Power Steering Fluid and its Purposes

Power steering fluid reservoir of a car showing the min and max fill lines

Like brake fluid, power steering fluid is a hydraulic fluid. Its primary function is to transfer force to a hydraulic piston in the steering rack to assist in steering the car. It is used in most vehicles that have hydraulic power steering systems. Cars with electric-assist power steering do not use it.

Some cars use automatic transmission fluid (ATF) as the power steering fluid. When this is the case, the power steering system usually (but not always) uses the same flavor of ATF that the car uses for the automatic transmission.

As with all automotive fluids, it's critically important to use the proper fluid for your vehicle. Using the wrong fluid in your vehicle's power steering system could cause damage ranging from premature wear to sudden, catastrophic failure of the power steering system. Also avoid power steering fluid "sealants" or "leak stop" additives until a qualified mechanic has checked out the system. Some leaks in power steering systems can be stopped by rejuvenating the seals, but others cannot. Let a mechanic take a look at the problem.

You can check your car's owner's manual or the filler cap on the fluid reservoir to determine the proper power steering fluid for your car. If all it says is "Power Steering Fluid," with no additional specifications, then you can use any standard power steering fluid (including synthetic power steering fluid).

Synthetic Power Steering Fluid

As is the trend with automotive fluids in general, synthetic power steering fluids are starting to replace fluids based on mineral oils. The benefits to using synthetic power include:

A lower pour point, which improves initial steering response and provides better initial lubrication of the steering system on very cold days.
Better temperature stability provides more consistent steering response regardless of outside temperatures.
Better lubricity reduces wear in the power steering pump.
Better dynamic viscosity (that is, it can be pumped through the system more easily) usually results in easier steering.
You may experience slightly better fuel economy due to reduced drag on the engine from the power steering pump.

Adding synthetic power steering fluid to mineral-based power steering fluid when the level is low won't hurt anything, but it won't result in much benefit over using mineral-based fluid. To experience the maximum benefit of synthetic power steering fluid, you need to drain the existing fluid and replace it with synthetic. If that's not something you want to do, then there's little reason to use synthetic for top-offs unless that's what's already in the steering system.

How to Check and Maintain the Power Steering Fluid Level

Most power steering fluid reservoirs are translucent, as in the picture on the top of this page, so you can check the fluid without removing the cap. You just eyeball the colored fluid through the reservoir. Other reservoirs have dipsticks built into the cap, and you check them the same way you would check the engine oil. In either case, the power steering fluid should between the MIN and MAX levels, which will be denoted by words, crosshatching, or pictures.

Many power steering reservoirs, such as the one in the picture on the top of this page, will have two sets of marking labeled HOT and COLD or using pictographs to suggest hot and cold. This is to compensate for thermal expansion of the fluid at high temperatures. If the car is at operating temperature when you check the fluid, then use the HOT markings. If not, then use the COLD markings.

I recommend checking the power steering fluid level weekly (along with all the other fluids under the hood) or more often if you know it's leaking.

Power Steering Fluid Leaks

A properly-functioning power steering system with no leaks should rarely, if ever require fluid to be added. A very tiny amount of loss may occur at the piston seals and be considered normal; but once the loss is enough to cause a noticeable fluid level drop in the reservoir that requires adding fluid more often than once a year or so, it qualifies as a leak. Power steering leaks usually occur either in the pump, the hoses, an O-ring, or the seals behind the rubber boots on the power steering rack.

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If the leak is in a hose, then the leaking hose must be replaced. Trying to repair a power steering hose is usually a waste of time. Once the hose has started to deteriorate, it will just sprout a leak somewhere else. In fact, I recommend replacing all the power steering hoses if one starts to leak.

If the leak is coming from an O-ring, you can try just replacing the O-ring. It usually works.

If the leak is in the piston seals (in the rack), then a power steering stop leak product like those made by Lucas or ATP may solve the problem quite inexpensively, possibly for a long time. Replacing the power steering rack would be a better solution, but it's a fairly difficult and usually expensive job. At a minimum, have a mechanic at least look at the system before trying additives.

If the leak is in the power steering pump, then stop-leak products are much less likely to work. You can give it a try, but I wouldn't get my hopes up. Fortunately, on most cars, replacing the power steering pump isn't nearly as hard a job as replacing the rack; so check the car's service manual or look for a YouTube video by someone who did the job on your make and model car. On most cars, replacing the power steering pump is not that difficult a job.

Changing the Power Steering Fluid

Power steering fluid doesn't last forever. Over time its additives wear out, the contaminant levels increase, and debris from worn seals enter the fluid.

Unfortunately, many vehicles don't have a specified power steering fluid change interval, and professionals have vastly different opinions on the question of how often it should be changed. My personal rule of thumb is that the fluid should be changed if it has turned brown, smells burnt, has visible debris in it, is more than three years old, or has been in use for more than about 45,000 miles (72420 kilometers).

Before changing your own power steering fluid, check the car's service manual for specific instructions. The process explained here is general in nature. Also, this is the kind of job that you may want to have done professionally simply because it's usually not a very expensive job. It's one of those jobs that's a lot easier to do if you have a lift.

If you do decide to change your own power steering fluid, make sure to observe good safety practices. Use jack stands if you have to get under the car, wheel chocks to keep the car from rolling, chemical-resistant gloves, and eye protection.

Steps in a Typical Power Steering Fluid Change

If the car can be driven, drive it around for a few minutes, making a few turns to warm the fluid.
Park and shut down the car and loosen the power steering fluid reservoir cap.
Unless the system has a drain valve or plug (which is rare), locate the lowest hose connection in the system.
Wipe the outside of the connection and the immediate area with a shop rag to remove any oil, grease, or other debris.
Disconnect the hose at the lowest point and let all the fluid drain out into a oil drain pan. If it's convenient to disconnect two hoses, then better yet.
When all the fluid has drained, turn the engine over for no more than a second or two to move any fluid out of the pump. Let any remaining fluid drain out into the drain pan. Most municipalities consider spent power steering fluid to be engine oil for recycling purposes.
Reattach the hoses hand-tight and then tighten them to the manufacturer-specified torque using a torque wrench.
Fill the reservoir to the MAX line (use the HOT MAX line if the reservoir or dipstick has one) with the appropriate fluid and start the car.
Turn the steering wheel from lock to lock several times, then shut the car down.
Repeat steps 8 and 9 until the fluid level in the reservoir no longer drops.

A few cars have bleed valves in the power steering systems that will have to be bled; so again, be sure to check your car's service manual.

It's not unusual for the power steering fluid level to drop for a few days after changing the fluid, so check it frequently and add fluid as necessary. If it keeps dropping, however, then you most likely have a leak.