The intake manifold in a car is the part of the engine that distributes the air flow between the cylinders. 

Intake manifold.

Often an intake manifold holds the throttle valve (throttle body) and some other components. An intake manifold consists of the plenum and runners, see the photo. In some V6 and V8 engines, an intake manifold can be made of several separate sections or parts.

How the intake manifold works: The intake air flows through the air filter, intake boot (snorkel), then through the throttle body, into the intake manifold plenum, then through the runners and into the cylinders (see diagram).

The throttle valve (body) controls the engine rpm by adjusting the amount of air flow. 

Intake air flow.

In modern cars, the engine idle speed is also controlled by the throttle body: at idle, it opens at a very small angle. Because the throttle body is almost closed when the engine runs at idle, there is vacuum inside the intake manifold. If there is a vacuum leak somewhere in the manifold, the engine will run rough at idle. Many of the problems with intake manifolds are related to vacuum leaks, read more below.

The engine performance can be adjusted by varying the size of the intake plenum and the length or opening size of the runners. For this reason, modern cars have variable intake manifolds where special tuning valves change the air flow through the manifold depending on the engine speed and power demand.

Intake manifold problems

Common problems with intake manifolds include vacuum, coolant or oil leaks, reduced flow due to carbon build-up and issues with the intake tuning valves. In some engines, an intake manifold can corrode or crack causing either vacuum or coolant leaks. A cracked manifold must be replaced if it cannot be safely repaired.

Coolant leaks: In some cars, there are coolant passages inside the intake manifold that can leak, often because of bad gaskets or other damage. For example, this problem was fairly common in older GM V6 engines. If the manifold is not damaged and mating surfaces are in good shape, replacing the gaskets or re-sealing the manifold is usually enough to solve the problem. If the manifold is damaged, it must be replaced.

Intake manifold problems.

Vacuum leaks: Worn-out intake manifold gaskets (in the photo) often cause vacuum leaks. This can cause rough idle, stalling, as well as the Check Engine light coming on, although the engine may run fine at higher rpm. For example, the OBD-II trouble codes P0171 and P0174 are often caused by intake manifold vacuum leaks. If the leaks are caused by bad gaskets, the repair involves removing the intake manifold, checking and cleaning the mounting surfaces and replacing the gaskets. See, for example, these YouTube videos of this repair in a Ford V6 engine.

Often the source of the vacuum leak could be a cracked vacuum hose or line that connects to the intake manifold. In this case, a broken vacuum hose or line must be replaced. Sometimes an intake manifold can warp causing the gaskets not to seal properly. A warped intake manifold must be replaced. In some cars, a vacuum leak can be identified by a hissing sound coming from under the hood. Read more: Vacuum leaks: common sources, symptoms, repairs.
Carbon buildup: In some engines, for example, Volkswagen TDI Diesel, carbon build-up inside the intake manifold, can cause a lack of power, misfiring, smoke and poor fuel economy. Issues with carbon buildup are more common in turbocharged engines. One of the main symptoms is a lack of power. A clogged-up intake manifold might need to be removed and cleaned manually. In some cases, replacing the intake manifold might be a more sensible solution than cleaning it. There are many hidden areas inside the manifold that cannot be cleaned.

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.

WHAT IS AN INTAKE MANIFOLD?

In order to better understand the function and form of the intake manifold gasket, let’s first examine the job of the intake manifold. Found on any vehicle with an internal combustion engine, the intake manifold sits on top of the engine and plays a key role in the combustion process.

Designed to run on three timed components, air mixed fuel, spark, and combustion; the internal combustion engine relies upon the intake manifold to help it breathe. Featuring a series of tubes, the intake manifold ensures that the air coming into the engine is evenly distributed to all the cylinders. This air is used during the first stroke of the combustion process.

The intake manifold also helps cool down the cylinders to prevent the engine from overheating. Coolant runs through the manifold to the cylinder heads where the coolant absorbs the heat and reduces engine temperature.

On modern engines, an intake manifold, sometimes known as an inlet manifold, distributes air to the engine’s cylinders, and on many cars, also holds the fuel injectors, just above the intake port. On older cars without fuel injection or with throttle body injection, the manifold takes in the fuel-air mixture from the carburetor/throttle body, to the cylinder heads.

The air is admitted to the combustion chamber on the intake stroke and is mixed with fuel from the injector, after which the combustion cycle continues.

