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Discussion Starter · #1 · (Edited)
So, I, like many of us, am plaged by intermittent running issues:

  • rough idle
  • missing
  • lack of power
  • poor fuel econ
  • sulfur smell
  • etc

Bring it in and it works just fine or techs say there's nothing wrong when it's clearly driving like doodoo

I've been constantly trying to find a leak or bad coil or whatever and nothing seems to make the slightest difference until today.

I decided to see what happens if I pull the gas cap and drive. I realize this is not recommended... I figured this would throw something off and I'd be able to see it in the fuel trims. If the evap system is pulling in extra air, it should make the fuel trims go up and maybe throw a code. Neither happened...

What did happen was, for the first time ever, my fuel trims leveled out, my idle got smoother, casual acceleration felt more crisp, etc.

Admittedly, I don't know a lot about how the evap system works in these cars. Some casual research led me to some things that say if your tank isn't ventilated right, it can create as vacuum causing your fuel pump to not work as efficiently and have trouble pulling fuel through. This seems to track.

I know the fuel filler cap is supposed to relieve pressure in the tank, but how does it relieve vacuum? Is it through the single valve that runs to the intake or is there another part that let's air IN to the tank that could be clogged causing a strong vacuum?

Or I'm crazy and have no idea what's going on. Grasping at straws here. Been pulling my hair out for nearly 2 years
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