How To Replace Exhaust Tubing

| December 12, 2008

TOOLBOX

Combination wrenches
3/-inch socket set
1/-inch socket set
Air-powered carbon cutting disk
Assorted hammers
Assorted screwdrivers
Rubber mallet
Chisels
Pry bars
Wire brushes
Eye protection
Coveralls
Shop towels


FOR MORE INFO

Air Flow Systems
(503) 659-9120
airflo.com

Donaldson
(800) 374-1374
donaldson.com

Dynaflex
(800) 334-3363
dynaflexproducts.com

Fleetguard/Nelson
(800) 223-4583
fleetguard.com

Tenneco Walker
(734) 384-7834
tenneco-automotivehd.com

TEXIS Truck Exhaust
(800) 267-4740
texisexhaust.com

For many truckers, the occasional faint scent of spent diesel fuel is a pleasant reminder of their occupation and the mechanical power they command. The fragrance is much less appealing, however, when an exhaust leak opens, filling a cab with noxious fumes that foul clothing, carpeting and even cello-wrapped sandwiches.

Cab and sleeper interiors are particularly susceptible to exhaust leaks because most of the system’s plumbing is routed just below the floorboards. Left unattended, these problems only get bigger, adding more noise and odor to a driver’s working environment.

Flex tubing is an exhaust system’s highest mortality part. It usually lasts one to two years, about half the life expectancy of solid tubing. Flex is designed to isolate vibration of components, such as the turbo and mufflers. Over time, though, its accordion-like structure corrodes and stiffens, eventually cracking because of the constant movement of the connected parts.

One big contributor to premature flex-tube failure is misuse, says Clark Lewis, head of product management for Donaldson in Minneapolis. “If an exhaust system isn’t properly designed, truck makers will sometimes use flex to take up the misalignment [of solid tubes],” he says. “Any bends in the tubing will limit its ability to flex, causing it to crack in a very short time.”

Another factor in the longevity of flex tubing – and all exhaust components – is metal quality. Most original-equipment parts, from turbo to tailpipe, are now made from aluminized steel, a combined metal that lacks shine but offers good corrosion resistance. Stainless steel and chromed cold-rolled steel are popular for exterior pipes. Chrome is slightly shinier than stainless, but it doesn’t last quite as long, typically rotting from the inside out. The shortest life cycle is that of plain cold-rolled steel, used for low-end aftermarket solid tubes and some mounting hardware. Galvanized steel, the material of cheap flex, is only slightly better. But in using it, you risk the additional problem of galvanic corrosion, a chemical reaction that occurs between dissimilar metals when they’re clamped together.

Obviously, the purpose of an exhaust system is twofold: route engine gasses to an appropriate location – downwind of the driver – and quell the roar of combustion. These goals, however, are not equally important in the minds of some owner-operators. A portion of the trucking public likes the sound of high-decibel horsepower, preferably delivered through dual straight pipes the size of culverts.

This sort of customization, featuring monster stacks with or without any sound attenuation, is impressive on the truck-show circuit, but it’s not necessarily good for engine performance – or public image. Many in the loud-and-proud crowd use single short mufflers to subdue some of their trucks’ volume. Although the technique might get them within arm’s length of federal vehicular sound regulations – 85 dBA above 35 mph (measured at 50 feet) – it can also raise exhaust backpressure to unacceptable levels, cutting into power and fuel economy.

All original equipment mufflers and most aftermarket replacements have flow ratings indicating their exhaust-handling capacity, measured in cubic feet per minute. The numbers, available from suppliers and manufacturers, enable truckers and shop personnel to closely match mufflers with specific engines, all of which also have flow ratings.

Unfortunately, the process of getting the correct replacement muffler isn’t always exact, says Dale Zuhse, account manager at Fleetguard/Nelson in Stoughton, Wis.

“A person working at an aftermarket parts counter might look in a catalog and choose the first product that seems to match a trucker’s requirement, in terms of inlet and outlet diameter and physical size,” he says. “There could actually be eight or 10 mufflers with those dimensions, but each one has a unique intended application. It’s important that the parts man understands the difference and does a little research. That doesn’t mean he must use the OE product, but he should at least know whether the muffler he’s selling is for a single or dual system, and he should know the engine model and horsepower. He should also know if the original setup had any unique features. This information is going to be critical as we move into catalytic mufflers.”

