Auto Questions: How Camshafts Work

This article discusses how a camshaft works, examines the reasons they may fail, and lists typical signs of a bad camshaft.

A camshaft is used in most engines that rely on a valve system, but many people don't understand how it works or why it sometimes goes bad. This article will discuss how a camshaft works, discuss some of the reasons they may fail, and list typical signs of a bad camshaft.

Most of us have seen a camshaft before, but not everyone knows what the "lobes" do or how they affect performance. A camshaft (or cam) runs the length of an engine's head and has uneven looking "lobes" set apart at regular intervals. A camshaft may be made of solid steel billet, low-grade steel or even iron. The weight and lobe lifts vary from cam to cam.

The cam is responsible for the precise timing of the valves as they open and shut. The cam turns as the engine runs through its revolutions, usually by means of a chain or belt connected to the end of the crankshaft, pushing rods along the lobes. This applies pressure to the valves, forcing them open and allowing them to shut again once a stroke has ended.

All camshafts rotate at half crankshaft speed, regardless of all other variables. The shaft is located in the engine's head along with other valve train components, such as the valves, valve springs, rocker arms, and occasionally, on older engines, rods.

The shape of the lobes affects the engine's performance, since they regulate both engine timing and how long valves remain open. A wider lobe will allow valves to remain open longer than a narrower lobe. Duration refers to how long valves remain open due to the width of cam lobes. When you hear mechanics talk about different cam profiles, what they are actually referring to is how fat or narrow lobes are along the shaft.

The profile of a cam becomes even more important at faster speeds, as the time the air / fuel mix and exhaust strokes decreases with increased engine speed. Generally, the narrower the "valve lift," the better an engine will perform at higher revs. The tradeoff is that with a "hot" or performance cam your engine will appear more lumpy and uneven at lower revolutions. What you gain at a specific revolution you often lose at another engine speed or in general reliability.

How do you know when your camshaft isn't working properly?

Premature camshaft failure is actually usually symptomatic of another problem, and not an isolated event. More often than not a cam fails because of other problems in an engine's valve train. Cams can fail for a number of reasons. Oil can be starved in an engine's head, leading to increased friction where the cam and rods meet or excessive heat on the cam itself; the rocker arm assembly, which seesaws the set of rods along the cam, can be incorrectly adjusted and bind; or the valve springs can bind or break.

Additionally, cams will sometimes have manufacturer's errors or be damaged in shipping. Cams can also be pitted or scratched if the lubricants surrounding it become contaminated with iron fillings, sand, dirt or other foreign particles. Camshafts are under mechanical duress, and do wear over time, especially along the lobes. Under ordinary circumstances, however, camshafts should last many, many miles.

What should you expect if your camshaft isn't working?

If your engine suddenly develops a "popping" sound, poor gas mileage or bad performance, it could be a problem in your valve train. If your engine backfires or "pops" it could be a sign that its stroke is off, a valve or valve spring damaged, or the cam is worn.

Most of the symptoms of cam or valve train failure are very subtle - loss in performance, difficulty in starting, backfires - that gets progressively worse over time. You may hear more valve clatter or notice a dark, oily exhaust, depending on the level of the damage. A catastrophic valve train failure will render a vehicle immobile, binding up the very system that is responsible for fuel delivery and exhaust.

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