If you hit a bump or a pothole, is your car bouncing all the way home? Does it look like you're carrying a ton of bricks in your trunk? When you turn a corner, do you feel like your car is going to slide right off the frame? If your answer to any of these questions is a "yes", then it's likely you need to replace your coil springs. Excessive bounce or sagging in your vehicle's ride is indicative of a suspension problem, one that can usually be traced back to your springs. Isolating which coil springs need to be replaced can be determined by looking at the car when it's parked. Wherever your car is riding too low (either front or rear) is going to be where the springs need to be swapped out. Failing to do this when needed will significantly add to the amount of abuse you are giving your shocks and struts. This will reduce their lifespan as well, forcing you to replace them sooner. So if you suspect your coil springs are worn out or broken, it is best to replace them with new ones right away.
Most early automotive suspensions were supported by leaf springs. This changed in the 1930's when coil springs found their way onto independent front suspensions (http://www.motorera.com/history/hist08.htm). Not only was this a significant weight savings over earlier designs, but coil springs require little to no maintenance. They also provide much firmer road-handling capabilities than leafs, however were not favored for large, heavy vehicles such as pickup trucks and cars over 2-tons. So coil springs did not find wide range popularity or use on rear suspensions until vehicles became lighter. Still, they remain our most efficient method of springing vehicle weight today.
How do I replace a coil spring? If you have to ask, it will probably not go very well. It can be very dangerous to remove and install a new coil spring without a spring compressor. Trying to change a spring without one could result in severe bodily harm, which is why we recommend professional installation. However, if you are feeling confident, here's how you would do it: