It's not uncommon for people to use the wrong fluids in their cars, and the results of doing so can vary from irritating to deadly. For example, adding antifreeze to the windshield-washer reservoir might just create a slimy mess. But filling it with only water creates a good breeding ground for the bacterium that causes Legionnaires' disease, according to a study conducted by the British Health Protection Agency. This can increase your chances of becoming ill.
Here's what else could happen if you use the wrong fluids:
The brand of motor oil matters little, but its viscosity grade (10W-30, for example) is important. Use only what the owner's manual specifies. Using the wrong oil can lead to reduced lubrication and shorter engine life. If the manual says to use synthetic oil, do so. Contrary to what some believe, adding a synthetic oil to regular oil won't harm the engine, but there's also no benefit in doing so.
Some car batteries have accessible individual cells that might need replenishing with a little water to cover the lead plates. Only use distilled water, which contains no salts or minerals. If tap water is added to a battery's electrolyte liquid, it can allow minerals from the water to build up on the battery's internal lead plates, which will reduce the battery's power and shorten its life.
A car's cooling system uses a blend of water and antifreeze, properly called coolant, at concentrations (typically 50/50) designed to keep it from freezing on a cold day and boiling on a hot one. Adding too much water to the mix can make it more susceptible to freezing and boiling. That can keep the car from starting when it's freezing and cause overheating in warmer weather. Tap water could also lead to mineral buildup in the cooling system, reducing its effectiveness. Too much antifreeze in the coolant can cause corrosion, water pump failure, and increased engine wear.
Adding diesel fuel to a gasoline-powered car's tank will make the engine stumble and knock if it runs at all. Fortunately, diesel pumps have oversized nozzles, so that mistake is hard to make. Depending on the quantity of gasoline that's added to a diesel vehicle's tank, it could do little harm or it could damage the fuel pump, injectors, and other parts. If the mix-up is caught soon enough, a technician can limit the damage by draining the contaminated fuel. Meanwhile, don't run the engine.
Brake systems use hydraulic fluid that's specially formulated for the purpose. Substituting transmission or power-steering fluid, which are similar to each other, can affect the seals, damage the system, and possibly cause brake failure. Note that if the brake fluid is low, your vehicle probably needs brake-system service anyway. Either the brakes are worn or there's a leak.
Automatic transmissions must only use the fluid specified by the automaker, such as General Motors' Dexron series or Toyota's Type T. Using the wrong fluid can cause poor lubrication, overheating, and possibly transmission failure. A mechanic might not be able to reverse the damage, even by flushing the transmission. Mistakenly adding motor oil or brake fluid can also destroy your transmission.
In addition to creating the perfect environment for deadly bacteria, water doesn't clean as well as washer fluid does and is subject to freezing. Using household glass cleaners or ammonia can leave suds on the windshield, damage a car's finish, and get into the air-intake system and create a potentially noxious environment in the cabin.