GM 90570541 Engine Exhaust Manifold Assembly

Disconnect the cable from the negative battery terminal. If equipped, remove the AIR valve bracket nut, then remove the bolts and disconnect the AIR pipe from the exhaust manifold. Note: It isn't necessary to detach the hose from the AIR valve. Remove the exhaust manifold heat shield. Raise the vehicle and support it on jackstands. Detach the exhaust pipe from the manifold. Follow the lead from the oxygen sensor up to its electrical connector, then unplug the connector. Also detach the lead from its retaining clip. Remove the exhaust manifold mounting nuts and detach the manifold from the cylinder head. Using a scraper, thoroughly clean the mating surfaces on the cylinder head, manifold and exhaust pipe. Remove the residue with a solvent such as acetone or lacquer thinner. Check that the mating surfaces are perfectly flat and not damaged in any way. A warped or damaged manifold may require machining or, if severe enough, replacement. Install the new gasket to the cylinder head studs and place the manifold on the cylinder head. Tighten the nuts evenly, working from the center outwards, to the specific torque. Connect the exhaust pipe to the manifold and tighten the nuts evenly to the specific torque. The remainder of installation is the reverse of the removal steps.

Check the mating surface for flatness and damage; if warped or damaged, machining or replacement may be necessary.

Penetrating oil should be applied to the exhaust pipe-to-manifold studs and nuts.