How to Diagnose Vehicle Engine Leaks

By Deanna Sclar from Auto Repair For Dummies, 2nd Edition

Pay close attention to leaks from your automobile's engine. Running a vehicle that’s drastically low on a vital fluid can cause severe damage. After you find the source of the leak, the following information will help you decide whether you can handle it yourself or you need professional help.

If water is getting into your vehicle’s passenger compartment, check the rubber gaskets and weatherstripping around the windows, doors, and sunroof.

Unless your vehicle has a protective shield under the engine area, here’s an easy way to see whether anything is leaking out from under your vehicle and a few pointers to help you decide what to do about it:

Park your vehicle overnight on a clean patch of pavement or a large, clean piece of white paper.

Either tape some sheets of paper together or buy a roll of plain white commercial wrapping paper. Newspaper is too absorbent and can change the color of the stains.
Place marks on the paper.

Show where each of the four wheels is resting, and indicate the front and rear ends of the vehicle.
In the morning, move the vehicle and look for small puddles or traces of liquid on the ground or paper.

Touch and smell each puddle or trace of liquid.

Here’s how to decipher the evidence:

If it’s clear, watery, and under the air conditioner: It’s probably normal condensation if you’ve used the air conditioner recently.
If it’s black or dark brown, greasy, and located under the engine area: It’s probably oil. Figure out which part of the vehicle was over the spot. Look under the hood around the oil filter and the engine and under the vehicle for leaks around the oil drain plug, the crankcase, and the oil pan below it.
If it’s thick, black or tan oily liquid: Gear oil may be leaking from a manual transmission, the differential, an axle, or the steering gears. Any of these leaks needs immediate attention.
If it’s red, pink, or reddish-brown and greasy and you have an automatic transmission: It’s probably transmission fluid. Check the transmission dipstick, and if the level is low, top it off with the proper transmission fluid. Then check the dipstick again in a day or two. If it’s low again, have a professional check the transmission to make sure that the seals are intact.
If it’s watery or slippery; green, red, blue, or yellow; and is coming from under the radiator or engine: It’s probably coolant. Check the radiator, pressure cap, engine, and hoses for leaks.
If it’s oily; pink, red, or clear; and you find it toward the front bumper (usually on the driver’s side): It’s probably power-steering fluid. The power-steering system is sealed and shouldn’t lose fluid.

If it’s a light-colored or clear fluid: It may be brake fluid. Even if the leaks have dried, the stains should be visible. Leaky brakes are too dangerous to leave unattended. Have a professional repair any brake fluid leaks immediately.
If it smells like rotten eggs: It’s battery acid. Avoid getting it on your hands or clothes and have the battery replaced.
If it smells like gasoline: It probably is! If the smell is coming from under the hood, check around the fuel pump and the fuel injectors — or the carburetor if your vehicle has one. If the leak seems to be under the center of the vehicle, check the fuel lines. If it’s under the rear end, check the fuel tank. (Don’t smoke while you do this!)