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.