Given the risk of misfit, it’s probably lucky for truckers that mufflers usually last a long time: four or more years when mounted vertically, as they are on about 90 percent of Class 8 trucks. Horizontally mounted mufflers, often called “weed burners” because of their proximity to the ground, last about three years.

Working on today’s exhaust systems can be challenging, given the limited space and multiple fairings of modern trucks. Replacing solid tubing, when it’s routed above the frame, sometimes requires the combined skills of a body man, plumber, mechanic and contortionist. Assess your abilities and tool arsenal before starting such a job. Once you’ve decided to do it yourself, make sure you have an assistant to help lift and guide the heavier parts into place. Here are a few more things to consider:

Loose band clamps will dig into the tubes they’re holding, causing wear ridges and eventually holes in the metal.

FAIRING REMOVAL. Unless your truck is a cabover or has its exhaust routed below the frame, you’ll probably need to remove some body panels to reach the tubing mounted under the cab and sleeper. Although this is an extra step, it is a worthwhile effort that will save you a lot of time and frustration when removing and installing submerged components.

DISASSEMBLY. A carbon disk on a small, high-speed air grinder is the best tool for disassembling corrosion-bonded exhaust tubes. Used skillfully – with hand and eye protection – the cutting disk will slice through corroded flex tubing without damaging the solid tubes within. One shallow, lengthwise cut from each end of the flex is usually enough to free even the most stubborn joined parts.

BUYING AND INSTALLING FLEX. The ideal span for flex tubing is more than 10 inches, but less than 20, according to Dan Hrodzicky, owner of TEXIS Truck Exhaust in Mississauga, Ontario. He also says that flex should be in a relaxed position when installed: not stretched out and not squeezed together. The same rule applies when you’re buying it. Hrodzicky says some unscrupulous vendors try to boost their profits a little by selling flexed in a fully extended position.

SYSTEM TESTING. Unless you are planning to replace your entire exhaust system, it’s a good idea to test the integrity of parts you intend to reuse. This is easily accomplished by tapping on solid tubes with a plastic hammer. Reasonably robust metal will give off a clear tone, but areas of deterioration will sound raspy and dull. You should also watch for signs of small leaks, revealed as thin sooty streaks along the tubes. Replace any marginal parts, even those with a few more miles left in them.

CLAMPS. Stepped band clamps are the best devices for joining flex and solid tubing. They apply firm, even pressure around both tube ends, sealing the connection. Unlike guillotine clamps, though, band clamps – step or flat – cannot be reused.

Most cracks in flex occur within an inch of the clamps holding them, an area receiving the greatest stress from moving, connected parts.

VIBRATION. Exhaust systems endure vibration from the road and engine. Any excess movement in the tubing will result in wear and, ultimately, leaks. It’s important that all exhaust mounts are intact and secure. These should be checked during each PM, and tightened or replaced promptly, if necessary. Operators in severe applications should consider adding extra mounts if some part of the system is repeatedly failing.

EXTRA SOUND CONTROL. Many truckers today prefer the sound of their stereo systems to that of their engines. There are a number of products on the market that will reduce exhaust noise below the original-equipment level. About three years ago, Donaldson introduced its Silent Partner muffler, designed to minimize the rasp of engine brakes. Al Hovda, an application engineer at Donaldson, says the Silent Partner offers 20 dBA of noise attenuation – more than double the amount of generic mufflers. Tenneco Walker introduced a similar product early last year, calling it the NoiseBraker. In addition to premium mufflers, truckers can also buy resonators and glass-packed stacks, the combination of which will lower exhaust noise by 6 to 10 dBA.

HEAT PROTECTION. It’s important to shield exhaust tubing from anything that would melt, burn or transmit heat into the cab. Most well-stocked parts outlets carry a variety of devices for this purpose. These should be installed after the system is assembled, but before it’s covered with side fairings.

Exhaust systems require no preventive maintenance other than periodic inspections. During the winter, they should be pressure washed if you’ve had the misfortune to drive on roads treated with magnesium chloride or other corrosives. The only way to ensure maximum longevity, though, is with good, brand-name replacement parts. These will cost more than low-grade generics, but you won’t spend nearly as many weekends replacing them.

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