When you get that intake manifold from the junkyard take a good look inside. You should be able to move the flap valves ( don't know their correct name, but they open and close which gives you the long or short runners for the air routing to the cylinders) from the front of the manifold where the solenoid attaches. Be sure that it flips back and forth smoothly opening and closing those door type valves. You may even want to spray a bunch of degreaser into the manifold and finish up with brake cleaner to help clean the interior of the manifold. It is actually two pieces that can come apart and give you access to the interior, but you may not want to go that far. Inspect your current manifold when you have it off and compare it to the junkyard as to cleanliness and operation. An endoscope would be helpful here. You'll need new gaskets.