Together, your intake and exhaust valves play a critical role in the operation of your engine if a valve fails, your engine will experience problems. Each cylinder has at least one pair. During the first stroke of the combustion process, the intake valve (IV) opens to allow fuel and air into the combustion chamber. When the chamber is full, the valve closes and completely seals the opening. A tight seal is important because it allows the air-fuel mixture to be compressed for ignition.
After the mixture has been ignited, vapors expand rapidly and push the piston to the bottom of the chamber. The cylinder’s exhaust valve (EV) opens to let the vapors escape.
For this reason, a failing valve needs to be replaced or repaired as soon as possible. The challenge is determining the root cause of the failure. Unless the reason is identified and resolved, a replacement IV or EV will meet the same fate (e.g. breakage, premature wearing, etc.). In this article, we’ll describe the most common reasons engine valves fail.
Surprisingly, a large portion of failures is due to defects in the valves. For example, metal impurities may be present, which reduces their durability and causes them to wear more quickly. There may also be blemishes – such as cracks – in the design. These will weaken the metal and make it more susceptible to breaking.
Many IVs and EVs are designed as two-piece components where the stems and heads must be welded together. If the welding is done poorly, they can separate.
Ideally, a mechanic will examine each valve that is being replaced prior to installing it. Defects, such as small cracks, are often easy to see with a quick inspection. By identifying such problems early, you can avoid having to bring your vehicle back when a defective valve fails.
Between the two valves, EVs are far more vulnerable to heat stress than IVs. The reason is because the vapors create from the ignition of the air-fuel mixture expand at extremely high temperatures. They rush past the EV continuously. Over time, the intense heat (up to 1,350 degrees Fahrenheit) causes thermal stress in the EV.
This is the reason exhaust valves are often designed with steel as opposed to a weaker material. The steel is more durable against the high temperatures. That said, the heat still takes a toll and can eventually cause failure.
Mechanical fatigue can cause the intake and exhaust valves to break. There are two main areas where breakage typically occurs. The first is between the head and the stem (where two-piece valves are welded together). The second is at the end of the stem near the keeper grooves.
Fatigue can occur for a couple of reasons. For example, high temperatures can cause the metal to stretch. This breaks down the material over time and makes it more susceptible to breaking. It can also happen when there is too much clearance between the valve and the camshaft. This can lead to stress as the valve closes with excessive force.
In some cases, the IV or EV can become bent due to the piston slamming against it. This can happen if the valve springs are failing, your engine is operating at extremely high RPMs, or if a lack of oil is causing the valve to stick.
The valves can also fail due to premature wearing in the stems. Factors that can contribute to this problem include a lack of oil and excessively high operating temperatures (for example, when your engine overheats). Bad seals can also lead to wearing because they regulate the amount of oil available to the guides.
Intake and exhaust valve failures are normally expensive to fix. The most effective way to prolong their life is to have engine-related problems resolved before they sustain stress, fatigue, and ultimately, damage.