Hey guys, I could use a few ideas on this one. My wife drives a 2005 Chevy Impala with the N/A 3800, and it started misfiring yesterday. Today it got worse and started throwing P0300 for multiple/random misfires. I test-drove it and I can only get it to misfire at about 75-100% throttle, and even then only some of the time. Took maybe a dozen hard acceleration runs tonight before I got it to show up. My wife said it misfired more when she was accelerating too. The car still pulls as hard as ever, but you can feel it stumble when the misfires begin. I have scanned it and found that it is misfiring on all six cylinders when it happens. There is no problem with rough idle or hard starting.
My first thought was fueling since it showed up on all cylinders and only under load, so I tested the fuel pressure with key on/engine off, at idle, and while revving it. Everything looked to be within spec and never dropped below 46 psi. Pulled the vacuum line off the fuel pressure regulator and found no gas leaking.
My next thought was ignition. I replaced the original spark plugs with NGK TR55 plugs about a year ago, so I pulled two of the front plugs to check for fouling, thinking maybe it was too cold a plug. They looked normal though. I ran the car at night and used a spray bottle to get water mist around the plug wires to help look for arcing, but found nothing.
The only other ignition problem I've had was also about a year ago when I stupidly sprayed water on my hot engine at the car wash, which killed one of my coil packs. I replaced it and also replaced the other two recently for good measure, so all the coil packs are new. The only major work I've done on the car recently is replacing the transmission, which I did myself a few weeks ago.
I'm unsure where to go from here. I know there are many things it could be, but I can't afford to throw parts at it right now (spent all my money on the tranny). Any suggestions?
