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Fluids/Oil - Consistency and Colors for Diagnosis -- gives causes general text below
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 LIGHT BROWN OR BLACK STAINS - MEDIUM CONSISTENCY Ah, you're thinking, "That's easy ... engine oil!" You're right, we'll  start simple and stay simple -- we're all familiar with this the brown/blackish color of motor oil. Because of the oil system has many gaskets, lots of  heat (weakens seals and gaskets over time), and lots of pressure,  engine oil tends to be a frequent visitor. While a little oil seepage is not a  problem, any drips should be investigated. Pay particular attention to any lines, filters, or coolers external to the engine. Replacing an O-ring or  pressure line that is starting to leak is a wise move, as it offers peace of mind. Engine gaskets that leak need careful assessment -- they are not likely to fail completely and may be damn difficult to replace in-situ. Note: Did  you add new oil to the engine recently? Sometimes a simple mistake can lead to oil spills appearing later. Oil leaks that have continued for some time should lead to the buildup of dirt -- can you see any on your block or  sump?

 REDDISH, LIGHT BROWN OR BLACK - THINNER OR THICKER CONSISTENCY Stains, drips or puddles that look similar to engine oil, but are thicker  to touch and located near the center of the car, are usually from the transmission. While automatic transmissions used to use a reddish fluid thinner than engine oil, recent  units  employ a reddish fluid that is thicker. Note: overfilling the transmission with fluid seems to be a  common source of leaks -- check the temperature/ volume relationship.  Actual  leaks are most likely to come from the clutch side shaft seal or the output  shaft seal. Leaks are also possible from the pan gasket. Some engines (M70) have external coolers with connection lines and these can also leak.

 REDDISH OR LIGHT BROWN - THIN CONSISTENCY Sound like a familiar fluid? It should. If your car is equipped with power steering, then it has a fluid in the steering system that is either identical or similar to your car's automatic transmission fluid. Luckily  the power steering system is localized so any leaks should be easy to spot.

 CLEAR, SLIGHTLY BROWN AND SLICK - MEDIUM CONSISTENCY This is that critical fluid for your brake hydraulic system. When brake fluid is new, it is usually either clear (like mineral oil) or tinted slightly brown. But after time and with the accumulation of dust, rust particles and other contaminants, it can turn entirely brown. What you  need to remember is that brake fluid is characteristically slippery to the  touch -- more so than engine oil or automatic transmission fluid.

 DIFFERENTIAL FLUID - THICK CONSISTENCY Your car's differential uses a fluid that is similar to the standard transmission fluid. The key to distinguishing the fluid origin is where is drips. If it drips from the rear of your car, then the chances are that it is from the differential. Since this fluid is not pressurized, leaks indicate that something is significantly loose.

 YELLOW, GREEN PINK AND SLIMY - THIN CONSISTENCY Here it is: coolant. Slimy to the touch and one of the easiest fluids to identify, coolants come in a range of bright colors. Older cars simply dumped the coolant on the road or driveway when the engine got too hot. Of course, overheating maybe a sign of some other problem. Today's cars have closed systems that capture and recycle any blow-off. So, if you have a  late model car you shouldn't ever see coolant on your driveway or on your  carpet. If you do, it's a sign of potential trouble (e.g., leaky water pump or hoses).

 CLEAR, SHINY LIQUID - VERY THIN CONSISTENCY ... LIKE WATER? If you ever come back to your car in your driveway or in a shopping center parking lot and see a puddle of clear water under the front and on the passenger side, you don't need to panic. But, if you've been running your car's air conditioner, then the spot is most likely water that has  condensed on the air conditioning hardware and is harmless. The air conditioner has  a water drain tube to drip this innocuous condensed water out to the road -- and there's no problem. If all fluids were only so innocent!