I have a 2001 GMC Yukon 5.3L. It has both codes P0174 and P0171. The only problem is a very hard start every time. And also lack of throttle response and power. I cleaned the EGR valve and TBI and also ran additives through the gas tank. Also does have a clean KN air filter in it. Thanks for ur time, Stephen.
what octane fuel are you using? low octane fuel could cause this problem, fill up 91 octane from Shell using a good fuel system cleaner like BG 44k or Chevron with techron fuel system cleaner, and maybe do an intake flush also.
You may check for an intake leak, you can spray brake cleaner around the intake manifold and intake tubing and see/hear if there is an RPM change. Don't spray brake cleaner on hoses and wires, just hard material.
here is a youtube video showing
other than if that does not fix the issue oxygen sensors will have to be checked, and fuel pressure which is supposed to be 55-62 psi with the key on and the engine not running. You may also consider doing a tune like replacing spark plugs and etc.
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