Red means stop and green means go — but what are the colors under the hood of your car trying to tell you?
Engine oil
Fresh oil is usually light to dark amber and translucent. Like honey. This is how the oil should appear on the dipstick immediately after an oil change. If you paid someone else to do this job, pull the dipstick and check before you drive off. If the oil on the dipstick is not light to dark amber, the color of honey, they may not have changed it.
Over time, oil will turn darker (brownish) in color due to contaminants being held in suspension. This is normal. The oil is doing what it’s supposed to be doing.
When the oil has turned black, it is probably time for an oil change — or getting close to it.
Watch out for: Milky or cloudy-looking oil. This is a clue that water (engine coolant) is getting into the oil, possibly from a leaking intake manifold gasket or more serious problem such as a failing head gasket.
Get the car to a shop ASAP.
Automatic transmission fluid
Most, though not all, automatic transmission fluid is reddish in color when fresh and like engine oil, gets darker over time. But unlike engine oil, it should never look brown or (worse) black. If it does, it is likely the transmission overheated at some point. This is your cue to have the car checked out. A running-hot transmission will usually be a short-lived transmission.
Watch out for: Burnt smell or small metallic shavings held in suspension in the fluid or present on the dipstick. Wipe the (cool) dipstick between your thumb and index finger. If you feel any grit, it is very likely the transmission will need work — possible a major overhaul — in the near future. A burnt smell is a clue the transmission is running hot. Get it checked as soon as possible.
Engine coolant
Coolant (anti-freeze) used to be neon green in color but many late-model cars also use so-called “long-life” coolant that can be orange in color. What you want to be wary of is coolant that is brown or oily-looking. If the coolant just looks dirty, all the system may need is a thorough flush and refill with fresh coolant. If there’s oil in the coolant, it could be a more serious problem that might involve major work to the engine as well as the cooling system. Look out for air bubbles in the coolant, which you can check for by removing the radiator cap (engine cold!) and then running the engine for a few minutes while you watch the filler neck. Bubbles in the coolant are a clue that an intake manifold or head gasket may be leaking.
Watch for: The manufacturer’s recommendations about which type of coolant to use. If your car came from the factory with the orange-red colored long-life coolant, it may not be ok to top off or refill the system with standard green coolant. If it’s an emergency and nothing else is available, use straight water — ideally distilled water.
Have the car serviced as soon as possible.
Crankcase breather element
In addition to the air filter, inside the air cleaner assembly in most late-model cars you will find a small, replaceable breather element. It filters the air that’s normally sucked into the engine’s crankcase as part of the Positive Crankcase Ventilation (PVC) system. It’s normal for this small filter element to look dusty and dirty — and when it does, it’s probably time to replace it. What’s not normal is for the breather element to look black and be sopping wet with oil. If it is, there may be a problem with the PCV system (minor and easily fixed) or the engine might be getting tired; oil is leaking past piston rings up into the air cleaner assembly.
Many owners forget to service this small but important little filter. Check its condition at least once a year and replace the breather element when indicated.
Brake fluid
Like engine oil, brake fluid looks clear to honey-colored when fresh and gets darker over time, as contaminants build up. Unlike engine oil, the contaminants in brake fluid don’t get caught by a filter.
Brake fluid also tends to attract water, which can rust out the lines (and master cylinder, if it’s metal) from the inside. This is why it is especially important to keep track of the condition of the brake fluid and have it changed out as necessary (usually, once every 3-4 years at least). Otherwise, you risk damaging expensive components. Braking performance will also be reduced, especially if any air has gotten into the system.
Be certain to use only the correct type of fluid for your vehicle (e.g., DOT 3, 4 or 5) as specified on the master cylinder cap. Use of incorrect/incompatible fluid can cause problems you don’t want.
Watch for: Spongy or soft pedal feel. This indicates low brake fluid level, or air in the system — either of which are potentially dangerous. Reduce your speed and leave extra stopping room between your car and other vehicles, just in case.
Have the car looked at a soon as possible.
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