Do you have a steering gear box or a rack and pinion assembly? The gear boxes rarely are the cause of loss of power assist. Rack and pinion assemblies can develop internal leakage that allows the pressurized fluid to bypass the power piston but the clue is that always affects turning in one direction only at first, then the other direction weeks or months later. When the loss of assist affects turning in both directions at the same time, the pump is the cause.
This pump appears to be a version of the "ZF"-style pump that caused a lot of trouble in the early '90s. Due to internal wear they developed low pressure at low speeds. The clue was the power assist always came back when raising engine speed a little. When the cause is internal leakage in the rack, it doesn't matter what the engine speed is or how much pressure there is. The seals will still leak and there won't be power assist at any engine speed. The additional clue is the seals expand after a few minutes when the power steering fluid heats up, and they'll seal so the power assist at idle comes back. When the pump is weak it won't get any better after the engine has been running a while.
Tuesday, June 11th, 2013 AT 10:06 PM