P0171 issue

Hi Guys,My 2000 manual trans with CAL emissions with 135k miles showing P0171 code. "System too lean,(AF lean, bank 1)". I have changed MAF and upstream O2 sensor. Cleared code, then it reappeared after 4 miles on uphill test run. Hoses seem ok. It starts and runs smooth but mileage seems down. My google search shows it could be, fuel filter (has original), vacuum leak, fuel pump, downstream O2 sensor, pcv valve, injector fouled.
Any experience with clearing this would be appreciated. Thanks.

 

Did you keep your MAF & O2 sensor that you removed? And did you replace with OEM or at least Denso?
What is the status of your hoses? Ever been replaced in total? They're 24 years old potentially. Get a mirror and inspect the under sides of them. Also pay attention to any of them that attach to plastic parts and check those attachment points for integrity also.
Have you recently done any work that would require intake removal? You can try a smoke test (see you tube) or spray around the hoses with a) starter fluid which will cause the engine to race if it gets pulled in but runs the risk of catching your engine bay on fire if you get careless b) water mist which will cause a stumble and can harm electronics if you get careless.

Fuel filter. Probably not the cause, but cheap and easy to change and at 24 years, what the heck, let the poor guy retire. Unless you live in rusty land...then proceed with caution, because again, that probably ain't it.

Injector, could be but that would more likely produce a misfire code. Plus you're kinda low mileage for that.

I'd focus on the vacuum lines.

 

Kept old parts. Replaced with denso. Also replaced one small hose that was cracked. Inspection of others looked ok but old. Will try the carb cleaner test next for unseen leaks. Thanks

 

Good luck and be careful

 

I'd also look at the downstream sensor. Upstream ones don't seem to go nearly as bad as often.