Exhausting work: How to fix those noisy pipes

Exhausts haven’t changed all that much since Étienne Lenior’s first internal combustion contraption clattered down the street causing oldsters to wave their canes and yell “TURN THAT THING DOWN!”

Hey, progress is a bitch, and gas and air exploding dozens of times per second can hardly be expected to be quiet. But since engine noise is viewed by all but die-hard motorsport fans as an undesirable byproduct of combustion, the gasoline engine has been muzzled by an exhaust system since the dawn of the motoring age.

The plumbing of an exhaust system is pretty straightforward. A short cast-iron or stainless steel exhaust manifold bolts directly to a set of studs on the side of the head and shepherds exhaust gasses from multiple cylinders into a single outlet. The other end of the manifold attaches to a downpipe that angles beneath the car, mating to a center resonator (or, on a modern car, a catalytic converter), and terminates in a muffler and tailpipe. Four-cylinder engines usually have a single exhaust manifold. Inline sixes may have two. V-6, V-8, and V-12 engines have a manifold and downpipe for each cylinder head, which may continue all the way back as dual exhausts (yay!) or merge via a Y-pipe (boo!). On high-performance cars, the manifold and downpipe pairs may be replaced with “bundle of snakes”—like exhaust headers whose individual pipes have lengths designed to let the gasses from each cylinder pass without fighting against each other to get out.

Other than inspecting the rubber hangers holding the sections to the underbody, exhausts are, for the most part, maintenance-free. Unfortunately, at some point, age, heat, moisture, and corrosion do their thing and suddenly it’s LOUD. Since the manifold is usually thick cast iron, it is unlikely to rust through, and as long as it doesn’t crack it often lasts the life of the car. But the other pieces are poster children for being normal-wear-and-tear auto parts.

If, when you hear a loud exhaust, you find a hole in the side of the muffler big enough to stick your hand through, the cause of the problem is obvious, but small exhaust leaks can sometimes be surprisingly difficult to locate.

The old-school technique is to take a rag, fold it over multiple times so you won’t get burned, and with the engine running, press it over the tailpipe. If the pressure from the exhaust gas pushes your hand away, the system is tight. If not, it’s leaking. You then cajole a friend into rag duty, crawl beneath the car and listen for the leak. Sometimes you need to pass your hands (carefully) around the exhaust in order to feel the presence of escaping gas. If there aren’t any obvious holes, it’s likely that the leak is coming from the flange connecting two sections. You should be able to verify it by putting your hands around it to feel the escaping gas, but again, be very careful. If you’re fortunate, the problem will be that the sealing ring has failed, and if you’re double fortunate, you may simply be able to replace it and get the old flares and flanges to seal against the new ring.

Sometimes, however, when you disassemble it you find that the exhaust pipe itself has rusted away at the flared lip. If the exhaust is loud on startup but soon quiets down, and you can’t find a hole anywhere, it’s possible you have a cracked manifold that seals as it heats up. But the overwhelming majority of the time, a leaky exhaust is due to the thinner metal sections or the sealing rings at the flanges connecting them.

So… you fix it. Exhaust work is a slam-dunk, do-it-yourself repair. You usually look at the cost of an OEM exhaust from the dealer and say, “Well, I’m not going to pay that,” and instead select an aftermarket exhaust system with sound and performance characteristics to your liking for much less. You get to be a real man, lying beneath your car for hours, swearing like a sailor with rope burns, rust falling in your eyes, and jagged metal severing skin and threatening to take out your tendons—just livin’ the dream. Then you hang the replacement exhaust yourself and save several Benjamins in the process.

This is all still valid, but the presence of the catalytic converter complicates things. Here’s why. If there’s a hole anywhere in the exhaust, odds are the other sections are weak too. And with all that rust, you can bet that the fasteners and flanges are corroded, and the sections won’t disassemble without a fight. The big nuts and studs holding the downpipe to the manifold usually surrender with heat, wax, and torque, but the small bolts on the triangular flanges that squeeze the downpipe and cat flares against their metal sealing rings often need to be cut. And the tubular sections where the muffler pipe (when new) slid flawlessly inside the resonator or cat pipe is now fused tighter than a clam with lockjaw—reason No. 1 why I’ve been driving my ’72 BMW Bavaria for three years with chimney flashing hose-clamped around the muffler.

If the sections won’t separate, and if the exhaust path has one of the pipes running above the rear subframe, you have no choice but to cut it somewhere to get it out. And even if you can pull the sections apart, a new piece might not seal completely against an old corroded or bent one, no matter how hard you torque things down. Back in the day, you’d simply blitzkrieg it and destroy the old exhaust, go all Jack Nicholson on it with a Sawzall, and replace everything, installing new downpipes, resonator, and muffler. But on most post-1975 cars, replacing everything means replacing the catalytic converters, and they’re expensive. Re-using the cats requires one to holster the Sawzall and keep one’s “Here’s Johnny” impulses in check. Unfortunately, the problem of new-muffler-won’t-seal-against-old-cat-flanges can be absolutely maddening.

If this happens, you may be able to get a muffler shop to save both your bacon and your cat by having them cut off the old flange pipes and splice new ones in that mate cleanly with the next section. And if you want them to do it all, they can weld custom-bended sections of pipe to new mufflers, resonators, and cats of the appropriate size. This is a good option if your car is old enough that the original exhaust is stratospherically expensive and aftermarket options are no longer available—reason No. 2 I’ve been driving my ’72 BMW Bavaria for three years with chimney flashing hose-clamped around the muffler.

If you want to be able to try to disassemble the exhaust later to get the transmission or driveshaft out, a good muffler shop can also splice in flange connections. And you can tell them if you want the system Lexus-quiet, a little more aggressive, or downright nasty.

After all, there’s a long tradition of annoying the public to be maintained. Remember, the other guy’s exhaust is loud. Yours is throaty.

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Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel Magazine for 30 years. His new book, Ran When Parked: How I Road-Tripped a Decade-Dead BMW 2002tii a Thousand Miles Back Home, and How You Can, Too, is available here on Amazon. In addition, he is the author of Memoirs of a Hack Mechanic and The Hack Mechanic™ Guide to European Automotive Electrical Systems. Both are available from Bentley Publishers and Amazon. Or you can order personally inscribed copies through Rob’s website: www.robsiegel.com.