Air is supplied to the manifold from the air cleaner assembly, which contains an air filter.

It’s vital that the filter is changed on a regular basis because it stops dust and other foreign bodies from entering and damaging the engine.

Inlet manifolds are made from aluminium or cast iron, although some cars use plastic manifolds.

Sometimes intake manifolds split and develop a leak. If you lift the bonnet and listen to the engine when idling you’ll be able to hear a whistling or hissing noise, and the engine itself may idle roughly or stall when idling.

If the split is small it can normally be repaired, but replacement is usually advised.

Manifolds on modern engines are easily replaced with new parts, but parts for older models out of production will be harder to come by.

Source these from scrapyards and junkyards, from wrecked cars, via eBay or specialist car parts collectors.

Sometimes replacement inlet manifolds simply don’t exist and you’ll have to either carry out an extensive repair on the old one or you’ll have to source a specialist who can make one for you. However, this will be expensive.

Problems with inlet manifolds are relatively rare, but some diesel car models’ manifolds incorporate what are known as swirl flaps, fitted before the intake ports.

They are designed to improve airflow at lower engine speeds but the flaps can become fouled by the exhaust gas recirculation process, and stick, or may sheer off altogether and be ingested into one or more engine cylinders, causing catastrophic engine damage.

Intake Manifold Leak Symptoms

Intake manifold leaks aren’t extremely common, but they do happen. You would think that the result of a leak in the intake manifold would be air escaping and less air making its way to your car’s cylinders. Actually, it’s precisely the opposite that happens. Because the air pressure inside the manifold is lower than that in the ambient air surrounding the engine, the manifold will actually suck additional air through the leak. This will put too much air into the cylinders and decrease the amount of gasoline that can be squeezed in alongside it, which will make for less efficient combustion. Remember that every time one of those small explosions takes place inside one of your car’s cylinders, it turns the crankshaft. So if there’s too much air and not enough gasoline for the combustion process, the explosions will become weaker and your engine will have to work harder to turn the crankshaft. So if you notice that your car is responding more sluggishly every time you press down on the accelerator, a leak in the intake manifold could be the culprit.

But there are plenty of other possible causes for sluggish acceleration in a car, too. So how do you know if your car’s reluctance to speed up when you tell it to is caused by a leaky intake manifold? One way is to simply listen to your engine. Your car may be trying to tell you that it has a problem, so pause for a moment and try to understand what it’s saying to you. In fact, you’ll literally need to pause, because you can usually only hear the problem while the engine is idling. What you’ll hear has been variously described as a hissing, whistling, sucking, gulping or even slurping noise. The car may also feel rough while idling and the engine may even stall completely at slow speeds. Or, when you turn off the car’s ignition, it may keep on running for a while longer than it should. All of these can be signs of an intake manifold leak. Some experts even suggest spraying small amounts of starter fluid on the seals of the intake manifold while the engine is idling. If the engine reacts to this in any way — for instance, by speeding up briefly — then the fluid is slipping in through the leaks. All of these signs are a warning that you should be paying a visit to your local auto mechanic for a definitive leak check.

There’s a second way in which intake manifolds can leak. In some models of car, the intake manifold has a double use as a conduit for coolant fluid. If the leak is in a coolant seal, you may start noticing loss of coolant and distinct puddles of coolant beneath the car after it’s been sitting in one place for a few minutes. Once again, this is a sign that you should get your car looked at by someone at your favorite auto shop.

 How does an exhaust manifold work?

Answer: The exhaust manifold is the first part of your vehicle’s exhaust system. It is connected to your vehicle’s engine and collects your engine’s emissions. The exhaust manifold receives the air/fuel mixture from the multiple cylinders in your vehicle’s engine. It collects the fuel/air mixture from each cylinder, whether you have four, six, or eight cylinders. Not only does the exhaust manifold receive all of the burnt engine gases, but also it completely burns any unused or incomplete burnt gases using its very high temperature. The manifold also houses the first oxygen sensor in your exhaust system to inspect the amount of oxygen entering the system. The oxygen sensor monitors the amount of oxygen and will tell the fuel injection system to increase or decrease the amount of oxygen used in the fuel/air mixture used to power the engine. Basically, the exhaust manifold acts as a funnel and is used to collect all of the engine’s emissions (from however many cylinders your vehicle has). Then once they are in one place and completely burnt, the manifold sends the emissions into the rest of the exhaust system.