The Symptoms of a Blown Strutby Chris Stevenson
Struts, commonly referred to McPherson or coil-over shocks, have the responsibility of keeping vehicle tires flat on the ground over all conditions. They also serve as dampening devices that control and lessen the transfer impact between the chassis and road. Struts provide comfort while driving over the road, assist in smooth and even cornering, reduce tire wear, maintain curb height and keep suspension parts from vibration and shock damage. A vehicle owner has several things to look for in finding a blown strut.
Excessive Bounce and Rebound
Struts that have lost their ability to cushion and rebound will bounce noticeably on the affected wheel. Plac the vehicle in "Park" with the emergency brake set, and put your full weight on the bumper at the corner of the vehicle where you suspect a blown strut. Bounce up and down hard, then release your weight. If the vehicle rebounds more than one-and-a-half times, it indicates a bad strut cylinder or damaged strut piston rod.
Diving and Squatting
Look for a bad strut when you suddenly and firmly apply the brakes. A bad front strut will nosedive the hood downward, particularly over the side of the vehicle that has a bad strut. During a quick, hard acceleration, look for the rear end of the vehicle to noticeably squat down or dip more than normal. A single blown strut on the rear end will dip down further than the other side.
Visually inspect the shock housing, which is inside the coil spring. Any visual signs of oil, or a gummy film, will indicate the strut cylinder has lost its hydraulic oil. The oil usually passes through a seal at either end of the shock but will appear in larger amounts on the bottom of the shock housing or in the coil spring mounting plate. The oil does not have to look fresh; it can look like a oily film covered with dust.
A blown strut will allow the upper part of the vehicle body to lean during a cornering maneuver. This will be apparent on the rear or front of the vehicle. The affected side the vehicle will also be slower to retain its normal height once the turn has been completed, and it might affect the steering response, adding some extra load to the steering wheel response. The vehicle might also wobble a bit after recovering from a strong, tight turn.
Struts that have lost their power to cushion the weight of a vehicle will emit a muffled or loud metallic clunk when the vehicle drives over potholes, deep dips or curbs. The clunking noise is actually the chassis contacting or bottoming out against the suspension parts or frame. The noise can originate from the front or rear of the vehicle, but will assuredly happen over rough road surfaces.
Blown struts lose their ability to keep downward pressure on the tire. When the tire rebounds upward, even after a small or medium-sized hole or bump, it does not return to the ground immediately. This lag time causes the tire tread to skip or scuff over the road's surface, especially at higher speeds, tearing out small cup-shaped depressions in the rubber. You can see these scalloped cups by examining the bottom of the tire tread. Such scalloping indicates a blown, or useless strut.
A blown strut will show evidence of structural damage. During visual inspection, symptoms will include broken or missing strut tower plate nuts or bolts, a disconnected or broken upper or lower shock mount connection or a bent, broken or jammed coil-over spring. The cylinder piston shaft (shiny part) might also be noticeably bent, in relation to the shock housing. Attempting to firmly move the strut shock or coil-over spring by hand is usually enough to determine if the parts have broken or loosened.
Use a tape measure to measure the distance from the top of each wheel well to the ground. The distance should be approximately the same for all the wheels, give or take 1/2 inch or so. If one of the measurements reads significantly less than the other wheels, it indicates the curb height has dropped due to a bad strut. Struts that have lost pressure will let the car sag at their location. If you have a low reading on both struts on the same axle, perform a rebound test to make sure the suspension has not been deliberately set lower for this axle. If both struts fail the rebound test or show signs of leaking, then both are defective.
Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.