Gases will pass through between the valve and the valve seat and thus lose compression, and eventually power. Additionally, uneven wear can start to occur thus reducing the life span of both the valve and the valve seat itself. Eventually, that cylinder will have a significant reduction in power as well as increase heat which could result in a number of other issues for the engine down the road.
There are two main reasons why valves can get stuck to their seats: debris buildup and faulty or loose components. They can cause a great deal of problems and keep your vehicle from running properly, which is why it's important to detect early. Debris can build up by hardened oil, dirt, and gas. A solvent should be used to remove and clean both the valve and valve seats for a tight efficient seal. If faulty components are located, they should be either tightened or replaced completely. If not, crooked or broken valves can affect the spring, affecting the rocker arms, leading to a snowball effect throughout the engine.
Incorrect valve timing often results in bent valves. If the engine valves are bent they can result in lots of wasted unburnt fuel/air mixture, due to not opening and closing at the right times. In addition, the valves can also collide with the pistons and not only wreck the valves but the pistons as well which can be a costly repair.
Tight valves are less common as loose valves with a bad seal, but can be even more of a hassle. If the valves in your engine are too tight, you should be able to detect a profound clicking or tapping. If your valves are within 0.003 mm of the pistons, they will wear down twice as fast, leading to inefficiency or even worse, cracking and leaking. You may also lose some compression if the valves cannot function properly and fully seat in when closed. If you begin to detect sounds of over tightened valves, you should consult a mechanic to get them properly fitted as soon as possible.
When valves don't provide a complete seal, due to incorrect timing, being too tight, etc., the hot gases are forced past the valve which eat away or burn the edges of the circumference of the valve due to concentrated heat and pressure. Factors that play a part in this issue also include improper cylinder head cooling as well as carbon deposits on the valves due to poor fuel quality. Burnt valves can lead to a poor seal, thus leading to countless more problems such as cylinder power imbalance, intake flow issues with other cylinders, and damaged valve springs.
Just like any other mechanical component, valves need to be kept clean to prevent inefficiency or further problems. When valves are not kept clean, they are the source of many problems down the road such as sticky or burnt valves, as well as bending or cracking. The most common way valves get dirty is by carbon buildup caused by a rich fuel mixture or oil passing through a worn valve guide. If you see carbon deposits on your valves, check your valve guides for wear, make sure your ignition system is operating properly, and check for a rich fuel mixture. Carbon deposits can be easily cleaned with a water based solvent, but it is a simple step many of us overlook. Keeping clean valves will save you and your wallet from countless headaches down the road.
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