Well I know this is an apples and oranges thing but here is my .02 worth of information...hope it helps. I am an instructor for BMW of North America and on BMWs, Secondary Air Injection is a system that pumps air into the exhaust stream when the engine is cold to "fan the flames" so to speak and heat up the cats quicker. The system requires an electric motor/pump, a valve to let the air into the exhaust stream and prevent exhaust gas from going back to the pump, and some plumbing to connect it all togeher. If the Chevy is anything like that, see if you can figure out some way to make the pump run or just check it when the truck is cold. See if there is air being pumped out toward the exhaust manifolds. If it is, move on to the next thing. If you can get the valve off, get the pump to run again and see if the valve is allowing air to go through (on some there may be a vacuum hose to get the valve to open, if so apply vaccum). If the valve passes air, the problem may be clogged passages in the exahust manifold or in some cases, the passages are in the cylinder heads. If this is the case, the O2 sensors will not pick up the lean mixture and that is how the system is checked. Unfortunatly, the only way to unclog those passages would be to take the manifolds (or cylinder heads) off so you can run something through them. Maybe on the Chevy though you could get lucky and be able to do it on the engine with little or no disassembly. Well that's it....that's what I know about Secondary Air Injection...Good Luck! Hope it helps. Dwayne
