2002 Avalanche 5.3, P0171. Performed troubleshooting, I'm stumped.

[Redacted]

Hey everyone,

 

I recently purchased a 2002 Avalanche. In the process of taking care of all the wonderful deferred maintenance I've come to expect from used vehicles, I noticed a combination P0171/P0174 (Lean Banks 1/2) come up. In pursuit of taking care of these issues, I first went to look at replacing the fuel filter and noticed a great deal of rust on the lines. I had all the fuel lines replaced, from the rails all the way back to the pump. Line replacement had no effect on the lean conditions. Mechanic noted good fuel pressure at the rails, however volume could be a bit low. Next, I replaced the intake gaskets. When I pulled the manifold, I noticed that the manual calls for 89 in/lbs on the 10 bolts- all but the back two were nearly hand-tight upon removal. Someone had attempted an intake gasket repair before me, as I found RTV silicone smeared all over the heads. Everything was put back together correctly, and P0174 disappeared.

 

So now I've got to figure out what causes P0171 to show up on its own. I've replaced the MAF with a new Delphi unit, no change. Long-term fuel trims around 23%. Short-term fuel trims oscillate +/- 5% as they should. O2 sensors refresh at the appropriate intervals. 19.9 in/hq vacuum (I assume that's a MAP reading). I'm unable to find any miscellaneous vacuum leaks. I've even gone so far as to take the truck to a local dealership for diagnostic. Unfortunately, I'm pretty sure that they saw me as an easy mark and simply charged me $90.00 to clear my DTCs and fuel trims, which did exactly nothing.

 

To recap:

 

Replaced Fuel Lines

Fuel pressure verified OK

Replaced intake gaskets

Replaced MAF

 

The sensor values I listed were acquired from Torque Pro. I'm going to datalog a trip tomorrow evening.

 

I would appreciate any insight that any of you could give regarding my issue. Thanks in advance.

 

[pickardracing]

I'm chasing the exact same issue. Mine is combined with a surging idle. From what I've read, many guys are pointing towards a intake manifold leak that is exacerbated by the cold weather here. I can't give you an exact answer as I'm still researching, but if I come up with a fix, I will post up. My first diagnostic test will involve spraying brake/carb cleaner around the suspect areas and see if it changes the idle.

[Redacted]

I'm chasing the exact same issue. Mine is combined with a surging idle. From what I've read, many guys are pointing towards a intake manifold leak that is exacerbated by the cold weather here. I can't give you an exact answer as I'm still researching, but if I come up with a fix, I will post up. My first diagnostic test will involve spraying brake/carb cleaner around the suspect areas and see if it changes the idle.

 

Yeah, it does seem as though the problem is worse in colder weather. I went ahead and put an upstream O2 sensor on order for gits and shiggles. I was wondering about a leak in the manifold itself as well, but I would think that a leak in the common air supply would affect both banks, that is unless the leak is in a runner or something. My carb cleaner testing yielded zero results, though I didn't coat the manifold.