2000 tahoe, rough idle, P0171 & p0174, engine stumble when cold
This is my first post, but I thought I'd let you know the successful resolution to my problem since I found your forum helpful in my trouble shooting. I hope this might help others.
2000 Tahoe, 5.3L, 4wd, 103,000 mi.
Symptoms:2000 tahoe, rough idle, P0171 & P0174, engine stumbles off idle when cold.
Essential Tool: Auterra Dyna-Scan OBDII software on Palm 71. Used this for determining stored codes, resetting codes, monitoring O2 sensor voltage (graphed in realtime), monitored short and long fuel trim (very helpful, especially long term which was at +25% continually when I began troubleshooting, indicating a lean condition), MAF, and others.
Procedure: Checked easiest to hardest:
Fuel Infectors: Used two bottles for Chevron Techron in two successive tanks of fuel. No change. Therefore injectors probably not clogged. (This stuff is good).
MAF: read .55 lbs/min at 600rpm (warm idle). Cleaned and rechecked. no change. OK
Vacuum connections: disconnected and plugged all vacuum lines sequentially to rule out problem with power brake booster, pvc valve, fuel pressure reg, MAP sensor, canister purge valve, and that is all the vacuum connections I could find. Cleaned PVC valve. All OK.
Checked intake runners individually with brake cleaner and then propane to check for intake gasket leak. Saw immediate richening on half of the intake runners. Knew I had found a leak. Snugged the 10 bolts, MAF went up to .62 lbs/min indicating an improvement, but still had rough idle and lean condition.
Decided to replace both upstream O2 sensors anyway since they've been working for 100,000 miles and they are easier than the intake gasket.
The O2 sensors cleared the lean condition (long term fuel trim began coming down, and p0171 and p0174 codes stopped coming back. The stumble off idle when cold ceased too.
Still have rough idle, but this condition will clear up with a new intake gasket I'm sure.
My conclusion and the interesting point this is even though the vacuum leak was present all the time, most of the symptoms were caused by sluggish O2 sensors even though they appeared to be doing their job when I monitored them on scan tool. They gave a nice waveform that appeared normal, but was actually askew. Two things needed to be fixed to clear all the symptoms.