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Maintenance Q&A
Exhaust leaks, solving stud failures

By Paul Abelson, senior technical editor

Q. I first noticed I was waking up with headaches a few months ago. I used to idle my 2003 Freightliner Century Class when I slept in it. But last year I got an auxiliary power unit so I could stop idling. When the headaches became bad enough, I went to the clinic at my favorite truck stop. They’re a branch of the local hospital. They ran blood tests and told me I had abnormally high levels of carbon monoxide.

When we used to idle all night, the exhaust went up the stacks and out over the trucks. I can’t remember anyone complaining about getting headaches. Now they’re shortening the stacks for better aerodynamics and replacing them with “weed burner” exhausts near the ground. And a lot of trucks now have APUs.

Are all these exhausts that are low to the ground increasing CO poisoning? If so, what can we do?

A. OOIDA reports hearing a number of similar accounts. At the February TMC meeting, I checked with a number of fleet maintenance directors about problems with exhaust getting into cabs. All of them told me that even with under-cab exhausts, they had few complaints. However, there were a few reports of exhaust system problems in the APUs themselves.

Problem areas seemed to be with cracked exhaust manifolds, exhaust gaskets and flex-tubing in the exhaust. Tubing can be damaged from impact or from loose clamps allowing vibration.

Damage wasn’t confined to the APUs. Some trucks’ main engines had exhaust leaks that could make their way into cabs. These stemmed from around the turbocharger and, on at least one truck, at the EGR assembly.

You need to be alert to what’s happening in your own truck and get a carbon monoxide detector.

Carbon monoxide alarms can be found for as little as $40 at major retailers. RV carbon monoxide monitors are designed specifically to handle motion and stress of a moving vehicle. UL, the not-for-profit product safety testing and certification organization, actually lists certain carbon monoxide monitors as rated for recreational vehicles and unconditioned areas (areas with no heat or air conditioning).

Check regularly for exhaust leaks. Anytime you hear a telltale hissing with the engine (main or APU) running, check whether it goes away with the engine off. If so, have a shop check your exhaust.

If you ever start getting unexplained, persistent headaches, get checked by qualified medical personnel. Symptoms of carbon monoxide poisoning should never be put off. The effects could cause severe medical problems and could be fatal.

Q. All of a sudden, I started breaking wheel studs on my 2005 Kenworth T2000. They started on my right steer tire a few weeks after I had a road service call. Two studs were broken. I thought it was the tire company after the road service. But after I had my brakes changed at a local shop, studs on two of my four drive wheels broke.

Last year I switched to Michelin X-One wide single tires, and they ran fine for more than a year. Are they causing studs to break? What should I do?

A. I’m happy to hear that you noticed the broken studs during your pre-trip inspection and not as a result of a wheel coming off. That just points out the importance of thorough pre-trips.

Let’s address the single tires first. You didn’t mention if your 14-inch wide wheels were offset or not. The 1- or 2-inch offset compensates for differences in track. The offset could put an added stress on your wheel end bearings, but should not affect studs.

Since your truck is seven years old, I’m sure you’ve had the wheels off and on numerous times, if only to change tires. Every time you do, the studs are stretched. That’s how they generate the clamping force that holds things together. The principle applies to all studs, nuts and bolts. It’s like a solid bungee cord that tries to pull its hooks back into itself.

With a stud, the head is at one end, bearing against the brake drums. The nut serves two functions. Its threads stretch the stud, while its washer assembly bears on the wheel surface. This generates a clamping force that holds the wheel and brake drum together.

Steel is elastic. When stretched, it wants to return to its original shape, but only if its elastic limit has not been reached. Picture a rubber band that stretches and springs back. But stretch it too far and it breaks. So what can stretch a stud? Two things: fatigue and excessive torque.

Each time a stud is used, it loses a little of its clamping force. When used enough times, it may generate only 50 to 60 percent of its original clamping force, even when torqued to specification. That’s why many TMC member fleets replace studs after five or six wheel removals or every few years.

Improper torquing of wheel nuts is the greatest contributor to stud failures and wheel-off-incidents. In a good, brand-name tire shop, air wrenches are set to stop turning before required torque is achieved. For a truck tire with a 500 ft.-lbs. of torque requirement, that could be 300 to 400 ft.-lbs. The nut will be snug on the stud, but final torquing should be done by hand using a calibrated torque wrench.

A careful tire technician will treat his torque wrench like the crown jewels, storing it in its padded box and never letting it fall onto a hard surface. If it is jarred, he will recalibrate it as soon as possible.

Unfortunately, far too many shops take shortcuts. They use only air wrenches, setting the torque to the required final loading. They assume the torque setting is accurate, even if the wrench hasn’t been checked for a year and has bounced around in the back of a service truck.

When the wheel nut is firm, they give it just a few more seconds to make sure it’s really snug. This can severely over-torque studs, often beyond their elastic limit. On many occasions, TMC members have reported that to remove wheel nuts, it is not unusual to see torque readings in excess of 1,000 ft.-lbs.

As a result, even though the nuts are tight, they have little clamping force left. The stretched studs allow motion that eventually breaks the studs, leading to wheel-off incidents.

A quick way to determine if a stud has been damaged is with a thread gauge. If the gauge fits the stud with no gaps, the stud is probably good-to-go. If the gauge does not fit, the studs have been over torqued. Replace them. LL


Senior Technical Editor Paul Abelson is a life member of OOIDA, holds an Illinois CDL, is active in the Technology & Maintenance Council, and is a recipient of the Lifetime Achievement Award from the Truck Writers of North America.

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