I figured I would share my experience from last week working on our '98 Camry. The car has 180k miles on it, and was still running well. This ended about two weeks ago when the CEL came on, the code I retrieved was P0171, bank 1 too lean. This coincided with a slight cold engine missfire, not enough for a second code however. Numerous sources including service manuals suggest MAF sensor cleaning or replacement, 02 sensor replacement, fuel pressure, plugged injector or intake manifold gaskets. I connected my code reading tool to view 02 sensor plots and while under the hood observed a hissing sound from drivers side engine compartment. A quick run test our other 2001 camry confirmed this was unusual.
While systematically disconnecting one vacuum hose at a time and plugging it of with my finger, I monitored 02 sensor plots and listened for the hissing to let up. The result was the brake vacuum booster leaking internally. Once I disconnected the brake booster vacuum hose at the booster and plugged, the hissing stopped and car idle changed.
Ordered a reman vacuum booster and repair is complete. Hopefully someone else may find this info helpful.


Robert

 

Thanks for the information, it definitely might help someone out down the road!