Upgraded fulel pressure regulator? hmmm seems like that may be a problem. The pressure/purge systems in an MB are very sensitive, I mean very sensitive, these types of problems are very common at kicking off CEL lights. Lots of times the purge valve fails, but in your case if you are getting incorrect tank pressure that could be due to the pressure regulator not being in sync with the rest of the system. What this system does it monitor the pressure in the fuel tank, once it gets to a certain level it opens the purge valve and bleeds off the pressure through the charcoal cannister where it stores vapors, these vapors are burned through the intake system. The pressure has to be maintained properly to prevent fuel tank colapse and over-pressure. I would suggest checking the purge valve. That new of a car the cannister is probably ok, but the purge valve can cause purge flow pressure issues, I had a CEL light come on several times, each time incorrect pressure and tried several things, I finally replace the valve over a $100 for the part. Also if that doesn't fix it i would get the operating parameters of your upgraded fuel pressure regulator and compare them to spec, that could cause a problem as well. Good luck!
B

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Brian W. Heitman

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