My '02 V6 gave me a bit of trouble yesterday, since resolved. Two fairly uncommon codes (P1131 and P1151) were eventually flagged. I thought I'd share a heartwarming tale of what I found and how I fixed it. I'm thinking this would make for a wonderful Hallmark movie, because at first I wasn't sure I was going to make it home without a tow truck. Still working on the title, as "Home for An Afternoon Nap in the Comfy Chair" just doesn't quite have that ring to it...
I was driving home from work when I stopped for gas, about halfway home. When I first tried to restart the engine, I heard a muffled pop from somewhere under the hood, and saw just a smidge of smoke or dust blown up from the back edge of the hood. The engine started on the second try, but raced high for a few seconds and then the RPM dropped low enough for the battery light to flicker a few times. I moved the car clear of the pumps and immediately noticed the brakes required a LOT of force on the pedal. Revving the engine a couple of times in neutral seemed to help just a bit, and the idle no longer dropped so low. The brakes still felt iffy, but I quickly realized the hydraulic portion was okay. It was only a lack of boost at the pedal.
A quick poke under the hood found one vacuum line loose, at the throttle body. The line had probably popped off when I heard that muffled backfire. The line wasn't entirely disconnected, and was being forced somewhat back into place by the hard plastic line. I reseated the connection fully and it helped a little, but not much. I decided to make a go of it and was able to nurse the car home for further troubleshooting. Above 20 MPH or so, the car behaved moderately well. Pulling up to a traffic light was fun, as the engine sporadically tended to rev up a little. After a bit of experimentation, I slipped the transmission into N as I coasted to a stop, and then the behavior was manageable. The brakes themselves were okay, but I had to be aware the boost effort was not always 100% and to be prepared. Throttle response was also a bit iffy, as if controlled by a rubber band linkage.
Eventually I found a larger elbow blown off at the back of the intake manifold. Once reattached, all was good. I had to get my nose right in there to realize this, as the elbow was sitting loose right above the bare protruding fitting. From most angles, it appeared to be installed properly. Curiously, I was hearing a hissing noise that seemed to originate near the throttle body, but it was really coming from the back of the intake manifold.
Initially no fault codes were set. Only after driving about 5 miles did I get P1131 and P1151. You can read about them here, and they indirectly point to a lean condition because the O2 sensors are not cycling properly:
I didn't find much in the forum about these codes. For a vacuum leak, it seems P0171 and P0174 are much more common. I'm not sure what is the difference, but I think those codes are typically set over a greater period of time as long term fuel trims go high trying to compensate. My fault had a sudden onset, and the long term fuel trims had barely budged with just a few miles of driving.
I'm still not sure what caused that initial backfire I heard and popped those lines loose. Never heard anything like that before. Hopefully it was a one-time thing. Here's a picture showing the two lines, circled in red:
