What Is Exhaust Manifold and How does It Work?

What is an Exhaust Manifold?

In automotive engineering, an exhaust manifold collects the exhaust gases from multiple cylinders into one pipe.

The exhaust manifold is exposed to the extremes – it’s heating and cooling, which causes constant expansion and contraction. Manifolds can crack over time due to the stress from constant, extreme temperature changes.

Exhaust manifolds are generally simple cast iron or stainless steel units which collect engine exhaust gas from multiple cylinders and deliver it to the exhaust pipe. For many engines, there are aftermarket tubular exhaust manifolds known as headers in American English, extractor manifolds in British and Australian English, and simply as “tubular manifolds” in British English.

These consist of individual exhaust head pipes for each cylinder, which then usually converge into one tube called a collector. Headers that do not have collectors are called zoomie headers.

The most common types of aftermarket headers are made of mild steel or stainless steel tubing for the primary tubes along with flat flanges and possibly a larger diameter collector made of a similar material as the primaries.

They may be coated with a ceramic-type finish (sometimes both inside and outside), or painted with a heat-resistant finish, or bare. Chrome-plated headers are available but these tend to be blue after use. Polished stainless steel will also color (usually a yellow tint), but less than chrome in most cases.

Another form of modification used is to insulate a standard or aftermarket manifold. This decreases the amount of heat given off into the engine bay, therefore reducing the intake manifold temperature. There are a few types of thermal insulation but three are particularly common:

  • Ceramic paint is sprayed or brushed onto the manifold and then cured in an oven. These are usually thin, so have little insulatory properties; however, they reduce engine bay heating by lessening the heat output via radiation.
  • A ceramic mixture is bonded to the manifold via thermal spraying to give a tough ceramic coating with very good thermal insulation. This is often used on performance production cars and track-only racers.
  • Exhaust wrap is wrapped completely around the manifold. Although this is cheap and fairly simple, it can lead to premature degradation of the manifold.

The goal of performance exhaust headers is mainly to decrease flow resistance (back pressure), and to increase the volumetric efficiency of an engine, resulting in a gain in power output. The processes occurring can be explained by the gas laws, specifically the ideal gas law and the combined gas law.

Exhaust Manifold

Why The Exhaust Manifold Gasket Matters?

The exhaust manifold gasket, as with any gasket, acts as a sealing surface sandwiched between two different metal components. The gasket is made of layers of metal or composite materials and prevents leaks as the two metal surfaces go through countless heating/cooling expansion and contraction cycles over an engine’s operational life.

During the exhaust stroke, the exhaust valve opens to allow spent gases to exit the combustion chamber through the exhaust port in the cylinder head. The gasket is an airtight seal against the head, so the gasses pass into the exhaust manifold and onto the rest of the exhaust system.

The gasket prevents noxious gases and incredibly high temperatures from escaping the engine at the wrong point.

Signs Of Exhaust Manifold Gasket Failure

Generally, exhaust manifold gaskets are hardy items, often lasting well over 100,000 miles. A lack of engine maintenance and carbon buildup, however, can cause hot spots in the combustion chambers and exhaust ports. This can cause a gasket to catch fire or expand outward. This is a good reminder to use fuel system cleaners to remove carbon buildup.

A failed gasket also allows fresh air into the exhaust system at a point it wasn’t designed for, which could lead to burned exhaust valves. This is an expensive repair and can lead to more damage, so it’s best to replace a bad exhaust gasket as soon as possible.

When exhaust manifold gaskets fail like this, the symptoms are rather obvious. Here’s what to look for.

  • Increased engine noise. Since it isn’t passing downstream to the mufflers, the engine noise isn’t muffled. This makes a Civic sound like a cammed big block.
  • Smells or visible exhaust emissions. A leak lets blasts of untreated exhaust into the engine bay. You can sometimes see this as smoke vapors. It’s also incredibly hot and can melt surrounding engine bay plastics.
  • Loss of performance. The exhaust and emissions systems are part of a fine-tuned engine on a modern vehicle. With a sudden lack of backpressure and exhaust sensor readings out of norm, the engine’s performance suffers.
  • Gas mileage hit. Just like the horsepower, the MPGs take a hit, too. When the ECM can’t adapt enough to compensate for the failure, the engine uses too much gas.

How does the exhaust system work?

The exhaust system is responsible for moving the gases coming out of the combustion system away from the car. Moreover, it muffles the noises associated with high speeds and explosions in the combustion system. Finally, it cools down the toxic gases and converts them into less harmful gases before exiting the car.

With all this onerous responsibility, the exhaust system consists of specific parts to help it do the job correctly. These parts include manifolds, heat shields, joints, pipes, mufflers, and flexible unions. The exhaust system experiences the roughest conditions as compared to the other parts of the vehicle. It must cool temperature from as high as 1,200 degrees to the ambient temperature.

During cold weather, water might come as a byproduct of the combustions system. This water can cause the exhaust system to rust over time.  Moreover, the exhaust system is located towards the bottom of the vehicle, which forces it to face all the different road conditions (e.g., snow, ice, dirt, salt, etc.)

That been said, it is not surprising that you might need to do regular repairs and maintenance to the exhaust system more often than other parts of the vehicle.

Exhaust manifold leak symptoms

Knowing that you have an exhaust manifold leak is easy to catch; however, it is more challenging to determine where the leak is coming from exactly.

There are general symptoms that can tell you there is a leak in your exhaust system, including:

  • Strange noises in your engine
  • Poor fuel economy
  • Engine bay smells like burn
  • A reduction in the acceleration power

If you experienced any of the previous symptoms, you might need to confirm the leaks using visual inspection.

Before doing any visual inspection, you need to keep in mind that the exhaust system can be boiling, especially if the car was running. Therefore, before doing any investigation, you need to make sure that the vehicle has cooled down for at least a couple of hours. It is a good idea to wear safety gloves when doing the inspection.

Once your vehicle cooled down, open the hood and find the manifold. It might be a little tricky to find the exhaust manifold as it could be hidden under heat shields. The manifold is usually close to the exhaust pipe (one of the multiple rusty pipes).

Follow the exhaust pipes and check for leaks or cracks, as you follow. Make sure to check all the exhaust system components (e.g., catalytic converters, muffles, resonators, etc.) check for leaks from the bottom of the top using a mechanic mirror.

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
  • 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?

In the automobile field, and maintenance cost is divided into parts costs and labor costs, which is measured by the number of hours needed. The labor cost differs significantly by the location of the repair (e.g., dealership versus repair shop).

In general, fixing the exhaust manifold leak requires two to three hours of repair. The average labor cost per hour ranges from $80 to $90, which means that fixing the manifold leak will cost from $160 to $270 on labor only. However, this number can go up to $330 if you want to repair your manifold leak in a dealership where the labor cost per hour is about $110.

On the other hand, you can eliminate the entire labor cost if you fix your manifold leak on your own.

The parts cost depends on the vehicle’s make, model, year, and the number of cylinders in the motor (e.g., four or six or eight). For example, the gasket cost for a Chevrolet Impala ranges from $18 to $28 if you buy it from a local auto repair shop. However, the gasket for the same Chevrolet Impala can cost $50 if you buy it from a dealership.

The bottom line, the cost of fixing your exhaust manifold leak could range from $18 if you set it on your own to $400 if you fixed it at a dealership.