2001 1500 with 318. Started with a p0456 code (small emission leak). Found my 15 year old locking gas cap was broken where it screwed into the fill tube. Replaced with a new cap put about 100 miles on the truck and check engine light returned with p442 code (medium leak). Found a hole in my rusty fuel fill tube. Used some vulco wrap and a zip tie to get me by until I had time to replace. Put about 200 miles on it this time no problem. Replaced the fill tube yesterday but the one they shipped me uses a threaded cap and there is no flap in the tube to prevent the wrong fuel from being put in. Bought another cap which cross referenced to 1999 or 2000 model. Put about 20 miles on it and now have the check engine with p0455 (large leak).
Can I not use this type of fill tube on my model year or is it a problem with the cap?
Thanks

 

Make sure all the lines for the EVAP system are in good shape. They hang out under the truck, so, get the worst of the road crap...... they dry out, and crack. If you can, have the system smoke-tested.... makes it easy to find bad lines that way.

Welcome to DF!

 

I pressure tested it this afternoon. It's leaking at the gas cap. I rechecked to make sure I bought the right cap. It's either the gasket in the cap or the new fill tube is defective in some way. I'll return the cap and try a new one. Hopefully it's just a defective gasket. Thanks

 

Is it a vented cap? Not sure if it should be or not (though it sounds like "no") but maybe that is the issue.

 

It's not vented but does have pressure relief. I'm only putting 1.5 psi to it so it shouldn't be opening. I'm going back to the walmart today as I cross referenced another part number for the same filler neck. ( I think)

 

Finally found the smoking gun. The leak wasn't coming from the fit of the gas cap. It was coming from a defective fill neck. It was leaking from the joint between the metal and plastic surface. It was made in Taiwan so I'm not really surprised. I'll be ordering from a different parts store this round.
Thanks for the input!