Guys, I've been going nuts trying to figure this one out. I did the intake manifold gasket using the Felpro set on my 98 C1500 5.0L. I also replaced the spider, so all new injectors. I soon got P0171 and P0174 codes. I checked for vacuum leaks via smoke machine, propane, and throttle body cleaner. I can't find any leaks. Engine coolant seems to be staying consistent so I don't believe I did a bad manifold gasket job. I've put in new spark plugs, new O2 sensors, new PCV valve and tube, cleaned the MAF, and new fuel pump (old one was low psi). Looking at the scanner it appears the MAF, AIT, and engine coolant sensors are working correctly. MAF is giving me 6 grams/second, with the in-spec being 5.8g/s so I think the MAF is good. Only thing I can think of is the computer needs to be reprogrammed because of the new injectors. Does that need to be done ? 'd really appreciate any help here guys. Thanks.