Question: I recently got into a discussion with a friend about the volatility of fuels. He claimed that diesel fuel is highly volatile but gasoline is not. I told him it's the other way around. He said he knows that gasoline isn't volatile because his cousin Bubba puts out his cigarettes in a can of gasoline. I responded that what Bubba does sounds dangerous and highly unlikely. Now I'm afraid that my friend will try to duplicate Bubba's stunt. What should I advise him? — Ken
Ray: Advise him to take out a nice, large life-insurance policy on Bubba, naming you and him as co-beneficiaries, Ken.
Tom: Here's why: The burning tip of a cigarette is somewhere between 750 and 1,100 degrees Fahrenheit, depending on whether you measure it on the side or in the middle, where it's hottest. The ignition point of gasoline is only 500 to 860 degrees F. So you do the math, Ken. Cigarette + gasoline = kaboom!
Ray: The truth is, it is possible to throw a lit cigarette into a bucket of liquid gasoline and have the cigarette go out. If the gasoline and/or ambient temperature is cool, and the flame is extinguished quickly, you might luck out. But here's why you're taking an awfully big risk . . . .
Tom: Since gasoline is highly volatile (and you are correct, Ken; it's more volatile than diesel), it's continually giving off gasoline vapor. And gasoline vapor has a lower ignition point than liquid gasoline. So if your lit cigarette spends an extra half a second in the gasoline vapor before getting into the liquid, or if it floats for a moment on top of the liquid before being extinguished, it could much more easily ignite the vapors, which would then ignite the whole thing. And that could lead to tragic — or in Bubba's case, Darwinian — consequences.
Ray: Diesel fuel is far less volatile, and it won't vaporize at room temperature. In fact, diesel is the same as home heating oil — and it was chosen as a home heating fuel because of its relative safety. So you won't see diesel vapors igniting in the same way as gasoline vapors.
Tom: But still, I would strongly — make that very strongly — suggest to your friend that the surgeon general has determined that smoking, in and of itself, is a bad idea. And extinguishing your cigarettes in any kind of flammable liquid is a pretty clear demonstration of world-class stupidity.
Ray: And just to be on the safe side, Ken, since your friend might share some genetic material with his cousin Bubba, if you ever hear your friend utter the words, "Hey guys, watch this!" run like hell.
Question: I have a 1996 Jeep Cherokee that I bought used last year. Recently, it has developed a shimmy at speeds over about 50 mph, and it feels like the tires are bouncing all over the road. On gentle turns, it feels out of control, like it is going to roll or leave the road. I've taken it to a tire-and-alignment place, where the mechanic said I simply needed to have my tires balanced. That didn't fix it. This really scares me. I'm a new driver (I'm 17), and I don't need anything else to worry about while driving. — Alex
Ray: This is not something you want to fool around with, Alex. A front-end vibration could be anything from a bad tire to a wheel getting ready to fall off. Do you want to wait and see which one it is?
Tom: We didn't think so. If you think the alignment-and-tire guys are good guys, you can go back to them and tell them that the wheel balancing didn't fix the problem. Have a mechanic drive the car with you, so you can say: "See? You feel that? That's the problem I'm talking about."
Ray: It could be many things, Alex, or a combination of things. For instance, the shimmy could be caused by a bad tire or a bent wheel from that curbstone you hit last summer (you forgot about that, didn't you?).
Tom: And the "bouncing all over the road" could be the result of severely overinflated tires. Some new drivers think that if 30 psi is good, then 45 psi must be better. That would cause very scary handling in a Cherokee.
Ray: Bad handling could also be explained if you had put the vehicle in four-wheel drive at some point and then forgot to take it out. The Cherokee has an old-style four-wheel-drive system that cannot be driven on dry pavement. Why? Because on dry pavement, it causes dangerous and erratic handling!
Tom: And if you happen to have a seized universal joint in your front axle, too, that could cause a shimmy when you're in four-wheel drive. So check those things first.
Ray: But it's just as likely to be a bad ball joint, a bad tie-rod end or something else that could lead to a wheel falling off. So you absolutely want to have a competent mechanic inspect the whole front end and find out what's going on under there.
Tom: You're right to be concerned, Alex. And you want to fix this — whatever it is — right away. And get back to your Cherokee's normal, slightly safer, lousy handling.
The Magliozzi brothers' radio show, "Car Talk," can be heard Saturdays at 10 a.m. and Sundays at noon on KUER FM 90.1, and on KCPW 88.3/105.1 FM Saturdays at 9 a.m. and Sundays at 10 a.m. If you have a question about cars, write to Click and Clack Talk Cars c/o King Features Syndicate, 235 East 45th St., New York, N.Y. 10017. You can e-mail them by visiting the Car Talk section of the Web site www.cars.com.