Nothing can be more frustrating than a broken-off bolt on your vehicle's exhaust manifold. Not only is this putting stress on the rest of the attaching bolts for the manifold and the manifold itself, dangerous carbon monoxide could be escaping from the manifold and working its way into the passenger compartment. It is very important that you correct a broken exhaust manifold bolt from a mechanical as well as a safety standpoint.
Use penetrating fluid on all of the exhaust manifold bolts, including the broken one. Let the penetrating fluid work in for several hours.
Loosen the remaining exhaust manifold bolts. Give the bolts another dose of penetrating fluid if one or more bolts still will not budge. Allow the fluid to soak in for an hour or so. Apply some heat with a propane torch if you still have any stuck bolts. Concentrate the heat onto the centre of the stubborn bolt until it is red hot. Allow the bolt to cool then loosen and remove it.
Remove the exhaust manifold to reach the broken bolt. Try using vice grips or an easy-out tool to twist the bolt out if part of the bolt is protruding from the engine block. Drill the bolt out if it is broken off flush with the engine block. Use a drill bit the same diameter as one of the exhaust manifold bolts without the threads.
Run a tap into the hole that is the same size as the exhaust manifold bolts to clean out any remaining remnants of the broken piece. Tap the remaining manifold bolt holes while you are at it to ensure the bolts will thread back in smoothly.
Replace the broken bolt with one of the same size and strength as the original. Take one of the good bolts to an auto parts or hardware store to make sure you get the correct replacement. Clean up the rust on the threads of the original bolts that you are going to reuse. Use a wire brush or a wire wheel installed on a bench grinder for this task.
Scrape off any remaining gasket material from the engine block and the exhaust manifold mounting surfaces with a gasket scraper or a putty knife. Install a new exhaust manifold gasket between the exhaust manifold and the engine block. Run in the manifold bolts finger tight. Tighten the bolts with a torque wrench. Find the torque specifications for your specific model in a shop manual or from your car dealer's service department.
Start your vehicle and check for exhaust leaks.
One of the first indications of a broken exhaust manifold bolt might be a louder exhaust note or a ticking noise coming from under the hood with the engine running. The noise may get louder under acceleration.
Repair any exhaust system leak immediately to keep carbon monoxide fumes from entering the passenger compartment. Keep a fire extinguisher handy any time you are using heat to extract a broken bolt.