Most people know not to stick their fingers through the bars at the zoo -- though some don't and get to learn from the experience. Likewise, there are things you should never do to your car or truck that can have consequences every bit as unpleasant -- and sometimes as permanent -- as offering your succulent fingertips to a hungry rhesus monkey.
Sunny day car wash: Dousing your car with water in bright sunlight can be ruinously bad for its finish. First, the sun heats up the metal to the point it's scalding hot to the touch. Then, you add some nice tepid tap water -- which is immediately boiled off the finish, along with caustic soap and whatever grime you didn't clear off -- while any remaining rivulets of moisture act like a magnifying glass, amping-up the power of the sun's murderous rays. The end result is not pretty -- and cosmetics aside, you've cut into the resale/trade-in value of your vehicle by flailing its finish. New cars with base/clear-coat paints are especially vulnerable to sun damage, because once the very thin clear topcoat is burned away or otherwise damaged, the paint will never shine again, no matter how hard you wax and polish. Only an expensive repaint will fix things. It’s far better to wash on cloudy, overcast days -- or at least in the shade, away from direct sunlight. A great time to wash a car is just after dawn -- and in the late afternoon, just as the sun is slipping past the horizon.
Pressure washing a modern car's engine: A grimy engine that runs properly is much better than a clean engine that won't -- which is what you risk if you force jets of water past rubber seals and into the sensitive electronics that are fitted to all modern, computer-controlled engines. There's a reason for the hood (and all those protective coverings). They're there to prevent moisture and contaminants from wreaking havoc with the sensors, wiring harnesses, sending units and other components that like to be sprayed with water about as much as your typical house cat. Excess moisture can short-out electrical parts, cause intermittent malfunctions (including stalling for no reason, hard-starting, rough-running) and make dashboard "check engine" light flash (or stay on) for no apparent reason. It's OK to degrease your engine with a garden hose if you like to keep it clean; just don't use high-pressure sprayers like you find in self-serve car washes as they can force water past rubber seals and weather stripping into places it isn't supposed to go.
Overloading the alternator/charging system: Teens used to be into exhaust headers and big Holley carburetors. Now they're into boom-boom audio systems -- bass reflectors and subwoofers that take up most of the trunk and create enough racket to be heard in a different time zone. Aesthetics aside, a common problem with installing this kind of gear is overtaxing the factory-installed alternator and charging system, which may not be able to handle the additional demand. What typically happens is the overtaxed alternator fails to keep the battery charged -- so it rapidly drains and the car can't be started one morning. Frequent replacement of the battery becomes necessary -- but that only crutches the problem. The alternator itself eventually fails prematurely due to the excess loads -- an expensive part to burn up for no good reason. And sometimes, the car doesn't run right -- or won't run at all -- because there is insufficient voltage to operate the electric fuel pump, fuel injection system and other components because of the excess "draw" of aftermarket audio gear. Those planning on putting in a monster stereo (or any high-load aftermarket electronics) should check whether they ought to also install a high-output alternator that's equal to the job. It beats having to buy a new battery every month.
"Universal fit" wheels: Replacing the wheels that came with the car is a popular way to personalize a vehicle. But don't make the mistake of buying "universal fit" wheels that are designed to fit multiple vehicles using shims and "make it fit" bolt patterns. This can be extremely dangerous, yet people do it all the time. Automotive wheels are not like generic aspirin; they're very specific to the application -- and it's critical that such things as backspacing and bolt pattern be correct for your particular vehicle. Shims of any kind are an extreme no-no. And don't try and fit metric rims on a car designed for no-metric -- and vice-versa. Before you buy any non-factory wheel for your vehicle, consult the manufacturer to make sure it was designed to fit. Use the correct lug nuts, too. (It's often the case that you must swap them along with the wheels for the changeover to be safe.) A good tire shop can be of assistance here.
Tow an automatic-equipped car with drive wheels down: If you want to destroy your automatic transmission or greatly reduce its service life, a fast way to do it is to allow the vehicle to be towed with its drive wheels down. An automatic transmission uses hydraulic fluid under high pressure to transfer the engine's power to the road. The hydraulic fluid also lubricates the transmission's internal parts -- but only when the transmission's torque converter is being turned by the running engine. When the engine's not running -- as when the vehicle is being towed -- the torque converter isn't pumping pressurized fluid through the transmission, so there's no lubrication. But if the drive wheels are down and turning, the transmission is being "run" -- without proper lubrication.
It's like running the engine without an oil pump, and the results can be just as ugly. Therefore, if you are about your automatic-equipped car or truck, insist on a rollback truck if you need a tow. Instead of dragging the vehicle, the entire car is winched aboard the rollback, tightly secured and carted off. More and more towing companies are using rollbacks rather than old-style tow trucks because they're safer -- and limit the potential for damage to the towed vehicle.