Mechanic’s Tale: Bad Vibes

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Mechanic’s Tale: Going, Going, Gone! by Douglas Flint (1/17/2005)
Auction hunting sounds good – until the bills roll in.


My formative years in auto repair were in the engine performance area, and it was there I discovered a lot of people — customers and mechanics — could not tell or did not understand the difference between an engine misfire (an abrupt jerking accompanied by a loss of power) and a vibration that might come from any number of places on the car. One of my favorite characters was an old-school mechanic who resembled Mayberry deputy Barney Fife on crack cocaine. He ran a family transmission shop and would frequently show up at my door seeking help with a car because “it’s got a miss.” More often than not it was a vibration, and I would do my best to help him solve it. By the time he retired he knew the difference between a miss and a vibration and I knew a whole lot more about transmissions.

Shake, rattle and roll

The biggest problem I had in those early years were customers brought up in the Sixties, the peak era of smooth-idling American V-8s, who sincerely believed if the radio antenna vibrated while the car was idling it was missing and out of tune. A word about engine design: the more cylinders you have, the smoother they run, because of decreased time between power strokes within a circle. Running a tad rich at idle helps too. That’s why Cadillac and Lincoln introduced V-12 and V-16 engines during the 1930s, competing for the smoothest engine. But it’s pretty hard even today to beat the smoothness of the well-designed American V-8s of the Sixties.

So when the customers brought in their Eighties-era V-6s, which were often created by hacking two cylinders off a V-8, thus causing them to have an uneven firing sequence and run lean rather than rich, they had a hard time accepting the natural vibration these engines had, particularly when idling in gear. Thankfully the engines improved somewhat. But more important, they started mounting them in extremely big soft rubber mounts. Frankly, a rod could be hanging out the side of the block and you wouldn’t feel it from the driver’s seat. Also, more people drove four-cylinder cars and even the best of them vibrate a little at idle, so people came to accept it. But not all vibrations are normal, and they are not all from the engine.

Vibration salvation

There are many simple tests to narrow down what area of the car a vibration is coming from. If a customer comes to me with a vibration that occurs at a certain speed, say 45 to 55 miles an hour, I will first try to duplicate the vibration. Once I have it vibrating I will put the car in neutral and flip the ignition switch back far enough to kill the engine but not far enough to lock the wheel. If the vibration continues I can be pretty sure it’s associated with the rolling of the wheels — wheel balance, a bent rim, or a bad tire.

Severely warped brake rotors can give you a good vibration even if you’re not applying the brakes. If the vibration stops when the car is in its “dead stick roll” then it is likely to be in the powertrain (engine, transmission, drive axles, etc). On a rear-wheel-drive platform such as a pickup truck, a frozen U-joint or bent drive shaft will give you a shudder that shakes the whole vehicle. Typically it’s amplified when the vehicle is in the highest gear. There is less slack in the drivetrain to hide it. On a front-wheel-drive platform a bad axle may feel more like the car has an egg-shaped wheel on it if the wheel is turned.

If a vehicle has a hard heavy vibration at idle that goes away as soon as the car starts driving, I look for a bad motor mount. The motor mounts are typically blocks of rubber that isolate the engine from the frame and body of the car. If worn, severely compressed, or eaten away by oil leaks, they may allow the engine to touch the frame at idle, transmitting the vibration. When you drive, the torque (twisting force) causes the engine to rise slightly and the vibration disappears. If your car vibrates a lot when idling with the transmission in drive, put it in reverse. If the vibration goes away you’ve got a bad mount. A motor mount can also split perfectly, with half the rubber staying on the frame and half with the engine. At idle it will still have the desired cushioning effect but step on the gas hard and the engine will leap up eight to twelve inches, causing a vibration, a clunk, and a tug on every vital hose and connection.

Transmission mounts behave in a similar fashion but are not quite so dramatic. Every now and then you get a car with an intermittent pitch vibration resonating through the body. Then I look at the exhaust system for a pipe or component just barely kissing the body or frame, transmitting the engine vibration. It doesn’t take much. In some cases the pipes miss the body by only a half inch, so a pothole or backing into a snow bank or even the tiniest bumper-to-bumper nudge can do it.

And last, if a car vibrates or shudders just once between 30 and 45 miles an hour it is most likely the torque converter in the transmission locking. It may be correctable with fluid additives or even reprogramming the computer to lock at a higher speed.

Learn to love it

If you have a ten-year-old Jeep with a minor vibration, have it checked. But if nothing’s found, don’t lose any sleep over it. There are an awful lot of things spinning this way and that down there, and a Jeep is really not designed for smoothness. Caravans from the Nineties pick up odd vibrations too. Since a van from that era is basically a shoebox with four wheels and not much frame, it doesn’t take much to set off a vibration.

I remember one that had an awful vibration. Since I owned the same vehicle, I switched out everything from mine to hers in an attempt to find the problem. Wheels, CV axles, motor and trans mounts, brake drums and rotors. I never found the source, but I suspect it was in the final-drive part of the transaxle. If it ever fails completely, as most usually do, we will find out.

Sometimes that’s the only way to know for sure.


Doug Flint owns and operates Tune-Up Technology, a garage in Alexandria, Virginia.




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