Understanding Your Vehicle’s Fluids

We all know fluids are important, but let’s be honest, so long as our car or truck is running smoothly, we’re probably never even opening the hood.  Which is fine until our vehicle betrays us.

So today, let’s chat about fluids.  Your vehicle relies on several of them for various reasons and knowing those fluid’s properties and limitations can save you future headaches.  But don’t worry, you won’t have to memorize anything about them so long as you have the Great Driving Scripture of Travel – your owner’s manual.  It will tell you what kind of fluid it is, where it goes, and any specifics you need to know if you ever have to add some. 

Gloved hand holding an oil dipstick.

Motor Oil

Probably the most well-known fluid your vehicle requires is the oil that lubricates and cools the moving parts of your engine.  In modern vehicles, it actually does a lot more than that.  It also provides hydraulic pressure in many cases to activate vital components, ensuring your engine runs smoothly and optimally.

Motor oil is also the fluid that requires replacement more often than any other.  Heat and contaminants generated from the engine cause the oil to break down over time, causing the lubricant to lose its ability to do its job. If the engine is not lubricated properly, parts can break. Expensive parts. So, it’s important to have your oil changed on a regular basis.   Those intervals can be anywhere from 3000 miles to 15000 miles, depending on oil specification, manufacturer, driving conditions and habits.

Engines burn small amounts of oil over time.  Some vehicle manufacturers have even published that oil loss of up to 1 quart every 750 miles is considered “normal.”  If your engine’s oil capacity is 4-5 quarts, you may not make it to your oil change interval before serious damage can occur.  Checking your engine’s oil level on a regular basis may prevent a breakdown and/or considerable repair expenses.

Coolant

Coolant is made up of basically two ingredients – antifreeze and water.  Water has excellent cooling properties but it also freezes and boils, neither of which is helpful for your vehicle.  Adding antifreeze (ethylene or propylene glycol) lowers the boiling point of water and also raises the freezing point making it perfectly suited to keep your engine at an ideal operating temperature.  Mixture ratios can vary on climate, but most consist of a 50/50 blend.

The pH or acidity/alkalinity of coolant will change as it breaks down over time.  This can cause deterioration of metal components in the cooling system – radiator, heater core, engine block, etc.  Changing the coolant on a regular basis can prevent costly repairs in the future.  Intervals can range from 30,000 miles to as much as 150,000 miles.  

Coolant level is crucial to keeping your engine cool and running properly.  A low coolant level can cause loss of cabin heat, engine overheating, and eventually, catastrophic engine failure.  Checking your coolant level is a simple way to avoid disaster.  For the majority of vehicles, it is as simple as making sure the fluid level is between a “min” and a “max” line on a transparent reservoir. 

When adding fluid, never open a cooling system when the engine is hot as serious skin injury can occur.  If no coolant mixture is available in an emergency situation, you can add plain water, keeping in mind that it will change the mixture ratio limiting its function and your entire system will require flushing when you arrive at your destination.

Red stop sign.

Brake Fluid

Stopping is important.  Surely, we can all agree on this.  A braking system relies on hydraulic pressure being applied to the wheels in order to get them to stop.  Brake fluid is the active component that applies that pressure.  Brake fluids have different ratings and can be identified as DOT 3, DOT 4, DOT 5, and DOT 5.1.  Most vehicles will use either DOT 3 or DOT 4.  Check your owner’s manual before adding or simply look at the reservoir cap as it will likely have the required fluid imprinted on it.

Here’s the thing about brake fluid level – it’s complicated.  A brake fluid reservoir will have a “min” and a “max” line, but if its close to the “min” line you shouldn’t necessarily add fluid.  Low brake fluid will usually indicate a problem in the braking system such as a fluid leak or worn brake pads.  In the event of a leak, adding fluid is the appropriate step, however adding fluid to a system that has low brake pads will cause an over-full condition when those brake pads are replaced.  So long as the fluid is in-between the “min” and “max” lines, adding fluid is not recommended.

Brake fluid is also hygroscopic, meaning it absorbs water – moisture from the air.  This moisture will have corrosive effects on the braking systems’ metal components.  This is why manufacturers typically recommend replacing brake fluid every 20,000 - 30,000 miles.  The exception would be DOT 5 which is silicone based and non-hygroscopic.  DOT 5 is not compatible with DOT 3 or DOT 4, so be aware when adding fluid.

Transmission Fluid

Once upon a time, in a garage far, far away, vehicles would have transmission fluid dipsticks, allowing the owner to check the fluid level.  Unfortunately, manufacturers have made it much more difficult for the average owner to verify proper transmission fluid level.  In addition, there are numerous variations of transmission fluid and determining which fluid is compatible with your vehicle can be a daunting task.  The best practice when considering this fluid is to ask your mechanic to check your transmission fluid level and condition on a regular basis, such as every 15,000 – 30,000 miles.

Suggested fluid replacement intervals can be anywhere from 30,000 miles to “lifetime” fluid designation.  The definition of a “lifetime” fluid is often disputed, especially when considering driving habits and conditions.  The inescapable fact vehicle manufacturers must consider is that heat breaks down fluid and automatic transmissions generate heat.  Consequently, there is no such thing as a “lifetime” fluid.  Check your owner’s manual for suggested intervals or ask your mechanic for recommendations. 

Hands holding a steering wheel in a car.

Power Steering Fluid

If you can’t find this one under your hood, that might be because you have an electric power steering system and there is no fluid.  Many vehicle manufacturers are moving away from hydraulic power steering systems, eliminating the need for many hydraulic components.  For those vehicles that do use hydraulic power steering systems, checking the fluid level is typically done by observing the reservoir level “min” and “max” lines.  Other reservoirs will have a dipstick, usually attached to the cap.  Check your owner’s manual for clarification as well as what type of fluid to add, if necessary.

It's rare to find a manufacturer listed interval for changing the power steering system fluid.  This is because steering systems do not generate as much heat as an engine or transmission making fluid breakdown less of an issue.  However, fluid can become contaminated over time, and most experts agree that changing the fluid every 50,000 miles or every 5 years is a good rule.  Having your mechanic inspect the fluid at each oil change is another option and changing the fluid on an “as needed” basis.

Hopefully learning about vehicle fluids has made you feel more empowered. If you have any questions your owner's manual can't answer, your mechanic is a great resource!