Exhaust manifolds are metal devices that attach the exhaust pipe to the engine. Each engine cylinder expels exhaust into the manifold, which keeps the exhaust gases hot for combustion in the catalytic converter. The exhaust manifold is prone to cracks and leaks due to exposure to extreme temperatures. Sometimes a knocking in the engine indicates an exhaust manifold leak. Other symptoms include periodic engine hissing noises during driving. Finding and resealing leaks prevents dangerous carbon dioxide exhaust from entering the passenger cabin.
Check to confirm that the engine is at ambient temperature. If you have driven recently, your car's engine may take several hours to cool down.
Don goggles and long-cuff gloves. Attach a 9/16-inch socket to a 3/8-inch drive ratchet with 3/8-inch extensions measuring either 6 or 12 inches long. Remove the 9/16-inch bolts that hold the manifold directly to the cylinder with the 9/16-inch socket. Soak the exhaust flange nuts with a rust-penetrating fluid overnight to penetrate rust if necessary. Turn on an air impact wrench, and apply it to the nuts at the low speed setting to remove them. Heat the nuts directly with a propane torch if not loosened by the air impact wrench, then remove them immediately with the 9/16-inch socket. Use a 14 mm six-point socket if corrosion causes the 9/16-inch socket to slip.
Note whether the manifold appears scaly. Heat the manifold with the propane torch, then hammer it lightly to knock loose scaly buildup, caused by leaking exhaust and condensation.
Ask your mechanic to grind the surface of the manifold to remove additional corrosion and improve flatness of attachment surfaces.
Heat the stud located inside the heat riser valve with the propane torch. Remove the stud with vise-grip pliers before it turns molten orange. Allow the stud to cool down a little if it does start to color. Carefully remove any parts of the stud that break off with a drill. Apply thread-tapping lubricant to new studs before inserting them.
Note the side of the metal-ringed gasket that reads "Out." Pop the metal-ringed gasket into the round heat riser valve on the exhaust manifold with the outside of the gasket facing out toward the opening. Check to ensure that the gasket creates a tight fit to seal the leak.
Reconnect the exhaust flange nuts, and bolt the exhaust manifold back to the cylinder. Use new bolts if the bolts you removed were corroded.
