After chasing a PO171 all around the mulberry bush for weeks, I finally was able to cure it and my truck passed smog. I did all the usual stuff, plugs, wires, cleaned the MAF, re-ran all the vacuum lines and checked every sensor, replaced upper intake and throttle body gasket, cleaned the throttle body. Here's what finally worked.
1. My scan tool told me I still had a small but persistent vacuum leak. I was confident that I had re-run all the hoses correctly. So, although I was pretty sure they weren't cracked, I peeled off the plastic conduit running from the EGR solenoid to the EGR valve and intake manifold, and there it was-a crack in the very small plastic tubing. I patched this with some blue goo and a piece of small vacuum line. And then, when I was replacing the fitting for the EGR valve, which runs in the same conduit, I noticed that there was no gasket between the EGR valve and upper intake manifold. I installed a gasket, buttoned 'er up, and erased the PCM with a jumper wire. Had to take about a 20-mile trip to get the cat hot and my sensors re-engaged, and all was well.
Pulled into the smog shop and hooked her up and put her on the rollers, and passed with flying colors! So, If you're having PO171 problems, be sure to look at the small plasic vacuum lines under the conduit.
YAY! I was beginning to get a tad frustrated. Persistence pays.