Valves in the car engine allow fresh gas to flow into the cylinders or guide exhaust gases to the outside. At the same time, the valve reliably seals the combustion chamber when required. Find out everything you need to know about valves, how they work and what to do if a valve is defective.

Function of the valves in the engine

The valves in the combustion engine enable a smooth gas exchange. Each cylinder has at least one intake valve through which air or a gas-air mixture flows into the combustion chamber as fresh gas. The residues produced during combustion leave the engine as exhaust gases via at least one exhaust valve. Depending on the engine, the valves are controlled via one or more camshafts.

Valves in the four-stroke engine

Intake and exhaust valves are essential for the operation of a four-stroke engine. In the first stroke, the intake stroke, the intake valve is open and the exhaust valve is closed. The fresh gas flowing in via ducts is finally compressed and ignited by the piston in the second stroke. In the third stroke, the power stroke, the gases burn in the cylinder and the force released acts on the piston. In the fourth stroke, the so-called exhaust stroke, the exhaust gases are expelled from the cylinder. This takes place via the exhaust valve, which is now open. Depending on the engine, the intake valve opens at the end of this cycle, even before the exhaust valve is closed.

The valve control

Reliable valve control is necessary to ensure that the valves open and close at the right time. The valves are controlled by the engine itself via the valve train. For this purpose, the crankshaft is connected to the camshaft via a toothed belt or timing chain and drives it. The cams on the shaft usually act on the valve stem ends via bucket tappets and open the valve. The valves are closed by valve springs.

Structure of the valves

The valves in the engine are made of special alloys due to the loads they are subjected to. In modern four-stroke engines, poppet valves are almost always used. These valves have a valve head that fits into the valve seat or valve seat ring of the cylinder head. The valve stem is fitted with a valve stem seal that seals the combustion chamber to the cylinder head. At the same time, the seal allows the valve guides to be lubricated.

Valve defects and diagnosis

The valves are exposed to high loads during engine operation. At high engine speeds, the intake and exhaust valves open and close many times per second. The temperatures and aggressive combustion residues that occur also cause constant wear on the components. If the valves are defective, reliable gas exchange is no longer possible. Reliable diagnosis of defective valves is not always easy. Professional workshops have equipment that can be used to detect valve defects. For example, an endoscopy with a video endoscope can quickly clarify the condition of the valves and combustion chamber. It is also possible to measure the compression of the engine.

Symptoms of a defective valve

If one or more valves are damaged, the combustion chamber is no longer reliably sealed. As a result, gases can escape uncontrollably and cause the engine to malfunction. Defective valves often lead to symptoms such as uneven engine running or reduced engine performance. The compression of a cylinder is often greatly reduced.

This is why the valves are defective

A valve defect can have many causes. In addition to typical wear, inadequate lubrication of the engine leads to increased friction and thus to the defect. Impermissibly high engine speeds or combustion faults can also damage the valve. Last but not least, broken timing chains or timing belts often cause bent valves.

Repairing and replacing defective valves

Defective and worn valves should be replaced to ensure that the engine functions properly. However, replacing valves is not straightforward and often requires special tools. Only expert mechanics can carry out the repair themselves. The procedure differs depending on the engine type.

1. Removing the valve cover: To gain access to the camshaft and the valves, the valve cover must be removed. It protects the components and prevents oil from escaping.
2. Remove the cylinder head: To replace the valves, it is usually necessary to remove the head from the engine block. The component is then stored in a stable position on a workbench.
3. Removing the camshaft: In modern engines, it is often necessary to remove the camshaft in order to access the valves. The same applies to camshaft bearings and hydraulic tappets.
4. Removing the valve springs: Before removing the valves, the valve springs and their spring retainers must first be removed. The parts are best removed using special tools.
5. Dismantling the valves: Once the remaining components have been loosened, the valves can be easily removed. It is advisable to arrange or mark the valves according to position and cylinder.
6. Check the valve: After removal, the valves must be checked individually and with particular care. A bent valve stem or a broken valve plate is often noticed.
7. Clean the cylinder head: After removing all components, it is a good idea to clean the surfaces of the cylinder head. The same applies to any remaining valves that are reinstalled.
8. Install valves: After cleaning and checking or procuring new ones, the valves are installed in the head. It is necessary to grind each valve into the valve seat beforehand.
9. Assemble the cylinder head: The remaining components can be reinstalled or reassembled after thorough inspection. Valve stem seals should be replaced.

The valve is not always defective

If symptoms indicate a valve defect, it is not always the valves themselves that are faulty. Sometimes a fault in the valve train or other defective components are the cause of the engine not working properly. When repairing the valves, the valve spring and valve seat inserts should be checked in addition to the valve itself. It is also advisable to inspect the cams of the camshaft and check for wear.

Buying spare parts and valve accessories

Depending on the engine, the number of valves installed varies, as does their design. For example, the valve stem diameter or the length of the valves can differ depending on the engine type. The same applies to the valve guide and valve actuation parts. It is often advisable to buy new components such as individual valves to be on the safe side. Sometimes, however, it can be worth buying a complete cylinder head as a used original part. The used part usually includes all components such as valves and camshaft.