Worn Piston Ring
A common problem with Ford Mondeo; Galaxy and S-Max power steering racks is a worn piston ring. The various symptoms can include a noisy power steering pump, a grinding noise from the steering rack or loss of assistance.
The reason is due to the metal-loaded PTFE piston ring in the steering rack wearing which allows the steel piston to rub the inside bore of the steering rack (sometimes this can be heard as a grinding noise). This metal-to-metal contact can cause small particles of metal to be shaved off both the piston and the steering rack bore. These particles and the particles from the worn piston ring are held in suspension in the power steering fluid and are carried around the system until they are collected in the filter at the bottom of the power steering reservoir. The particles build up to such an extent that they restrict the flow of fluid from the reservoir causing the pump to be noisy due to fluid starvation.
The solution is to thoroughly flush the system and replace the steering rack. In our experience it is necessary to replace the power steering reservoir at the same time. If the power steering pump has not been “dry-run” for long often it will not need to be replaced. When we remanufacture the power steering rack we use a larger piston ring of a virgin PTFE material which in our experience gives extended life.