GM 12588987 Engine Exhaust Manifold (Machining)

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Disconnect the cable from the negative battery terminal. Raise the vehicle and support it securely on jackstands. Apply penetrating oil to the studs and nuts that attach the catalytic converters to the exhaust manifolds as they're usually rusty. Remove the catalytic converter bolts and disconnect them from the exhaust manifolds. If you're working on a left (front) manifold, remove the oil dipstick tube. Disconnect the oxygen sensor electrical connectors, then unclip the wiring harness. Remove the exhaust manifold heat shield fasteners and shield. Remove the exhaust manifold mounting bolts. The lower bolts for the right (rear) manifold can be removed from under the vehicle. Lift off the exhaust manifolds and discard the gaskets. Check the manifold for cracks and make sure the bolt threads are clean and undamaged. The manifold and cylinder head mating surfaces must be clean before the manifolds are reinstalled-use a gasket scraper to remove all carbon deposits and gasket material. The cylinder heads are made of aluminum, therefore aggressive scraping is not suggested and will damage the sealing surfaces. Also, if any of the manifold bolts are broken, it indicates a warped manifold. Have the manifold machined at a cylinder head reconditioning shop prior to installing the manifold to the vehicle. If this is not done, the manifold may not seal properly and the new manifold bolts will probably break. Place the manifold on the cylinder head and install the mounting bolts finger light. When tightening the manifold bolts, work from the center to the ends and be sure to use a torque wrench. Tighten the bolts to the torque. If required, bend the exposed end of the exhaust manifold gasket back against the cylinder head. Apply an anti-seize compound to the threads of the outer heat shield bolts, then install the bolts and tighten them to the torque. The remainder of installation is the reverse of removal. Start the engine and check for exhaust leaks.

If you're working on the left (front) manifold, remove the oil dipstick tube.

If any of the manifold bolts are broken, it indicates a warped manifold. Have the manifold machined at a cylinder head reconditioning shop before reinstalling it on the vehicle. This is important for a proper seal, and it helps prevent new manifold bolts from breaking.