Signs of a Failing Exhaust Manifold

The exhaust manifold routes spent gases from the engine to the exhaust system. Engines with either a V-style or opposed cylinder (or “boxer”) layout have two exhaust manifolds: one on each side. Inline engines have only one exhaust manifold.

Most exhaust manifolds are made from either cast iron, heavy-duty sheet metal or stainless steel. Multiple passages (one for each cylinder) are built into the manifold and come together at its base.

Signs of a failing exhaust manifold

The manifold bolts to the engine cylinder head at one end and the exhaust pipe at the other. Gaskets are used to seal the manifold at the mating points. On some vehicles, other components — such as the exhaust gas recirculation valve and oxygen sensor — may be attached to the exhaust manifold, as well.

When exhaust manifolds fail, they typically either crack or warp, resulting in an exhaust leak. That can cause a number of problems, including the following:

• Noise: A leaking exhaust manifold can cause a tapping or rattling noise as exhaust gases escape. Often, the sound is most noticeable at startup, before the manifold heats up, expands and closes the crack.
• Illuminated check engine light: The oxygen sensor measures the amount of air in the exhaust stream. A leaking exhaust manifold can allow excess air into the system, fool the oxygen sensor into thinking the engine is running lean, and cause it to turn on the check engine light.
• Poor engine performance: Because a faulty exhaust manifold can throw off the oxygen sensor, the air-fuel mixture can get off balance, leading to poor engine performance.
• Increased fuel consumption and emissions: An oxygen sensor that is thrown off by a leaking manifold can affect fuel delivery. As a result, both fuel consumption and emissions may increase.
• Exhaust smell: There may also be a noticeable exhaust odor coming from the engine compartment.

How to fix the problem

A bad exhaust manifold should be fixed as soon as possible. Although rare, a failed manifold can cause additional damage to components such as the cylinder head and catalytic converter. Plus, noxious exhaust gases can enter the cabin.

Typically, exhaust manifolds are replaced, not repaired. While the job is never easy, on some vehicles it can be exceedingly difficult, and therefore, is best left to a professional.