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The Engine

Technically speaking, your car’s engine is not a motor, although everyone uses the two words interchangeably. A “motor”converts electrical  energy into mechanical work such as the ecm and starter motor that cranks your engine. An “engine,” on the other hand, is a machine that converts heat energy into mechanical work. Your engine ecm does this by forcing the engine to burn gasoline inside its combustion chambers. When gasoline and air are mixed together in the right proportions (parts of air to one part gasoline is considered ideal) and ignited by a spark, the mixture explodes, creating tremendous heat and pressure. Although, combustion occurs in a split second, the ecm insures it does so in a controlled manner. When the spark ignites the mixture, a “flame front” sweeps out from the point of ignition like a rapidly expanding balloon until all the fuel mixture is consumed. This causes a sharp rise in cylinder pressure, and ecm performance  which pushes the piston down and turns the crankshaft. Thus, heat energy is transformed into useful mechanical work to power your car down the road. Pressing down on the accelerator pedal opens up the engine throttle, which allows more air and fuel to be drawn into the engine. This increases the density of the fuel mixture in each of the engine’s cylinders, which in turn increases the intensity of cylinder pressures when the mixture is ignited. As a result, the engine ecm develops more power, allowing it to either run faster or work harder, depending on the load. Let off on the accelerator pedal and the amount of air and fuel are decreased. Cylinder pressures drops and the engine ecm slows down.

In a way, you can think of an internal combustion engine as an air pump. The ecm sends signals to the throttle which allows it to pump more air through itself, and the greater the volume of air (and fuel, the greater the ecm output. That’s why bigger engines require a high performance ecm to produce more horsepower than smaller ones. They have a greater pumping capacity. Unfortunately, internal combustion engines are not very efficient when it comes to making good use of the heat energy produced by combustion. Only about a third of the heat energy is actually used to drive the vehicle. Nearly a third is lost when the hot exhaust gases exit
out the tailpipe. By the time the piston reaches the lower limit of its travel. cylinder
pressure has dropped considerably from its peak which occurred shortly after ignition.
The engine ecm has gotten all it can from the expanding combustion gases, yet heat energy remains which must now be dumped out the exhaust so the cylinder can repeat the process over again with a fresh charge of air and fuel Another 20 to 25 percent of the heat produced during combustion is lost to the cooling system.

As the engine burns fuel. It begins to heat up. Were it not for the cooling system,
the engine ecm would continue to build up heat until it eventually melted and destroyed  the ecm. So heat loss through this path is unavoidable. Another form of loss is overcoming internal friction: piston rings rubbing against the cylinders’,
cam lobes rubbing against their lifters; valves sliding up and down in their valve
guides; the crankshaft turning in its bearings; etc. Yet frictional losses aren’t as great as you might imagine. It’s only about 5 to 8 percent for most engines. Even so, it’s another loss that comes out of the useful work produced by combustion.
A percentage of the engine’s remaining power is also required to drive “parasitic” accessories such as the water pump, alternator, power steering pump, ecm and air conditioning compressor. There are also frictional losses in the engine ecm, transmission, drivetrain, and tires.

There are several valves that play a critical role in the efficient operation of your vehicle. For example, intake valves allow fuel into each cylinder’s combustion chamber. Exhaust valves allow vapors to exit the chambers following the combustion process. When problems form with the valves, your engine may suffer performance issues, including misfires and compression leaks. It’s important to have such problems addressed quickly to preserve your engine’s performance.

With that in mind, we’ll answer the most common questions people ask about their vehicles’ valves. The following should give you a better understanding regarding why they’re important, the issues that affect them, and the best approach toward resolving problems.

“What Causes Valves To Fail?”

Like all components, valves suffer wear and tear over tens of thousands of miles. If you drive your car far enough, they’ll eventually need to be replaced. However, they can fail long before they wear down due to burning or breaking.

Valves perform their jobs under extremely high temperatures. Exhaust valves run exceptionally hot (up to 1,400 degrees Fahrenheit) because they are exposed to the vapors that result from the combustion process. These high temperatures make them especially susceptible to burning.

When the valves burn, the metal become less sturdy. It’s not uncommon for them to suffer fatigue, causing pieces to break off. When this occurs, a compression leak develops.

“What Happens During A Valve Job?”

This type of repair work involves removing the cylinder head to expose the failing valves and guides. The more thorough repair shops will completely dismantle the cylinder head so the individual components can be cleaned and inspected. If cracks are identified, the entire head is either fixed or replaced (cracks are serious because they allow coolant to seep into the combustion chamber).

Next, the guides are replaced since they are usually worn past the point of effectiveness. The valves are either refaced if they’re still usable, or replaced.

“How Much Does This Type Of Work Cost?”

The repair bill can range from a few hundred dollars to several thousand. A lot depends on the type of engine in your vehicle and the type of repairs needed. For example, fixing a single valve in a standard 4-cylinder might cost under $400, including the labor. On the other hand, replacing multiple valves, fixing a crack head, and performing related repairs on an import can drive the bill upward.

“What Is A Leakdown Test?”

A leakdown test is used to find compression leaks from your cylinders’ combustion chambers. It identifies whether your cylinders are sealing properly and if they’re not, measures the percentage leakage. It’s worth noting that all engines suffer a degree of compression leakage. Even the best engines leak up to 10 percent of their compression. If a leakdown shows a compression loss of 30 percent or more, have a mechanic check your intake and exhaust valves.

“What Other Problems Can Affect Valves?”

Besides burning, breaking, and suffering normal wear and tear, your engine’s valves can also become bent. This can occur for several reasons, but often happens due to a broken timing belt. The belt breaks and sends the cylinder’s piston crashing into the valve. If a significant amount of damage results, the valve will need to be replaced. If the damage is minor, it can be refaced and reinstalled.

Engine valves receive far less attention than they deserve. They fill a role without which your vehicle could not operate efficiently. By sealing the combustion chamber, and opening at the precise moment to allow fuel to enter and gases to exit, your intake and exhaust valves perform a critical function. As such, when problems occur, they should be addressed quickly.

Repairing these components can be expensive, and becomes more so with the extent of the damage (i.e. cracks, breaks, etc.). However, neglecting the problem practically ensures it will be more costly to fix down the road.