Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
07 GMC classic 2500 with a 6.0l and stumped with a p0171,174. i have repaired the following. intake gasket leak, purge valve no sealing, throttle body gasket leaking. smoked the intake, no smoke, smoked the evap, replaced an incorrect cap, no exhaust leaks. static fuel pressure is 58 psi and drops to 55 when relay cuts out. running fuel pressure at 59 psi. injector test, all drop 20 psi and are identical. i have done an injector clean. at idle MAF is 3.95 - 6.0, fuel trims are ST 27%, LT 25%. Pre cat o2's are pegged lean at .35mv, post cats are the same. raise to 1500 rpm and ST goes into negative and LT drops to 5% or close to it. total fuel trims are within 10%. o2's are switching as they should and it runs great. i should ad that there is no misfire at idle to be concerned about. a couple of cylinders will show the odd one but not consistent are far between. at idle injector pulse width are 5 to 6 ms but drop when rpm is raised to 2-3ms. under heavy load WOT MAF is 250 plus, MAP is raising to 10 or 12 PSI. MAP at idle is 5 to 6 psi.
You seem to be dealing with a vacuum leak. You can graph the o2s and do a WOT o2 test. They should go rich with a vacuum leak and lean with a MAF and fuel delivery issue.
