When the exhaust valves open, the exhaust gases are pushed out into the exhaust manifold by the upward movement of the piston. The escaping gases form a pulse in the exhaust manifold. This pulse has high pressure at the front and negative pressure behind it as it travels in the exhaust runner.
Imagine a small crack in the runner of the exhaust manifold. As the front of the pulse encounters the opening, some of the exhaust gas escapes. As the pulse passes, the low-pressure can suck in gases outside the manifold.
To your ears, it may sound like a pop or a puff. To the oxygen sensor, the change to the exhaust gases in the manifold can cause inaccurate readings. The escaping gases and the outside gases cause the sensor to read too lean.
The engine management system does not know there is a crack in the manifold. All it knows is that there is an incorrect ratio of oxygen in the exhaust gases. Consequently, the engine management system will add fuel to burn the oxygen. Depending on the leak’s size, the fuel trim will eventually reach a point where it can no longer compensate. At this point, a code will be set like P0170 or P0171 for a lean condition, and the long-term fuel trim will be maxed out.
Repairing the leak in the exhaust manifold is critical to restoring the engine management system’s ability to control what is happening inside the combustion chamber.
Most exhaust manifolds fail in the high-stress areas where the flange meets the head or where the runners converge into the collector. On some exhaust manifolds that have a connected catalytic converter, cracks can occur at the flange that might connect to the downstream catalytic converter.
Some DIY solutions are epoxy or putty to seal the leak. These never last due to the constant expansion and contraction of the manifold.
The other option is to weld the exhaust manifold. The problem with this approach is the hole or crack might not be accessible to the welder. Most manifold assemblies are either cast iron or stainless steel, two of the most challenging metals to weld. Also, to properly weld cast pieces or stainless steel requires a MIG or TIG welder and a skilled welder.
The other option is to replace the exhaust manifold. This means you have to deal with the fasteners that secure the manifold to the head. These fasteners are subjected to extreme heat, vibration and water. Always plan for broken studs and seized fasteners.
There are methods for removing these stuck or broken exhaust manifold studs. Some methods include penetrant chemicals applied to a hot or cold manifold. Others involve welding a bolt to a stud or tools to grip the stud. No matter how careful you are with the stud or your level of experience, you will eventually have a stud break off in an aluminum head.
The measure of a professional technician is not if they can avoid snapping off a stud, but how quickly they can extract and repair the threads. This typically involves tooling up for the task. There are kits available to help with the removal and repair of exhaust fasteners. These tools can be engine specific, like for the exhaust manifolds on a 4.6L Ford V8. These guides can align drill bits, extractors and thread repair tools.