A dipstick is a measuring tool used to gauge the amount of liquid in a container. Petrol stations use very long sticks to check underground fuel tanks, but the dipstick is best known in connection with the automobile.

Built-in dipsticks are designed so that when a car, lawnmower, tractor, etc is on level ground the stick will be partially immersed in the fluid that is to be checked. Marks on the stick show the safe operating range of the fluid. Information is sometimes stamped on the stick indicating fluid type or instructions.

People may also be referred to as a dipstick meaning ignorant or stupid, such as: 'That dipstick ruined his car 'cause he never checked the oil!'

Identifying the Dipstick

The most well-known dipstick is used to check the engine oil. It will commonly have a handle that is black, yellow, grey or unpainted metal. The colour of the automatic transmission fluid (ATF) dipstick handle is usually red but can be unpainted metal. Use caution not to confuse the two. If neither has a colour you may be able to tell whether the stick is positioned on the engine or the transmission. The ATF stick on a rear wheel drive automatic is often accessed near the firewall via a long tube. A last resort is to examine the fluid on the stick. ATF is red or brown with a strange smell while engine oil is usually black or dark brown. Autos with a manual transmission sometimes have a smaller stick that checks the gear oil level, though not all cars use this.

Finding the dipstick can be difficult especially in older cars or when the engine is very dirty. Designers of machinery tend to put the dipstick in a place that is not too difficult to access by hand and is not near dangerous moving parts.

It may help to take a step back and look at the bigger picture when a dipstick is especially hard to find. Some vehicles are designed with a remote fluid reservoir that may house the dipstick in an unusual place.

If the dipstick has been lost, the tube where it should be may be hard to find. The hollow tube sticking out near the bottom of the engine must then be found and the stick replaced. The vehicle should not be run without the dipstick in place because dirt or water may enter the oil and cause damage.

Anatomy of a Dipstick

Dipsticks usually have three main characteristics: the handle, the cap and the measuring stick. The handle and the measuring stick are often a single piece of metal that is curled at one end and marked with a safe range on the other. Newer cars usually have a plastic handle attached. The marks used to indicate the safe range vary from a stamped pattern, lines, notches or drilled holes on the stick. Keep in mind that if you don't see marks on the stick it may have broken. This makes it impossible to get an accurate reading and the dipstick must be replaced. The cap seals the dip stick tube during normal operation and also establishes the point from which the measurement is taken. Most caps are simple plugs or covers attached to the stick though some may be threaded into the case or a self-tightening bung.

Dipsticks are made in a wide variety of lengths that are calibrated for a very specific application. One dipstick may not be substituted for another unless they are exactly the same.

Using the Dipstick

• If there is any doubt, refer to the vehicle's manual to find dipstick locations and instructions for checking fluid levels.

• Dipsticks often sling hot oil onto even the smallest bit of unprotected skin. Gloves and eye protection help so always use caution.

• It is a good idea to run and drive the car before checking the fluids. This ensures that all filters and passages contain the right amount of fluid, and that it has reached operating temperature.

• Position the vehicle on level ground.

• Pay close attention when removing any dipstick so that it may be easily replaced in its tube. Some dipstick tubes can be tricky to find if they are lost. Take care not to let dirt get on the stick or into the dipstick tube while replacing it.

• Double- or triple-checking is recommended.

Engine Oil

The engine oil should be allowed to settle and cool for about five minutes before checking.

With the engine off, remove and wipe the dipstick with a clean cloth. You should check the dipstick for any special information, burns, strange deposits or bad smells when you first remove it. Replace the stick for a few seconds and remove. Keep the stick vertical, pointing down, so the fluid does not flow up and give a false reading. The fluid should completely wet the stick to a level within the safe range.

Often the safe range is equal to about a litre¹, but this is not always the case. When this is the case, adding a quart of oil will bring the level from the lower mark to the higher. If the level is halfway between the marks, only half a litre is needed.

Automatic Transmission Fluid

Checking the fluid on an automatic transmission involves working around a running and warmed-up vehicle. Driving is the best way to properly warm up the transmission, idling will not do it. They are usually checked running because more fluid circulates in the transmission, giving a proper reading. Be very careful.

With the engine running and the car in park, pull the stick, wipe with a clean cloth and reinsert for a few seconds. The next time you pull the stick the transmission fluid should be within the marks. Be sure the stick is fully coated below the indicated level. Residual fluid in the tube can give a false reading. Unlike motor oil the range marked on the ATF stick often refers to half a litre².

Gear oil

Manual transmissions sometimes have a dipstick to check the gear oil level. It is usually less obvious than the engine oil and deeper in the works and located on the transmission rather than the engine. The checking process is the same as for engine oil (engine off).

Adding fluids

Do not overfill fluids. It's far easier to add more than to take it out. Too much oil in the engine will start to foam, and reduce the lubricating effect of the oil, thus damaging it. Don't panic if it's a little bit under.

Be sure that the proper fluid goes in the proper hole and that the corresponding dipstick is used. Engine oil has a fill opening located on the top of the motor while ATF must usually be added through its dipstick tube using a funnel and much patience.

Other fluid-related issues

Some fluids do not use a dipstick. Brake fluid, engine coolant and windshield washer fluid can be checked through translucent reservoirs that are usually clearly marked. The brake fluid level is sometimes difficult to see when it is clean and may be hidden by the level mark. Tapping or gently rocking the reservoir can disturb the surface of the fluid enough to be seen. If removing the cap on the brake fluid reservoir, it is necessary take care not to twist any wires connected to it. Other brake systems require the cover to be removed and the level checked on marks inside the reservoir. Clean the lid before removing and don't allow dirt to get into the fluid. Make sure any gaskets or seals are intact when replacing.

Coolant on cars without a reservoir can be checked by slowly removing the radiator cap to see that the fluid is near the top. Never open a hot radiator. Often some space for fluid expansion is left in the radiator: check the maker's directions or look for a level indicator.

Axles that contain gears and some manual transmissions have lubricant that is checked through a fill-hole. A fluid drain-hole and a fill-hole remain plugged during normal use. In this case, the fluid is at the proper level when excess fluid runs from the higher fill-hole. Checking the fluid level involves removing the upper plug and observing if fluid is released or if the level is very near the fill-hole opening. The process of checking such a system often requires special tools and knowledge and is not necessary from day-to-day as long as a leak is not suspected.

Windsheild washer fluid reservoirs can be mistaken for the coolant reservoir but usually contain a blue liquid. If there is confusion, trace the connections either to the radiator/motor or the windshield squirters.

Look for leaks

It is also good practice to notice if there is pool of fluid under the car when you don't have time to check the oil. Keep in mind that a busy parking area may be visited by many leaky cars, so it may not be from yours. If you suspect it is from your car, kneel down and look for fresh hanging drops or the dreaded rapid drip. If the puddle is very large, try not to drive until the problem is found. If you suspect the car is leaking fluid, at least check the dipsticks to see if there is enough to get to a place to refill.

It is helpful to note what colour and where abouts the puddle is. Green or watery liquid is usually coolant (though air-conditioning systems drip water during normal operation). Black is engine oil while red is transmission fluid. All these will be found near the centre of the engine area, usually. The most dangerous leak will be found near the wheels or on the inside of a tyre. This may be brake fluid and could indicate a very serious condition. If you see a spot like this and/or feel an unusual softness in the brakes the vehicle should not be driven. This could indicate a loss of hydraulic brake fluid that will rapidly deteriorate into loss of brake function.

Remember, when your vehicle always leaves a little spot and one day it doesn't, the car didn't fix itself: It's probably out of oil.