What are intake and exhaust valves?
Intake and exhaust valves are the final component in the valve train system. They are made of hardened metal that must withstand the extreme conditions of the combustion chamber. They are shaped to have a slim stem leading to a flatter face, with the intake valves often being larger than the exhaust. In many modern engines, each cylinder has two intake and two exhaust valves per cylinder.
Why are intake and exhaust valves important?
There are four events that must take place in an internal combustion engine to ensure operation. These are called strokes. The first stroke is the intake stroke. During the intake stroke, an air/fuel mixture is drawn into the combustion chamber by opening of the intake valves (with direct injection engines, the fuel is injected after the air has been drawn in). The next stroke is the compression stroke. Both the intake and exhaust valves close, trapping the air/fuel mixture in the combustion chamber. The piston is now forced upward compressing the air/fuel mixture causing it to become highly combustible. The next stroke is the power stroke. The compressed air/fuel mixture is ignited by a spark plug. With the exhaust and intake valves remaining closed, the ignited mixture rapidly expands, forcing the piston back downward in the cylinder. This in turn spins the crankshaft, which then through the power train eventually turns the wheels of the vehicle. The final stroke is the exhaust stroke. During the exhaust stroke, the piston will begin to head upward again, pushing the expended mixture through the now open exhaust valve. After the exhaust gases have escaped the cylinder, the exhaust valve will close and the intake stroke will occur again. This entire cycle occurs about 1,250 times per minute when driving at freeway speeds!
What can go wrong?
Although valves can fail for a number of reasons, the two most common types of failures are bent valves and burnt valves.
Bent valves occur when the piston makes contact with the valves. This causes the valve to be no longer able to seal the cylinder correctly. Many engines are interference engines, meaning the piston travels upward beyond the lowest point that the valves travel downward during normal operation. The engine is timed by a timing belt or chain via the crankshaft, which drives the pistons, and the camshaft, which drives the valves. Impact between the two is caused when this timing is disrupted by a form of timing failure.
Burnt valves occur when the valve can no longer seal correctly after excessive temperatures have damaged the material of the valve. The typical causes of burnt valves are allowing the vehicle to overheat or incorrect fuel being used.
Signs that work is required.
The main symptom with damaged valves is a rough engine operation, especially at idle.
What needs to be done when damaged valves are suspected?
The first step of diagnosis when damaged valves are suspected would be a leak down test. During this test, the engine is rotated until top dead center of the currently tested cylinder is achieved. The intake and exhaust valves will be closed at this time. Compressed air is injected into the cylinder through the spark plug hole. The ability of the cylinder to hold pressure is then measured using a leak down gauge. If significant leakage is detected coming from either the car’s throttle body or exhaust, the valves are leaking. In some cases, the valves can to be inspected using a borescope inserted into the cylinder. Removal of the valve cover to inspect the valve train may also be required. If all else fails, the cylinder head may need to be removed to check the valves.
How much do repairs cost?
Valve replacement is a very large job requiring engine tear down. Cost for valve repairs can reach above $5,000 in labor alone. This repair can take 35+ hours to complete, after diagnosis.
Because the engine is broken down during this repair, many gaskets will be replaced and many other parts will often be removed. If you are having this repair done, it may be a good time to have other needed repairs completed as overlapping labor discounts would likely be applied.