Wednesday, January 30, 2008

Under hood Maintenance: Power Steering Fluid

Most cars today are equipped with a power steering system. Many power steering systems use hydraulic power. These systems use a power steering pump driven by a belt from the crankshaft. The pump moves fluid under pressure through hoses to the steering gear. The pressure is used in the steering gear to reduce steering effort. A reservoir for fluid is attached to the rear of the pump. Checking the fluid level in this reservoir is a common underhood maintenance job.

The fluid in the power steering system provides lubrication as well as the power assist. Low fluid level can cause a lack of power assist, excessive noise, and rapid part wear. The power steering fluid level should be checked at regular intervals.

CAUTION: Check the fluid level with the engine off to prevent possible injury from moving parts.

The fluid is checked at the pump reservoir with a dipstick attached to the reservoir cap. . Before removing the reservoir cap, wipe the outside of the cap and reservoir to prevent dirt from falling into the reservoir. Pull the dipstick out and note the fluid reading. The fluid should be between the "hot" and "cold" mark on the dipstick. There are hot and cold marks because the fluid expands as it gets hot. If the level is below the "add" mark, you will have to add fluid to bring it up to the correct level. You should use only the type of fluid listed in the owner's or shop service manual. Older cars use automatic transmission fluid. Special power steering fluids are used on late-model cars. Add the correct amount of fluid and replace the dipstick.

Q & A: Power steering fluid


1. My power steering feels stiff when I first start my car, but then feels normal after I've driven the car awhile. How come?

This is called "morning sickness" and has nothing to do with being pregnant. The condition is caused by wear in the spool valve housing on certain power steering racks -- notably GM front-wheel drive cars.
When the car is first started, the rack is cold and clearances in the spool valve are at their greatest. Hydraulic pressure from the power steering pump leaks past grooves worn in the aluminum spool valve housing. This causes a loss of pressure and increases steering effort. The steering feels stiff with little or no power assist. As the car is driven, the rack warms up. This decreases the clearances inside the spool valve housing, which reduces the leakage past the grooves. More pressure goes to where it is supposed to go and the steering becomes easier as power assist returns.

The "fix" for this condition is to replace the rack with a new one (preferably with a cast iron spool valve housing) or a remanufactured rack that has a stainless steel sleeve pressed into the aluminum housing.


2. Does the power steering fluid ever need to be changed?

Not normally, but it should be if the steering rack or pump are ever replaced. Under normal circumstances, the fluid in the power steering system should last the life of the vehicle (or the life of the major power steering components, whichever comes first). But as the system accumulates miles, microscopic particles of metal and rubber can buildup in the fluid. These particles can act like an abrasive and accelerate pump and gear wear, so the fluid should be changed if the original pump or rack has failed to prevent contaminating the new parts with dirty fluid.


Even though the fluid in your power steering system does not normally require changing, it's a good idea to check the fluid level periodically (say once a month or when changing the engine oil and filter).

If the level is low, add fluid as needed to bring it up to the full level (hot or cold). Then inspect the hoses, pump and steering gear for leaks. More than a few ounces of fluid in the rubber bellows of a power steering rack indicates internal wear and leakage.

Always use the type of fluid specified by the vehicle manufacturer (Dexron II or a special blend of power steering fluid).

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