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27 October 2023 What an inlet manifold is and its failure symptoms
What is an intake manifold?
The intake manifold, aka the inlet manifold, essentially acts as an air distributor, supplying the engine with the air needed for combustion. Without this oxygen, the motor wouldn’t be able to convert fuel into power.
The car engine intake manifold consists of a plenum, where the air is held before distribution, and a series of tubes known as runners. The runners carry the air from the plenum directly to each cylinder head. In fuel-injected systems, the fuel is injected into the combustion chamber right before the air goes into the cylinders’ intake port. The length and diameter of the tubes has a direct impact on the air flow rate and therefore on the engine’s maximum horsepower and RPM range.
The components are typically made from aluminium or cast iron, although composite plastic manifolds have gained popularity in recent years.
How does the air intake manifold work?
The intake air goes through an air filter, then through the throttle body and into the plenum. From the plenum, the air is distributed evenly between the runners. During the engine’s intake stroke, a partial vacuum is formed in the cylinder due to low pressure. This vacuum sucks the air from the runners (where the pressure is higher) into the cylinders.
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Some modern cars feature a variable length intake manifold, which combines different sized runners to generate optimal power across a wider RPM range. For example, longer, narrower tubes provide increased torque at lower RPM by optimising air flow velocity. Shorter, wider tubes are better for higher speeds as they reduce air flow restriction.
Symptoms of a bad intake manifold
Intake manifold leak symptoms
The most common problems that occur in intake manifolds are usually related to the gaskets and seals. Over time these gaskets are worn down due to exposure to extreme temperatures, high pressures and excessive vibrations. If the gaskets or seals have gone bad, there is a risk of a coolant or vacuum leak.
In the case of a car engine coolant fluid leak, you may detect a distinctive sweet coolant smell and steam or pools of coolant liquid underneath the vehicle. The loss of cooling fluid may also cause the engine to overheat and consequently damage its key parts.
In the event of vacuum leak, the air and pressure will drop, subsequently affecting the air-fuel mixture in the cylinders. This can severely impact engine performance, potentially causing misfiring, poor acceleration, a loss of power, rough idling and decreased fuel efficiency.
Sometimes leakages occur due to cracked hoses or a warped intake manifold. You may be able to detect a vacuum leak by listening out for a hissing sound coming from the engine bay.
Carbon buildup
You may experience similar symptoms to those mentioned above if the engine gets clogged up due to a buildup of carbon deposits. Blockages will result in a loss of engine power and reduced performance.
Some diesel engines use swirl flaps, which are fitted before the intake port. They are supposed to improve airflow at low speeds but are also vulnerable to buildups of deposits from the exhaust gas recirculation system. This can cause the flaps to stick.car engine coolant fluid
It may be necessary to remove the intake manifold and clean it out using a petrol or diesel engine cleaner. In many cases, a replacement may be required.
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Symptoms of a cracked intake manifold
Intake manifolds are subjected to extreme and constant pressure and temperature fluctuations that can cause them to warp or crack. As a result, you may experience a decrease in the engine’s performance with issues such as rough idling and misfiring. Moreover, when damaged, the component should trigger the Check Engine warning light on the dashboard.
While in some cases, it may be possible to fix the engine by just replacing the manifold gasket or seals, other repairs require acompletely new intake manifold. To reduce repair costs, you can purchase your own aftermarket parts online. When searching for compatible parts, simply enter relevant keywords, e.g. "MX-5 intake manifold", or search using the part’s OEM number or the VIN.
How much does it cost to replace the intake manifold? UK
The cost of repairing and replacing the intake manifold can vary significantly depending on the vehicle make and model, the car’s year of manufacture, and the specific parts that need replacing. For example, the price of an intake manifold gasket replacement typically ranges from £100 to £150 including labour charges.
If you have to install an entirely new unit, it could get a lot more expensive due to the labour rates. The inlet manifold itself can cost anything from £70 to over £600.
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