There are different applications of coil spring suspension. On most front-wheel-drive cars, coil springs are assembled to the strut, coiling around it. Heavier-duty vehicles place coils between the chassis and the frame and feature shocks as a separate suspension component. Another popular item on the suspension market is coil-over shocks. Like a strut, the coil wraps around the shock, but it does not generally bolt to the steering knuckle like a strut does. You must replace shocks and struts more often than coil springs, but inspect the springs when you replace any suspension components.
Longevity of Coil Springs
Coil springs can and will become compromised after time. However, depending on their application to the vehicle, many coil springs can last the lifetime of the vehicle. There really is no expiration date on coil springs, but there are a few things to check for when inspecting your suspension.
While coil springs support the weight of the vehicle, it's the shocks' or struts' job to prevent the bouncing. Jounce the front suspension (and the rear, for that matter) and count how many bounces occur after you release the vehicle. Two or more full bounces means weak shocks or struts. But what caused it? Age is likely the determining culprit. Coil springs are much more durable than shocks and struts, and many times replacing the shocks or struts will return the vehicle back to its original level of performance.
However, weak shocks and struts will take their toll on coil springs if left ignored for a long period of time. As the coils weaken, you will notice clunking or bottoming-out sounds, because the coils are not able to provide stability to the weight of the vehicle.
In addition, weakened coils will damage shocks and struts, because their rebound distance will be much farther than the components were intended to perform. Weak coils will compromise the ride height of the vehicle, and this symptom will affect the vehicle's alignment. Measuring the ride height of the vehicle and comparing that with the specifications will help you determine whether the coils are working in conjunction with the shocks and struts.
Things to Look For
Some vehicles have a long history of snapping coil springs. This is a dangerous situation depending on the position of the break in the spring. Because coil springs are positioned near the tire, it is not all that uncommon for a broken coil spring to puncture or rub against the sidewall of a tire and cause a flat; or worse, a blowout.
When inspecting coil springs, run your hands all the way around the coils, front to back. Some coils employ a protective cover that can conceal the break; but if you run your hands around the symmetrical coil, you will feel any anomaly.
Another thing to listen and look for is creaking or binding of the strut bearing plate on strut applications. Do this by turning the steering wheel back and forth and listening for binding or creaking noises. Compromised bearing plates will manipulate the coil springs in ways they were not designed for and eventually wear them out.
What to Do and When to Do It
Should you replace coil springs each time you replace shocks or struts? Absolutely not. Inspect the coils every time you replace any suspension component. Ball joints, tie rod ends, idler arms, pitman arms and control arms that are compromised can all wreak havoc on the coil spring and shock or strut.
Measure the ride height of the vehicle, and then compare it with the specifications. Since the coil spring supports the weight of the vehicle, one that is sitting below its intended specifications may very well have compromised coil springs. Also, compare the ride height side to side on the same axle. The springs are separate components, and it's not unthinkable that one side can become weak while the other is OK. While this will make one side of the vehicle sag lower, replace both in this case. Always replace shocks, struts, coil springs and leaf springs in sets rather than individually.
Should you always replace shocks or struts when replacing coil springs? While many will argue the necessity of it, unless you know how long the coil spring has been compromised, you need to understand that the shock or strut has taken up the slack of the spring. If you want a more desirable ride, then yes; replace the shocks or struts. If you need to save money, then don't. But whether you're saving money in the long run is the real question. You must remove coil-over shocks and struts and compress them to remove the coil springs from the assemblies. Labor costs or time will be applied if you gamble.