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Automotive Engine Valves – Function – Failure

Automotive Engine Valves - Function - Failure - And, Testing
Automotive Engine Valves – Function – Failure – And, Testing

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

Above all, the valves in your cylinder head, are a vital part of your engine and undergo, enormous stresses.

So, the automotive engine valves, that let the air into the cylinder, are the intake valves. However, the automotive engine valves, that let the gases escape, are the exhaust valves.

To clarify, automotive engine valves, are designed to open and close, at precise moments.

Overhead Valve (OHV) – Overhead Camshaft (OHC)

The Overhead Valve Engine (OHV), operates by using pushrods.

(OHV) Engine Valves Are Operated By Pushrods
(OHV) Engine Valves Are Operated By Pushrods

The Overhead Camshaft Engine (OHC), operates, directly on bucket tappets. As a result, it needs fewer parts, to operate the valves.

(OHC) Engine Valves Run On Buckets or Lifters
(OHC) Engine Valves Run On Buckets or Lifters

This is why, automotive engine valves, play a pretty critical role, in an engines performance. So, the more air you can move in and out of the engine, the more efficient it will be.

So, intake valves handle cool, low pressure, low density gases.  However, exhaust valves handle hot, high pressure, high density gases.

Automotive Engine Valves
Automotive Engine Valves

Damaged Automotive Engine Valves, Can Result In:

Automotive Engine Valves – How They Can Fail

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

Burnt Valves

Exhaust valves, are the ones most likely to burn. Because, they run, hotter than the intakes. Most often, caused by combustion gases, leaking between the valve and valve seat. First, the hot combustion gases, are forced past the valve. After that, the heat starts to burn away, the edge of the valve.

Burnt Exhaust Valve In Cylinder Head
Burnt Exhaust Valve In Cylinder Head

So, a burnt valve, will cause issues, with your vehicle’s performance and fuel mileage. Consequently, rough idle, reduced power, backfiring and engine misfire, are all symptoms of burnt valves. Incoming air and fuel, cool the intake valves. And, that’s why they operate, at a much cooler temperature.

Bent Valves

The most common failure of valves, is bending or breaking. Most often, from contact with the pistons. The valves contacting the top of a piston, is due to wrong engine timing. And, can be caused by, a broken or damaged timing chain or belt. If you suspect your engine may have, bent valves, it is crucial, not to attempt to start the engine.

Bent Valves
Bent Valves

The bent valves above, are a result of a timing belt, that has broken. Your timing belt doesn’t last forever and needs to be replaced, according to the manufacturer’s specs. Replacing your timing belt is cheap insurance, against costly engine damage. Also, sticking valves from carbon buildup, can do the same thing.

How Are Automotive Engine Valves Cooled

So, the intake and exhaust valves, rely on physical contact with the valve seat and guide, for cooling. Because, the combustion heat is conducted away, through the valve seat and guides.

Machining Valve Seats For Proper Contact
Machining Valve Seats For Proper Contact

So, good valve seat contact, is essential, to prevent burning. Because, if the valve does not receive adequate cooling, it can overheat, burn and fail.

Testing Valves For Leaks

Leaking, Automotive Engine Valves

Using Cylinder Leak Down Tester
Using Cylinder Leak Down Tester

The cylinder leakdown test, is an excellent way to find out, where problems are, without tearing down the engine. Listening for where the air is leaking, by ear can isolate the problem. Cylinder Leak Down, Test Results.

Intake Valve : Air heard whistling out of the intake, carburetor or throttle body, means an intake valve is leaking.
Exhaust Valve : Air heard hissing out of the tail pipe, turbocharger or exhaust manifold, means an exhaust valve is leaking.

How Many Automotive Engine Valves Can You Have

A multi-valve engine design can have, three, four or even five valves per cylinder. Above all,  to achieve the best engine performance.

3 Valve Cylinder Head
3 Valve Cylinder Head

Three Valve, Head

So, 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, Head

So, this is the most common type of multi-valve head. It comes 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). As a result, this design is very fitting, for high power outputs.

Five Valve, Head

Finally, 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 for, 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). So, as you can see, automotive engine valves, come in many configurations.


So, valve problems, are one thing you should not ignore. Today, getting a valve job may save your entire engine. Because, most of the engine parts are now located, in the cylinder head. However, they are easy to prevent or at least delay, with proper engine maintenance. So, have your oil changed regularly. And, fix any other engine problems, ASAP.

Thank You !

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