What makes a cap fail smog and how to test it?? (1977 F250 351)
The last time I had a smog test (2 yrs ago) everthing passed but the gas cap.But they passed me anyway if I would get a new cap.So before this test I got a new cap. It was very hard to find one to fit.It fit very tight,But Faild .I took it back and they gave me another and it also failed.
Could it be the filler neck itself.It doesn't look bent or damaged in anyway.
I though of getting a new filler neck with a cap,But is there something else that would cause it to fail.

I am takeing it in for a smog Diagnostic test on Thur.Will this test cover the cap also?
Don

 

the gasket on the cap will make it fail. also the cap you get must be OEM type relief cap. some caps will fit but have the wrong pressure setting and that will make you fail

 

The gas cap will not make it run rich - they act like a one-way valve, opening and allowing air to enter the tank, but closing and not letting fuel vapors to escape. They seldom fail, and shouldn't go bad with just 2 year's use.

I think they take it off the truck and bench test it - but it has been a few years since I watched a test. Things could have changed.

Caps are also a safety device - keeps the gas in the tank if you roll over, but I don't think the EPA group cares about that!

 

I answered your post in the 73-79 forum