Bought a 2005 Cavalier 2dr base 5 speed a little over two weeks ago. Drove great for a few days, started stumbling/misfiring under fairly heavy throttle and setting combinations of the above codes. Gasoline odor near the hood, checked evap system and it was fine. Heard a sound near the fuel pressure regulator like a possible vacuum leak. My buddy (and this site) helped me build a list of possibilities (search for P0172 here to read those posts, not going to restate what has already been said well). In checking the car (don't have a book yet) figured out the MAP sensor was bad, based upon posts here. Then, checking for vacuum leaks, sprayed Gumout around vacuum lines. Sprayed the line for the Fuel Pressure Regulator, even thought that's not technically a vacuum line. No change in the running of the engine. Decided to see what it would sound like to get some Gumout into the intake, so I pulled the line to the Fuel Pressure Regulator line off the intake air hose, lo and behold gasoline came out of the line with the engine running! Pulled the Intake Air Temperature sensor (which is right underneath that line) and found it soaked with gasoline. Bought new MAP sensor and Fuel Pressure Regulator, went to put them in, and found at least a quart of LIQUID GASOLINE in the intake air tube as well as the resonator! So every time I hit the throttle hard, I was splashing liquid gasoline into the throttle body! Plus the gas that was evaporating into the intake air was too much for the engine management system to correct for, hence the long term fuel trim of -49%. No wonder I had a multiple cylinder misfire. As far as the exact cause of the MAP sensor failure, can't tell if it was the gasoline from the Fuel Pressure Regulator that made it fail, or it failed on its own. However, nobody (that I could find, anyway) posted the possibility of the Fuel Pressure Regulator dumping gasoline into the intake here for a P0172, so I thought I would. The vacuum leak sound turned out to be the power steering pump (never noticed it was located there at the end of the forward camshaft, no book yet), and a Chevy mech I know said this sound was pretty much normal and not a concern unless it got really loud. Called the original owner (I am owner #2) and he told me that he traded the car in at the dealer almost a year ago, so it sat. Stuff always messes up when a daily driver sits, right? So make SURE you pull that Fuel Pressure Regulator line if the FPR is on the list, to see if your are dumping gasoline into the intake.

Still love the car, not upset at all, it is back to its smooth as silk, great gas mileage, peppy (for what it is) self. Thanks to everyone here for helping. YMMV, and cheers! -Jim

Funny you said that James. I was so excited, when I would give the car a little gas I just knew I heard a vacuum leak by my evap purge valve. Ran out bought new lines, new seals...... was just the power steering pump bearings I was hearing. I was so excited I thought I had finally found my issue. NOPE

Well, spoke too soon. My car has "lot rot"... Poor thing sat for almost a year at the auction yard. The P0455 really is a separate problem. EVAP purge valves are both stuck, but at least my EcoTEC runs like a dream!