There is air in the power steering fluid.

Bleed the power steering system. Refer to Power Steering System Bleeding .

The fluid level is OK but still foams.

1. Remove the power steering pump.
2. Inspect the soft plug for cracks.
3. Inspect the housing for cracks.
4. Replace the pump as needed. Refer to Power Steering Pump Replacement or Power Steering Pump Replacement .

The internal pump is leaking.

Inspect the power steering pump for leakage.

Refer to Power Steering Gear and Pump Leaks .