P171 finally found - fuel pump seal failure
 

This post is just to help the next person who is searching thread titles. I have a separate post about my struggles to track down P171 and just wanted to say.... using a low pressure CO2 powered homemade smoke machine I found a leaking seal in the top of my 3 year old <10K fuel pump. Advanced Auto exchanged it under warranty I popped it in and the truck is SO much happier and I might have saved my cat from plugging up in the nick of time, because after driving it a bit the truck smells better too.

In another bit of weirdness, someone else said to test the evap system for leaks by unplugging the electrical connector from the EVAP Purge solenoid. It was expected this would throw a code for the electrical circuit but since the evap purge wasn't triggering if the evap system was the problem P171 would not show up. Voila! It worked, so I focused on the fuel system and evap system. The weird part is the leak is in the top of the fuel pump which operates whether the EVAP purge is open or closed but when it was closed the truck ran fine.... no lean code. I guess the vac leak in the top of the fuel pump was only letting atmosphere into the evap system, and not messing with the fuel delivery to the injectors. But I'm kinda making that up.

Anyway if you're struggling with P171 I hope that helps.

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Good info, thanks

Hard to figure out how EVAP system could cause lean code, so odd issue there

It wasn't really the evap system, I think. The leak was at that seal in the top of the pump housing itself.... wouldn't that make a vac leak, similar to driving without a gas cap?

No, not a "vacuum leak" that would effect engine performance or set Lean codes

The gas tank air is given a slight negative pressure, but not much, using engine vacuum, via the EVAP system
But if the EVAP valve was stuck wide open it could show up as a Lean code because then it would be a vacuum leak