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Location: West Midlands
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Always check the pavement or road for a spot or puddle of fluid beneath your car when parked overnight. If you develop this habit of checking for new leaks under the car, you can stay ahead of many problems before they become severe. You can identify the type of leak by the colour of the fluid accumulated on the spot of the leak.
Oily (Black) Fluid
Most likely to be engine oil. Check the oil level by removing the dipstick and measuring the fluid level. Refill the oil according to the mark on the dipstick and observe the level for the next few days. If you find the oil level has dropped and spot the puddle again, then your engine has an oil leak.
Oily (Red) Fluid
Most likely to be transmission or power steering fluid. Check the transmission and power steering levels by removing the dipsticks and measuring the fluid levels. Refill the transmission and power steering fluids according to the marks on the dipsticks and observe the levels for the next few days. If you find the levels have dropped and spot the puddle again, then your engine has a transmission or power steering fluid leak.
Oily (Clear) Fluid
Most likely to be brake fluid.
Green/Yellow/Red (Sweet Smelling) Fluid
Most likely to be coolant. Check the coolant level and top up with some coolant/water and observe the fluid levels for the next few days. If the coolant levels have dropped, then you have a coolant leak. Common causes of this are usually down to loose/worn coolant hoses going to the radiator/from the heater matrix, or the heater matrix itself may be blocked/damaged.
Most likely to be water from your windscreen washers or air conditioning system.
In all cases you should call your garage mechanic for a more in-depth diagnosis.
[Edited on 29-02-2012 by Ian]