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Normally power steering leaks will be present when the vehicle is running. The reason for that is because the system is under pressure while the engine is running. Older seals and gaskets will wear out over time and develop leaks when put under pressure, but once the vehicle is turned off, and the power steering system pressure subsides, the leak will stop. These types of leaks can often times be easy to diagnose, and fix because the leak is obvious.
A power steering system that leaks fluid while the engine is off can be difficult to diagnose because the pressure is not as severe, resulting in a leak that is not always obvious. If you find that your vehicle is leaking power steering fluid while off, it is best to first do a thorough examination of the power steering system and see if you can find any obvious leaks. If after thorough inspection is performed you cannot find where it is coming from, you will want to degrease or clean the system thoroughly in order to find the leak easier. Sometimes Ultra Violet dye will be used in conjunction with a black light in order to locate a leak.
On a rarer occasion you may develop a leak from the power steering reservoir cap when the vehicle is shut off. This type of leak will be more common after replacing a component in the vehicle like the power steering fluid cooler. The reason this type of leak develops is from too much air being in the system. What is happening is that while the vehicle is running the air is condensed and pressurized from the power steering pump and fluid being forced through many small cavities in the system, but once the system is turned off and the pressure is reduced the air has an opportunity to expand, causing fluid to push its way through the vent on the power steering reservoir cap. If you develop this type of power steering fluid leak, you need to bleed or purge your power steering system of any air. Often times vehicle manufacturers have specific bleeding or purging process required in order to remove all air from the power steering fluid system.
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Power Steering Q & A > What Is? > What Causes Power Steering Fluid to Leak When the Car Is Off?
What Causes Power Steering Fluid Leak When the Car Is Off
You’ve got questions on what causes a power steering fluid leak. Power steering fluid leaks can develop in a number of different places and be caused by many different factors in a traditional power steering system. When systems that contain hydro-boost assist or Magnasteer assist are added to the mix, you increase opportunities for developing a power steering fluid leak. Sometimes vehicles will develop strange or difficult to diagnose issues - one of them being a power steering fluid leak when the vehicle is shut off.