in 1998, light trucks in 2003 and heavier trucks in 2006), there was no good way to test for a leak at the cap-to-filler neck interface because of a check valve on the end of the filler neck. So, an adapter had to be used.
Since the introduction of ORVR, there is now a vapor recirculation tube that connects the vapor dome at the top of the tank to the upper end of the filler neck. The vapor recirculation tube allows the entire EVAP system to be leak tested from the front of the vehicle at the service port. This eliminates the need for installing the adapter to the cap for leak testing.
This test is more meaningful because it tests the actual cap-to-filler neck interface seal, rather than testing to see if the cap seals against an adapter surface.
Evaporative Emission System Tester
The best way to test the fuel cap is to leave it on the vehicle while using an Evaporative Emission System Tester (EEST), connected at the service port. By doing so, the critical cap-to-filler neck interface is tested for leakage. It is inappropriate to use a tool such as a fuel tank cap adapter (Fig. 10) (examples for GM vehicles include GE-41415-50 and GE-41415-50A) to test fuel caps on many of today's vehicles.
Fig. 10
Follow the Service Information instructions specific to the vehicle being worked on when connecting an Evaporative Emission System Tester.
Fuel Tank Cap Adapter
The fuel tank cap adapter cannot be used to test the cap on some vehicles because it may falsely show a leak at the cap. For these vehicles, a leaking cap on the test adapter does not necessarily indicate that the cap will leak on the filler neck. Use the adapter only when called out in the Service Information instructions.
Part of the adapter -- the filler neck end of the tool -- is still needed when testing for a restriction in the diagnostic for P0455 on certain vehicles. This is fully explained in the Service Information procedure for P0455.
The fuel tank cap adapter is also needed when there is a check-valve in the purge line; i.e., on some turbocharged vehicles.
- Thanks to Bill Coleman
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