You pull over to inspect your leg and the underside of your car, in that order. There are big, hot chunks of metal hanging loose under your chassis, and the exhaust system is nearly dragging. Today’s exhaust systems consist of a lot of expensive parts. There are one or more catalysts, an oxygen sensor, maybe even a 2-stage muffler. There’s piping that’s not only shaped to clear underbody lines, but also may be made of premium metals, in some cases dual-wall tubing.
There are sheetmetal underbody heat shields to prevent the exhaust from igniting dry grass. What’s holding everything together? A few clamp joints and some welds. What’s holding everything up? A few pieces of rubber with some brackets. What’s keeping everything aligned? Those same few pieces of rubber and brackets. Failures are common and they range from cracked rubber to failed welds. Before you suffer the same expensive fate we described earlier, take time to perform a regular underbody inspection—it’s easy. To get started, jack up the car and support it on safety stands front and rear. Now you’re ready for an in-depth look.
This project was originally published in the June 2002 issue of Popular Mechanics. You can find more great projects at Popular Mechanics DIY Central.
Step 1: Hangers
Today, the most popular design uses rods welded to the pipes and, often, to the vehicle underbody. These hangers also have a thick rubber section that resembles a flattened football or beach ball, with holes for a rod at each apex of the rubber. Welding positively locates all the parts, and although you may have to tug and pry, replacing the rubber piece when it becomes cracked is very straightforward. You can install a brand-new pipe if the rod comes off the pipe, but if the pipe itself is good, that’s an unnecessarily tough and expensive replacement.
The alternative is to install a universal hanger, though this may require a bit of jury-rigging. First, remove the broken hanger. Then look for a nearby hole, perhaps even from the old hanger. Take a universal hanger that can twist and tilt and has an adjustable length setup, and attach it to the underbody with a bolt (and if it isn’t threaded, also with a nut) through that hole. Next, make the connection around the pipe—a simple clamp and U-bolt usually works. Don’t be surprised if a part of the welded-on bracket (from the broken hanger) has to be cut or ground off to provide space for the clamp that will attach to the universal hanger.
You should try to make attachments very close to the locations of the original equipment setup. This will maintain system alignment and the balanced hang of the entire system. If there is noise from exhaust system contact with the underbody or an underbody part, you should check for damage underneath. Exhaust system clearances are limited, and if you see the need to increase one, it may be possible to bend or shim an original-equipment hanger. If a rod-type hanger is welded in place, see if you can bend it with a piece of pipe over the rod’s open end.
If you need more than 1⁄4 in. or so, look for an alternative. If the hanger is a bolt-on to the body, for example, you may be able to install washers as shims. Some older imports have rubber hangers that resemble thick rubber bands. If the old band has broken off, you can get a replacement, but installing it is not a matter of simply stretching it over the retaining tabs. The band is so thick and stiff you probably will have to pry pretty hard to get it on.
Many exhaust systems have a bracket to provide firm support close to the transmission. If the bracket cracks—or if it was removed for service and never reinstalled—that can account for a lot of exhaust system flex and eventual cracking. If you can’t get a replacement bracket, you may be able to get the old cracked one rewelded. But replacement is the best choice, and if the dealer doesn’t have it, the wrecking yard may.
Step 2: Shields
Step 3: Exhaust Joints
If a clamped joint isn’t cracked, but is leaking exhaust gas, remove the old clamp. If the pipe ends are badly distorted, you’ll have to rent a pipe expander to reshape them. Otherwise, separate the pipes, sand them to remove any rust, then apply a coat of exhaust pipe sealer. Reassemble and install a heavy-duty clamp, which can tolerate much more torque, to provide a leak-free joint. Many systems have a flange joint at the exhaust manifold or at the manifold’s Y-pipe.
That joint is held together with spring-loaded bolts. The heat may weaken the springs, and the joint will loosen. Try simply tightening the bolts, but if the springs have cracked or are weak, replace the bolts. They’re sold, with new springs, in auto parts stores. If a flange joint is badly rusted, it’s worth trying to free it up with penetrating solvent. Then, take it apart and see if it can be salvaged by sanding it clean, installing a new gasket with sealer and reassembling with new bolts and nuts (drill out rusted studs if necessary). Check at an auto parts store to see if a clamp-over repair fitting is available.