You can learn much about your vehicle’s health by periodically examining its engine oil, coolant, automatic transmission fluid, brake fluid, and battery fluid.
On older vehicles, oil can turn black between oil changes without any engine problem, but if it happens quickly, suspect a fuel system problem. On newer vehicles with lean burning and highly efficient combustion, oil will usually stay quite clean between recommended change intervals. If the oil turns black quickly, suspect a problem with the fuel injection or emission systems.
If the oil has a milky appearance when cold, it probably is just normal condensation that will go away when the engine is warmed up. If it remains when the engine is completely warmed up or is excessive, suspect coolant leaking into engine indicating internal engine problems like a cracked block or blown head gasket.
When the oil is changed look at the bottom of the drain pan and on the drain plug. If metal bits and chips are present, it could be a sign of internal mechanical problems. Smell the oil; a gasoline smell indicates a fuel system problem.
Engine coolant is usually greenish in color, though some of the new environmentally friendly and long-life coolants may have different colors. In any case, the coolant could be clean. Look for rust, scale, and crud, which usually can be corrected with a system flush and coolant replacement. If you see blobs of oil in the coolant, engine oil is probably leaking in from the engine, a sign of a more serious problem.
Automatic Transmission Fluid
Place a couple of drops of warm fluid on a white paper towel and wait a minute. If the fluid spreads and is light brown, red, or pink in color it is still good. AFT does get a little darker with use. However, if the spot remains small and is dark brown or even black, oxidation and varnish is a problem. It is definitely time to replace the fluid and have the transmission checked out. Smell the fluid. If it has a slight burnt odor, this is probably normal. However, if it has a strong odor like rotten eggs or burned electrical parts, there is definitely a problem. A milky pink look probably means engine coolant is leaking into the transmission, a condition that should be corrected immediately.
Used and worn-out brake fluid is black and should be changed. Indeed, brake systems should be flushed and replaced every two years. Moisture in the fluid could result in boiling and brake fade during hard braking.
If you have to add water to the battery frequently, suspect cracked cells or an excessive charging rate and a need for a charging system checkup. Other indications of charging system problems is acid buildup on the outside battery surfaces and on terminals.
To find the source of a suspected fluid leak, put clean newspaper, cardboard, or an old sheet under the car while it is parked. Examine for drops and puddles after a period of time. Not only can you determine the location of the leak, but the fluid type—brown or black for engine oil, reddish for ATF, green for engine coolant, etc. If the leak only occurs when the vehicle is warmed up, let the engine run, or better yet, drive for a while before parking over the newspaper, cardboard, or sheet and observing.