Functions of the exhaust manifold in car engine and symptoms of failure

What does the exhaust manifold do

What goes in must come out – what happens to exhaust gases after combustion.

The combustion process is a wonderful thing. The process of taking nothing more than air and fuel and using compression or ignition to create mechanical power is one of the most important scientific discoveries of all time. What comes along with this power, however, are exhaust gases that need to be gotten rid of through the exhaust and this is precisely the job of the exhaust manifold. This makes it one of the most important parts of your engine, even if it just sits there with gas moving inside it.

This vital piece of the engine is responsible in part for keeping the engine cool and allowing more combustion to take place. Without it, all that science would go to waste. But what exactly is it? How hot do exhaust manifolds get? Why do exhaust manifold bolts break? Read on to find out all you need to know about this important piece of your car’s engine and how to find some of the most common problems.

What is the exhaust manifold and why is it important?

What is the exhaust manifold and why is it important

A car’s exhaust manifold is used to take the exhaust gases from the engine cylinders to the exhaust pipe beneath the car body. Once they leave the exhaust manifold, the gases go through the car’s emissions system and mufflers and out the tail-pipe.

The manifold is a metal piece bolted to the side of the cylinder block on L-head engines and to the side of the cylinder head on I-head engines. Two-, three- and four-cylinder engines have one exhaust manifold because there’s only one bank of cylinders to extract exhaust gases from.

However, engines with a ‘V’ arrangement (V6, V8 and V12) have two manifolds, one for each bank. In some V-8 engines, each manifold is connected to a separate exhaust pipe, muffler, and tailpipe. On others, they are connected by a crossover pipe and exhaust through a shared muffler and tailpipe.

The exhaust valve is pivotal for removing waste products of the combustion process from the engine. When the engine’s intake valve opens and the piston goes down, which means the air-fuel mixture is sucked in through the intake valve, the exhaust valve is also slightly open. Without a manifold, all of the combustion gases would rush out quickly making the exhaust valve the path of least resistance for airflow into the cylinder. Along with air and fuel from the intake, the engine would suck cold air in through the exhaust valve, exponentially increasing combustion chamber temperature and quickly melting the exhaust valve, valve seat, and piston top. This phenomenon is known as ‘reversion’ and is known to ruin engines. A manifold is key for preventing this.

The gases in the exhaust manifolds are very hot, which increases the pressure. This high pressure in the exhaust manifold forces the gas to “squirt” out through the collector and into the exhaust pipe. Because the exhaust gases have mass, they also have inertia, creating a vacuum as they leave the manifold, in what is known as “scavenging”. This sucks the remaining gases out of the engine and leaves it as well prepared as possible for the next combustion cycle, thus more efficient. Standard cast iron and “log” type manifolds typically exhibit little of this power-building scavenging; the effect is generally most pronounced in tubular headers, which are designed to enhance scavenging.

What are manifolds made from?

Typically, manifolds are made from tubular steel, stainless steel, or iron. Stainless steel is the most expensive because it doesn’t rust and has great longevity, but tubular steel gives good gas flow and is also often used.

You will find that most cars, though, have cast iron manifolds. They’re cheap to produce in comparison to others, but are heavier than steel and become brittle with age and prone to cracking, which we will come onto later.

While most manifolds are simply bare metal, in some cases a ceramic coating can be applied to the manifold for insulation. This is expensive and often ‘exhaust wrap’ is used instead, which is relatively cheap. This ‘exhaust wrap’, however, shortens the life of the manifold.

Since exhaust manifolds get very hot, most of them are fitted with a metal heat shield to protect the other components under the bonnet. This stops any unnecessary engine melt!

If you are looking to replace your manifold you can choose between those from your manufacturer, aftermarket alternatives, and even used manifolds salvaged from other cars. Just be sure to check out what it is made with and how old it is first.

Symptoms of exhaust manifold problems

Problems with your exhaust manifold can have some serious consequences, such as reduced engine power, slow warm-up times, higher fuel consumption, and premature catalytic converter failure. To avoid this, it is important you know the signs and symptoms that might let you know your manifold is cracked, leaking, or has another problem.

An excessively noisy engine

Engine noises are a good indication that you have a leaking exhaust manifold gasket. The manifold gasket creates a seal between the manifold and the cylinder head to stop air escaping and a faulty manifold gasket sounds like a hissing or tapping. When you start the car from cold the sound will be at its loudest and it will increase when you accelerate.

Reduced power and acceleration

If your manifold gasket is leaking you will notice that your car isn’t performing as it was or should be. The backpressure provided by the manifold makes sure that the combustion process happens as smoothly as it can. If the vacuum isn’t created, then the process won’t be happening as efficiently as it should. Your car will be more sluggish and won’t accelerate as fast from stationary. Get this leak mended or the problem will just get worse. It should be noted, though, that a manifold is not the only reason for reduced power and acceleration.

