Tracking down Code P0171

[redzmonte]

so have been getting an intermintant code P0171 - Fuel Trim System Lean Bank 1 for a while now. it comes and goes, on my scans i do so a +16 LTFT which explains the code.. it seems to come on when im crusing 45-65mph. when im accelorating my LTFT's and STFT's close to 0 out. I have sprayed brake clean around the intake and S/C when idling and cant seem to locate a vac leak (engine doesnt stumble), I have a New OEM/AC Delco O2 sensor, Stock Cleaned MAF, Cardone New MAF and fresh tune up. I did a search and found a couple post... all say vac leak of some sort i found a post saying it was fixed with replacing "The P0171 was caused by a vaccuum leak in the Supercharger Bypass Valve Actuator." how can i test this? others have said the top engine vac harness, which is avalible at GM for like $30. if it helps, im seeing like 15-15.5mpg in the city and low 20's more on the highway.

viewtopic.php?f=18&t=13824&p=145557&hilit=p0171#p145557

getting annoyed with the light, any suggestions would help.

thanks,
S

[jedwards83]

Symptoms point to a vacuum leak. Unmetered air enters at idle or low loads through the leak, PCM has to add fuel to compensate. Upon acceleration, vacuum starts to go away, less unmetered air = better fuel trims.

Have you checked the fuel pressure? If you can't find a vacuum leak, maybe something is screwy with the fuel pressure regulator under vacuum. It should read around 52 PSI at idle IIRC.

[MiDMoTSI]

My P0171 was caused by a bad top engine vacuum harness. The vacuum lines were hard and brittle. The cracks were on the back side of the lines and were not readily visible from the front. Only after I pulled the harness off for a closer inspection, did I realize how bad the rubber lines were. I replaced the harness and no further issues.

[redzmonte]

I will check fuel pressure and replace the vac harness and see how she goes. looks like there are 2 common vac hoses that fail.. the one from the Map/LIM/SC BBV and one from the TB to the evap sensor... should i just replace both, the one on the TB appears to be a obvious crack when it fails.

ordered Vac harness from dealer PN: 24508742

S

[redzmonte]

Replaced the vac harness.. No change. i also checked th vac line from the TB to the evap thingy. all looks to be good. looked over my old vac harness well and didnt see anything. Sucked and blew into it and kept vac/pressure so. next step is to reflash back everything to stock tune and see if it is something still screwy in my tune (mostly stock settings anyhow) just to make sure its not something i jacked up. I got a P-Log on its way shortly, so i will also replace the exhaust gaskets while its out. if the problem is still there, i guess next step will to be pull the charger/intake and redo the gaskets (look to be newer when i got the car a few months ago and didnt stumble when i sprayed with carb cleaner).

S

[Archon]

Include the EGR pipe in your search. It can develop cracks, especially where it enters the LIM.

[redzmonte]

will do.

[redzmonte]

found leak at brake booster hose on the vac block ontop of the blower using the "Smoke" leak test mentioned up in another topic... hope this was the issue
"Smoking Engine":
http://www.pontiacbonnevilleclub.com/fo ... ml#p348660