I’ve been changing the power steering ATF in my 07 regularly using a battery filling bulb and noticed that I can’t completely empty the reservoir – the bulb tube doesn’t go further down than roughly the perimeter joint near the bottom.
I also was puzzled by recent power steering problems reported in the forum where aeration and overflows were not readily resolved by changing o-rings, the adapter, clamps or hoses.
This led me to wonder how the reservoir is constructed inside, and what impact it could have on the system.
I retrieved two 2005-9 Outback reservoirs from a recycling yard, and opened one to see what’s inside. The result might suggest that the reservoir could be a contributor to problems not resolved in the usual manner.
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This model reservoir was used in 2005-9, and a very similar one was used in the next generation until the change to electric-assisted steering.
Next is the upper part separated just above the perimeter joint. Here we see the inlet tube and three sections that make up the bottom part.:
The next three photos are close-ups of the three sections (chambers), two of which have screen filters on the bottom.
In the last photo above, we can just see, through the center chamber round opening, a similar screen below the center chamber. There’s also a black gummy substance on top of the chamber.
The following two photos show the position of the cap’s solid tube when the cap is in place (based on measurements). The tube blocks the opening in the top of the center chamber. Consequently, fluid being pushed into the reservoir from the inlet tube is forced down through the screen filter into the bottom of the reservoir.
A further separation at the perimeter joint reveals the path of the incoming fluid from the inlet pipe to the center chamber. All circulating fluid has to pass through the small round opening from the inlet tube into the center chamber.
The final disassembly was to remove the bottom to reveal three separate filter screens.
Some observations:
Incoming fluid from the inlet pipe goes into the center chamber, and with the cap tube in place, can only go down through the screen filter of that chamber. It goes into the bottom of the reservoir where it is drawn through the outlet tube to the power steering pump. That one screen is the only filter for fluid returning from the power steering rack and control valve to the power steering pump inlet. The other two chamber screens filter fluid that is in the reservoir above, as when filling the reservoir.
The reservoir cap is vented. It’s a small opening that normally will prevent any pressure or vacuum build up in the reservoir due, for example, to temperature-related expansion or contraction of the ATF.
The power steering system is made up mostly of solid materials – the rack, the tubing, the pump etc. There are no areas where the internal volume can change significantly. Moreover, fluids, including the power steering ATF, cannot be compressed to any degree. For every ounce/ml of fluid pushed out of the power steering pump toward the steering mechanism, an equal volume has to return to the reservoir and back into the pump on its inlet side. Even when the steering is being turned and fluid is pushed into one side of the rack to assist with steering, the same volume of fluid moves out of the other side of the rack. Consequently, the level in the reservoir should not change in normal operation.
The design of the inside leads me to wonder what would happen if the cap vent was blocked, or if the center chamber screen was clogged. In regard to the latter, there shouldn’t be a lot of material coming into the chamber from the rest of the system unless there’s a component failure. However, when the cap is off, the center chamber is open to the upper part of the reservoir, and to whatever is there (e.g. that black mass on the top of the chamber). Moreover, the chamber opening is directly below the fill opening of the reservoir, and debris can get into it while replenishing, or if the cap is left off for some time.
Could a clogged screen cause or exacerbate some of the problems attributed to leakage at the pump return fitting, o-ring, or hose?
Would it be prudent to remove and back flush the reservoir when replacing the o-ring, or better still just replace the reservoir, in addition to replacing the o-ring, when there’s aeration or unexplained overflow problems?
Food for thought . . .
I also was puzzled by recent power steering problems reported in the forum where aeration and overflows were not readily resolved by changing o-rings, the adapter, clamps or hoses.
This led me to wonder how the reservoir is constructed inside, and what impact it could have on the system.
I retrieved two 2005-9 Outback reservoirs from a recycling yard, and opened one to see what’s inside. The result might suggest that the reservoir could be a contributor to problems not resolved in the usual manner.
------------------------------------------------------------------
This model reservoir was used in 2005-9, and a very similar one was used in the next generation until the change to electric-assisted steering.
Next is the upper part separated just above the perimeter joint. Here we see the inlet tube and three sections that make up the bottom part.:
The next three photos are close-ups of the three sections (chambers), two of which have screen filters on the bottom.
In the last photo above, we can just see, through the center chamber round opening, a similar screen below the center chamber. There’s also a black gummy substance on top of the chamber.
The following two photos show the position of the cap’s solid tube when the cap is in place (based on measurements). The tube blocks the opening in the top of the center chamber. Consequently, fluid being pushed into the reservoir from the inlet tube is forced down through the screen filter into the bottom of the reservoir.
A further separation at the perimeter joint reveals the path of the incoming fluid from the inlet pipe to the center chamber. All circulating fluid has to pass through the small round opening from the inlet tube into the center chamber.
The final disassembly was to remove the bottom to reveal three separate filter screens.
Some observations:
Incoming fluid from the inlet pipe goes into the center chamber, and with the cap tube in place, can only go down through the screen filter of that chamber. It goes into the bottom of the reservoir where it is drawn through the outlet tube to the power steering pump. That one screen is the only filter for fluid returning from the power steering rack and control valve to the power steering pump inlet. The other two chamber screens filter fluid that is in the reservoir above, as when filling the reservoir.
The reservoir cap is vented. It’s a small opening that normally will prevent any pressure or vacuum build up in the reservoir due, for example, to temperature-related expansion or contraction of the ATF.
The power steering system is made up mostly of solid materials – the rack, the tubing, the pump etc. There are no areas where the internal volume can change significantly. Moreover, fluids, including the power steering ATF, cannot be compressed to any degree. For every ounce/ml of fluid pushed out of the power steering pump toward the steering mechanism, an equal volume has to return to the reservoir and back into the pump on its inlet side. Even when the steering is being turned and fluid is pushed into one side of the rack to assist with steering, the same volume of fluid moves out of the other side of the rack. Consequently, the level in the reservoir should not change in normal operation.
The design of the inside leads me to wonder what would happen if the cap vent was blocked, or if the center chamber screen was clogged. In regard to the latter, there shouldn’t be a lot of material coming into the chamber from the rest of the system unless there’s a component failure. However, when the cap is off, the center chamber is open to the upper part of the reservoir, and to whatever is there (e.g. that black mass on the top of the chamber). Moreover, the chamber opening is directly below the fill opening of the reservoir, and debris can get into it while replenishing, or if the cap is left off for some time.
Could a clogged screen cause or exacerbate some of the problems attributed to leakage at the pump return fitting, o-ring, or hose?
Would it be prudent to remove and back flush the reservoir when replacing the o-ring, or better still just replace the reservoir, in addition to replacing the o-ring, when there’s aeration or unexplained overflow problems?
Food for thought . . .