Reduced Fuel Efficiency

Fuel efficiency goes hand in hand with performance and as your car loses power, it is going to guzzle more gas. The car has to work harder and harder to keep the same level of performance it would have without exhaust problems. Although you may consider the cost of repairing any manifold problems, the cost of the extra fuel will outstrip this over time.

Rust visible on the manifold

Rust can appear on any metal parts, but especially those exposed to air (rust is caused by metal oxidizing). Since the manifold is metal, it can be prone to rust, especially if it is made of iron. The fact the system is close to the ground where it is exposed to moisture and gritty conditions means it is particularly vulnerable. If the rust is severe enough to cause holes or cracks to appear in the manifold and you will start to hear loud roaring engine noise or hissing as gas escapes. This will definitely need professional attention and potentially need the manifold to be replaced.

Visible Cracking

As well as possible noises and a drop in performance, the most obvious sign of a cracked manifold is, well, a visible crack on the manifold’s surface. To look for a crack, look closely at the manifold, particularly where it bolts up to the motor and where the most heat will be. A large crack will be relatively easy to spot, but a smaller, hairline fracture might be more difficult to locate. You may need to remove the manifold from the engine bay to inspect the entire surface. This type of problem will let you know when to replace an exhaust manifold.

Exhaust Odour

Symptoms of exhaust manifold problems

Exhaust odour is one of the most obvious exhaust manifold leak symptoms. Manifold cracks can also cause excessive exhaust odours since a portion of the exhaust gasses are evacuating out of the crack instead of out the end of the exhaust pipe. You might not be able to notice this exhaust smell from inside the vehicle, but if you open up the bonnet and the engine bay stinks, particularly around the manifold, this is a good indicator. If you can smell exhaust odour, this means the crack or leak must be quite serious and this can be harmful for your health.

How to replace your exhaust manifold and gasket

How to replace your exhaust manifold and gasket

For a cracked exhaust manifold, you don’t really have any other option than to replace it. Trying to seal cracks is a bad idea and using some sealants could even damage the performance of the manifold itself if not done properly.

Exhaust manifold gaskets can also be replaced in order to solve problems with leaking, but they can also be repaired. A ‘blown’ exhaust gasket can be replaced or you could use sealant to stop it from leaking and an exhaust manifold blowing is something you can easily repair yourself.

The exhaust manifold can be found attached to the engine block. Check your vehicle user’s manual to find the precise location of this in your vehicle. To replace the gasket or manifold, you will have to undo any nuts and bolts that attach the manifold to the cylinder head.Sometimes the gasket can stick to the manifold; tap it with a hammer to loosen it. If any manifold studs are broken or damaged, remove them using self-locking grips, two nuts and a spanner, or stud remover. With the manifold safely removed, carefully scrape all gasket-mounting surfaces clean of dirt and bits of the old gasket. Do not allow particles to fall into the manifold or the cylinder head as this will cause lots of problems later. This is a good opportunity to see if the manifold itself is cracked or damaged. Check that the surface is not warped.

If you are just changing the gasket, fit it in the position where the old gasket was, making sure that it is the right way round, with all holes lined up. On some engines a gasket may be in two or three pieces, or inserts may be fitted; be sure all parts are properly aligned. Finally, simply repeat these steps in reverse to reassemble your exhaust manifold. You should tighten the nuts on the manifold using a torque wrench and make sure that you adjust them to the setting recommended in the car service manual. You should tighten them in sequence starting from the centre of the manifold and working outwards towards the ends.

You might encounter problems here with the exhaust manifold bolts. It is important you know how to remove exhaust manifold bolts without breaking them and it can be a bit of a mystery as to why do exhaust manifold bolts break in the first place.

Because most manifolds are made from cast iron, as they are exposed to intense heat and then cool down they regularly expand and contract. This expansion happens naturally, and at first, the manifold and mounting bolts are in a state of “elastic deformation”, meaning they retain the original size and shape during these cycles and that they can be flexible, even as the expansion/contraction puts pressure on the bolts.

Over time, however, the expansion and contraction can affect the dimension of the manifold in subtle ways and this means that larger and larger tension forces stretch the manifold past its point of ‘elastic deformation’. This fractures the mounting bolts, leaving the manifold permanently deformed and dimensionally changed. This expansion and stretching of the manifold bolts over numerous duty cycles eventually causes too much tension on the bolt(s), stretching them beyond capacity and causing them to fail. If this has happened, when you remove the bolts to replace the manifold, it might look as though you have broken them off, but this fracturing has taken place long before.

If you need to remove an exhaust manifold and the bolts (or even the manifold itself) are rusty and corroded, you will want to remove the bolts without breaking them. If you break them while you are removing them, this could risk pieces going inside the engine itself and will require a lot more work. It is best to leave this to the experts, but it can also be done using an acetylene torch to cut open the shafts which hold the bolts. This will allow you to easily remove them, though will definitely mean you have to replace the manifold.

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