Shafted
Q: I have a '97 GMC Sierra that I've lifted 4 inches. I've also added 33-inch-tall tires and 10-inch-wide rims. The truck has a 350 small-block with an automatic transmission. It makes an annoying clunking sound when I accelerate briskly from a stop. I thought the U-joints were loose or failing, but I had them checked and they seem fine. There aren't any unusual vibrations such as those you'd expect with failing U-joints. Could it be bad wheel bearings? What else could be causing this problem? I don't remember the truck being as noisy before I lifted it and added the tall tires. I appreciate your help.
Lonnie Smalley
Provo, Utah

A: You were on the right track when you suspected the U-joints. Loose U-joints can make a clunking sound, especially when starting out. The slop in the joints can produce noise as the driveshaft turns against the excessive clearance. Your problem is most likely caused by the input yoke at the front of the driveshaft, where it engages the output shaft of the transmission. When you lifted your truck, you altered the driveshaft angle. Trucks such as yours that have more radical driveshaft angles are more prone to this problem than nonlifted trucks. As the angle of the driveshaft changes, the front yoke varies in how far it engages the transmission output shaft. When you stop (especially under hard braking), the forward momentum of the truck and movement of the rear springs can thrust the driveshaft yoke farther into the transmission output shaft. That can lead to increased binding between the mating splines of the driveshaft and corresponding splines in the transmission output shaft. When you suddenly apply power, such as when leaving a stoplight, the binding releases and causes the clunking sound you described. The more worn these splines are, the more room they have to unload and make noise. There are two possible cures for the noise. You could install a new front yoke and hope it's tighter than the old one, but whatever slop you previously had inside the transmission output shaft will remain. Changing the output shaft itself is a big job. An easier, less-expensive cure is to remove the input yoke and pack the splines with grease that is less susceptible to transmission fluid. You should be able to obtain the superior grease at your local GM dealership's parts department. Depending on how bad your problem is, the improved grease should keep things quiet for a few thousand miles before you'll need to remove the yoke and pack the splines again.

Shafted Again
Q: I have a '96 F-150 XLT 4x4 pickup that's been making clunking noise during starts and stops. I think the noise is coming from the driveline. I suspected the U-joints were going, so I got under the truck and tried twisting the driveshaft but couldn't detect anything loose. All the fasteners appeared secure. I inspected everything under the truck and didn't find anything amiss. I figured that since U-joints aren't very expensive, I would replace them, but the noise still didn't go away. Could something else be causing the clunking sounds? Thank you for your help.
J. K. Knowles
via e-mail

A: It sounds as though your problem is similar to the previous reader's with the '97 GMC pickup. Input or slip-yoke problems aren't specific to any single brand of truck. The basic dynamics of the problem are the same, regardless of what truck you're driving. Suspecting bad U-joints is a logical assumption, and it could be a contributing factor. In your case, we think it's the input yoke.

When the slip yoke doesn't slide smoothly in and out of the transmission output shaft, a clunking noise can be the result. Rear suspension wrap-up (such as under hard acceleration) can amplify the situation. The wrap-up overcomes the input yoke to output shaft friction, causing a sudden driveshaft movement. That movement generates the noise. Revised input yokes with part numbers E4TZ-4841-BB and F5TZ-4841-EB are available for '92-'96 F-series Fords. The difference is in the size of the original yoke. These revised yokes use a high-phosphate nickel coating on the splines. The coating improves the ability of the splines to smoothly slide in and out. It's also a good idea to install either new U-joint. Fords produced after August 1996 were equipped with the revised slip yokes and U-joint assemblies.

Thin Air
Q: I drive an '02 Chevy Silverado LS longbed, which I bought in early 2003. The truck has approximately 30,000 miles on it. I use the truck for work, so I haven't modified it. I build and install custom sound systems for homes, so I carry some weight in the bed. My problem is that two of my tires seem to have trouble holding air. The tires are on the left front and right rear, so it's not as though I'm overloading the rear tires. I keep the tires at the factory-recommended inflation pressure. I used to check the tires once a month, but since I noticed the leakage problem, I check them about once a week. I had the tires checked for nails or road debris and didn't find any. I used a soapy-water solution to see if there might be leaks around the valve stems, but there weren't. The tires still have plenty of tread left, so I'm reluctant to buy a new set of tires if I don't need them. I'm out of ideas, so I'm hoping you can help me.
Chad Wozniak
via e-mail

A: You didn't say what type of wheels you have, but judging by your problem and the fact that you state the truck is a totally stock work truck, we're assuming that you have the factory 16x6.5-inch steel wheels. A technical service bulletin regarding problems with these wheels has been issued. TSB 03-0310-001A addresses possible cracked welds. Such a problem would explain your steady air loss. This problem affects '99-'03 fullsize Chevy and GMC trucks with these wheels. In more severe cases, there have been reported incidents of wheel shimmy or vibration. Any truck with those symptoms should get the wheels checked immediately. The solution is to replace all the wheels with a revised rim. Your truck should still be under warranty, so discuss this problem with the service manager at the dealership where you bought the truck.