Automotive Engine Valves – What Are They – How Can They Fail

Automotive Engine Valves
Automotive Engine Valves

The main function of automotive engine valves is to let air in and out of the cylinders.

The automotive engine valve that lets the air into the cylinder is the intake valve.

The automotive engine valve that lets the gases escape are the exhaust valves.

Automotive engine valves are designed to open and close at precise moments.

The overhead valve system (OHV) system, operated by pushrods.

An overhead-cam (OHC) engine needs fewer parts to operate the valves. The cams act directly on bucket tappets.

This is why the automotive engine valve plays a pretty critical role in an engines performance.

The more air you can move air in and out of the engine the more efficient it will be.

Intake valves handle cool, low pressure, low density charges whereas exhaust valves handle hot, high pressure, high density gases.

Exhaust valves are exposed to more severe operating conditions than intake valves.

The valves in your cylinder head are a vital component of your engine and undergo enormous stresses.

Automotive Engine Valves
Automotive Engine Valves

Damaged automotive engine valves can result in:

  • Reduced power
  • Poor fuel consumption
  • Complete engine failure

Any valve will eventually wear out if driven enough miles. But many valves call it quits long before they should because of burning or bending.

Burnt Valves

Burnt Valve
Burnt Valve

Exhaust valves are the ones most likely to burn because they run hotter than the intakes. This is caused by combustion gases escaping between the valve and valve seat. The hot combustion gases are forced past the valve which starts to burn away the edge of the valve.

A burnt valve will cause issues with your vehicle’s performance and fuel consumption. Consequently, Rough idle, reduced power, backfiring, and misfires are all symptoms of burnt valves. Incoming air and fuel cool the intake valves. Consequently, They operate at a much cooler temperature.

Bent Valves

Bent Valves
Bent Valves

The most common failure of valves is bending or breaking as a result of contact with the pistons. The valves contacting the top of a piston is due to incorrect engine synchronization caused by timing chain/belt breakage. If you suspect your engine may have bent valves, it is crucial not to attempt to start the engine.

The bent valves above are a result of a fatigued timing belt that has broken. Your timing belt doesn’t last forever and needs to be replaced according to the manufacturer’s service guidelines. Replacing your timing belt is cheap insurance against costly engine damage.

Automotive Engine Valve Cooling

So, The intake and exhaust valves rely on physical contact with the valve seat and guide for cooling. The combustion heat is conducted away through the valve seat and guides. So, Good valve seat contact is essential to prevent burning. If the valve does not receive adequate cooling, it can overheat, burn and fail.

Valve Leakage (Cylinder Leakdown Test)

Cylinder Leak Down Test
Cylinder Leak Down Test

The cylinder leakdown test is an excellent way to pinpoint where problems are before tearing down the engine. Listening for where the air is escaping by ear can isolate the problem.

Intake valve : Air whistling out of the intake, carburetor or throttle body indicates a leak at the intake valve.
Exhaust valve : Air heard hissing out of the tailpipe, turbocharger or exhaust manifold means an exhaust valve leak.

A multi-valve engine design typically has three, four, or five valves per cylinder to achieve improved performance.

  • Three-valve cylinder head-This has a single large exhaust valve and two smaller intake valves. A three-valve layout allows better breathing than a two-valve head. Consequently, the large exhaust valve results in an RPM limit no higher than a two-valve head.
  • Four-valve cylinder head-This is the most common type of multi-valve head, with two exhaust valves and two similar (or slightly larger) inlet valves. This design allows similar breathing as compared to a three-valve head. The small exhaust valves allow high RPM, this design is very suitable for high power outputs.
  • Five-valve cylinder head-Less common is the five-valve head, with two exhaust valves and three inlet valves. All five valves are similar in size. This design allows excellent breathing, and, as every valve is small, high RPM and very high power outputs are theoretically available. Although, compared to a four-valve engine, a five-valve design should have a higher maximum RPM.


Valve problems are something you should not ignore because they can turn into even more serious and expensive problems down the road. However, they are easy to prevent, or at least delay, with proper engine maintenance. Have your oil changed regularly, and fix any other engine problems promptly.

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