What Is Exhaust Manifold and How Does It Work?

The exhaust manifold is bolted directly to the engine block and is the first section of a vehicle’s exhaust system. It collects exhaust gases from all the cylinders and directs them to the car’s catalytic converter. V-type engines have a separate manifold for each cylinder bank.

A leak in the exhaust manifold or its gasket can allow exhaust gases to escape, which poses a health hazard to the car’s occupants and can result in erroneous readings by the oxygen sensor, triggering a check engine light. Larger holes in a manifold will produce loud exhaust noise.

What is an Exhaust Manifold?

The car exhaust manifold is an integral component of the engine system that plays a vital role in ensuring the efficient functioning of the engine. Its main function is to collect exhaust gasses after combustion in the engine cylinders and direct them to the exhaust pipe. The system consists of pipes and tubes linking the internal combustion chamber with the exhaust system.

Since your exhaust system has a huge task at hand, there are a lot of different parts, including the manifolds, catalytic converter, heat shields, joints, tailpipe, flexible unions, mufflers, and more.

Many often refer to the exhaust manifold as just the manifold. However, we must inform you that a typical automotive system consists of the intake and exhaust manifold.

Intake and Exhaust Manifold

While the intake manifold is responsible for distributing fuel-air mixture into the internal cylinders for combustion, the exhaust manifold takes the by-product of the combustion – the gasses away. Therefore, it is important you do not mistake each component.

Besides acting as a channel and passage for exhaust gasses from the combustion chamber into the exhaust tubes, the exhaust manifold also helps improve engine output fuel efficiency and enhance overall car performance.

What is an Exhaust Manifold

Function Of Exhaust Manifolds

As the first component in the exhaust system, the exhaust manifold collects the exhaust fumes released from the engine cylinders and directs them to the catalytic converter. In turbocharged engines, the turbocharger is situated downstream of the exhaust manifold.

The design of the exhaust manifold has a significant influence on the performance and torque characteristics of an engine, as does the design of the connected pipes.

The fact that the individual exhaust tracts are merged into one ensures that pulsations, i.e. fluctuating pressure surges given off by a certain cylinder, do not prevent exhaust fumes being released from another cylinder. What’s more, waves of negative pressure are intended to promote the flow of exhaust fumes out of the cylinders.

Exhaust manifolds are often made of alloyed cast iron, which is able to withstand the high exhaust temperatures. Alternatively, exhaust manifolds made of stainless steel are also used.

How Does an Exhaust Manifold Work?

The exhaust manifold is designed to take away the exhaust gases from the individual cylinders. And that’s exactly why it’s curved. The hot exhaust gases from the cylinders flow through the turbo manifold and onwards to the turbocharger or a pipe, where they’re collected.

At this point, they’re cleaned and taken to the exhaust system. Exhaust gases from the engine usually enter the manifold horizontally and are ducted downwards from there.

In tuning, the goal is to streamline the manifold as much as possible so that it discharges the exhaust gases quickly and efficiently, preventing them from flowing back into the cylinders.

Manifold designs differ according to the ignition sequence because the exhaust gases flow into the manifold so fast that they create a vacuum that sucks them in and almost out of the engine. This applies particularly to sophisticated header designs.

Pipe length and diameter, pipe joints, and welds are key criteria in high-performance exhaust manifolds. These factors can have a big influence on the gas flow. For example, weld roots inside the pipes can cause turbulence. So the manifold interior has to be as smooth as possible to guarantee the optimum flow ratio.

Bad Exhaust Manifold Symptoms

If there is a leak in your exhaust manifold, you will certainly notice some of the signs. Check out our list of the most common symptoms of a leaking exhaust manifold and compare them to the symptoms you have seen in your own vehicle.

#1. Abnormal Noises.

If your engine is louder than normal, it could be a sign that your exhaust manifold has a leak. When there is a leak, un-muffled gas could escape from the exhaust system, causing the engine to sound louder or noisier than usual.

#2. Poor Fuel Economy.

A leak in the exhaust manifold can reduce your vehicle’s fuel efficiency. If you notice you need to take more trips to the gas station than you used to, you might need to have your exhaust manifold checked out by a professional.

#3. Strange Smells.

If there is a leak or crack in your exhaust manifold, the gases could be escaping into the outside air before it has been transformed into a less toxic gas. As a result, you might notice a strange smell coming from under your hood.

#4. Lack Of Acceleration Power.

Leaks can also cause a lack of engine power. If your car isn’t accelerating as it used to, don’t ignore it! This symptom can be mild at first, but if you put off your exhaust manifold replacement service for too long, it could worsen.

#5. Check Engine Light Turns On.

Sometimes a leak in your exhaust system will cause the sensors in your car to think there is a problem with your engine. If this happens, your check engine light will turn on.

If you notice your car is showing these symptoms and you think your exhaust manifold is leaking, book an appointment with a qualified professional today!

What does the exhaust manifold leak sound like?

It is a good idea to check for a manifold leak by listening to strange specific sounds.

The exhaust manifold leak sounds like tricking or puffing, and the sound gets louder when you get closer to the catalytic converter or the engine. Start the engine and have a friend rev in the motor and see if the sound gets louder or changes. Monitor the noises in the exhaust pipes.

Remember, the exhaust system can be boiling, therefore, never touch the pipes with your hands. Make sure to wear safety gloves.

Consequences of exhaust manifold leaks

As we mentioned before, if there is a leak in the exhaust manifold, the toxic gases will go back inside the car instead of leaving it.

Depending on the location and severity of the leak, the reversed behavior of the gases flow can cause many issues, including:

  • Disturbing the function of the oxygen sensor
  • Affecting the fuel trim
  • Causing improper functioning of the EGR valve
  • This might result in catalytic converter failure
  • Damaging the exhaust valves

What causes exhaust manifold leaks?

There are three leading causes for a manifold leak:

#1. A leak in the manifold gasket.

As we mentioned before, the gas flowing in the exhaust system can be extremely high. Then when the car cools down, the exhaust system’s temperature drops dramatically. This cycle of heating and cooling can cause all parts of the exhaust system to expand and shrink in a sequence.  The expansion and contractions can cause cracks and breaks in the gasket.

The manifold gasket is responsible for sealing the entire exhaust system. If the gasket got damaged or cracked, the gases are going to leak back into the car, which affects the car’s overall performance and causes strange noises in the engine.

#2. A crack in the manifold itself.

Like what might happen to the manifold gasket, the manifold itself might crack due to the heating and cooling cycles of the exhaust system. A small crack in the manifold might not be noticeable. It can cause strange sounds right when you start the engine.

These sounds are reduced by the time you drive as the metal parts expand with heat and close the cracks. However, if the cracks get bigger and bigger, the metal expansion cannot close the cracks and significant engine failures are likely.

#3. Broken or loose manifold bolts.

The manifold is connected to the exhaust system by two large bolts. These bolts might get loose or break due to the continuous heating and cooling or due to the wear and tear over time.

Loose or broken manifold bolts can cause holes and therefore leaks in the manifold, causing the toxic gases to reverse their flow direction.

How much does it cost to fix the exhaust manifold leak?

If you choose to have a professional replace your car’s exhaust manifold, you can usually expect to pay somewhere between $500 and $1500 to get the job done. Of course, the exact cost will depend on various factors, such as the year, make, and model of your vehicle. 

You can save money by replacing the exhaust manifold yourself if you have the tools and the know-how.