Why Power Steering Fluid Pushes Out of the Reservoir

It usually happens because the fluid is aerating and foaming—most often from air being sucked into the low‑pressure side of the system or from fluid overheating. Other contributors include an overfilled reservoir, a blocked return or reservoir filter, a faulty cap/vent, incorrect or contaminated fluid, a failing pump or rack, misrouted return lines (including hydroboost setups), or trapped air after recent service. Below is a clear breakdown of what’s going on, how to diagnose it, and how to fix it.

What’s Happening Inside the System

A power steering system is a closed hydraulic loop. The pump draws fluid from the reservoir through a low-pressure suction hose, pressurizes it to assist steering, then returns it to the reservoir through a return line. If air enters on the suction side or the fluid overheats, it can foam and expand. Foam increases volume and turbulence, which can push fluid up and out of the reservoir neck or past a vented cap.

Common Causes of Reservoir Overflow

The following are the most common, real-world reasons power steering fluid ends up pushing out of the reservoir, along with what typically drives each issue.

• Air ingestion on the suction side: Loose clamps, cracked or hardened suction/return hoses, or flattened hoses let air in. Failing or flattened O-rings at the pump inlet are notorious on many Honda/Acura, GM Type II, and some Ford systems.
• Overfilled reservoir: Fluid expands as it warms; over the “MAX” line invites overflow, especially after highway or heavy steering use.
• Incorrect or contaminated fluid: Using the wrong spec (e.g., ATF where a specific PSF is required, or vice versa) or water-contaminated fluid promotes foaming and boil-over.
• Clogged reservoir screen or return restriction: Many reservoirs (BMW, Mercedes, Volvo, etc.) have internal strainers; when clogged, return flow churns and aerates, causing a fountain effect.
• Faulty cap or blocked cap vent: A blocked vent can force fluid out around the seal; a damaged or missing cap gasket can let fluid slosh out.
• Overheating fluid: A plugged or undersized cooler, debris on cooler fins, towing, track use, or a sticking pump relief valve can push temperatures high enough to foam and expand fluid.
• Pump cavitation or failing pump: Worn pump vanes or a misbehaving pressure relief valve can create cavitation bubbles that expand in the reservoir.
• Internal rack/gear issues: Failing seals or internal bypassing can aerate fluid and surge return flow back into the reservoir.
• Misrouted return lines or hydroboost tee problems: On trucks and vehicles with hydroboost brakes, an incorrect tee or missing orifice causes turbulent return flow that splashes out of the reservoir.
• Post-service trapped air: After hose, pump, or rack work—or a fluid flush—entrained air can take time and proper bleeding to purge; during that time, foam may overflow.
• Full-lock operation and thermal expansion: Holding the wheel at full lock keeps the pump at relief pressure, overheating and aerating fluid; hot soak after shutdown can momentarily push fluid up the neck.

While any single issue can cause overflow, several often stack together—for example, a slightly loose suction clamp plus a partially clogged reservoir screen—making thorough inspection important.

How to Diagnose the Problem

These steps progress from simple checks to more involved tests. Use proper safety practices and consult vehicle-specific procedures where applicable.

1. Verify level correctly: Check hot vs. cold marks per the cap or service manual; many reservoirs have separate ranges. Correct any overfill first.
2. Inspect for aeration: Fresh foam or a milky appearance after running indicates air in the system. Note any whining from the pump.
3. Check cap and vent: Ensure the cap gasket is intact and the vent isn’t blocked. Replace suspect caps.
4. Examine suction and return hoses: Look for soft spots, cracks, kinks, or oil-slicked ends. Tighten or replace clamps; replace hardened hoses. Replace the pump inlet O-ring where applicable (common, inexpensive fix).
5. Dye and leak test: Add UV dye, run the engine, and inspect for seepage at the pump shaft seal, hose joints, and rack boots.
6. Bleed correctly: Jack up the front, engine off, turn lock-to-lock slowly 20–30 times; then vacuum bleed (hand vacuum on reservoir) if specified. Recheck after a test drive.
7. Assess reservoir filter/screen: If the reservoir has an internal screen and can’t be cleaned effectively, replace the reservoir.
8. Verify return routing: Ensure correct hose-to-port connections and, on hydroboost systems, the proper tee with the required orifice orientation.
9. Check the cooler: Confirm cooler lines aren’t pinched and cooler fins are clean; replace a clogged in-line cooler if fitted.
10. Temperature check: Use an infrared thermometer; typical operating temps are roughly 60–90°C (140–194°F). Consistently higher temps point to cooling or relief valve issues.
11. Pressure test the pump: With a gauge and shut-off valve per the service manual, verify pump output and relief pressure. A stuck or weak relief can overheat and aerate fluid.
12. Scan for faults on EHPS: Electric-hydraulic systems (e.g., some Mini, Subaru, certain hybrids) may set codes for overcurrent or overtemp caused by steering resistance.
13. Flush with the correct fluid: If fluid is wrong or degraded, perform a full flush with the manufacturer-specified fluid and rebleed.
14. Evaluate the rack/gear: Persistent aeration after the above may indicate internal rack leakage or valve-body wear requiring rebuild or replacement.

A careful, stepwise approach usually isolates the root cause without guesswork, saving both time and parts cost.

Fixes and Preventive Measures

Once you’ve identified the cause, these repairs and habits address the issue and reduce the chance of recurrence.

• Use the exact, spec’d fluid: Many systems are sensitive to fluid type; consult the cap or service manual.
• Set the level correctly: Keep fluid between the marks appropriate for temperature; avoid topping off to the brim.
• Renew soft parts: Replace suction/return hoses, clamps, and pump inlet O-rings; these are frequent air-ingress points.
• Replace the reservoir if it has an integral filter: New reservoirs restore proper return flow and reduce turbulence.
• Service or replace the pump: Address shaft seal leaks, vane wear, or a faulty relief valve if pressure/flow is out of spec.
• Repair or replace the steering rack/gear: Internal seal failures can continuously aerate fluid.
• Restore cooling capacity: Clean or upgrade the cooler; verify unobstructed airflow and undamaged lines.
• Fix the cap/vent: Install a correct, functioning cap with a sound gasket and vent pathway.
• Bleed thoroughly after any work: Vacuum bleeding plus careful lock-to-lock cycles minimizes residual air.
• Avoid prolonged full-lock: Don’t hold the wheel against the stop; it overheats the fluid and stresses components.
• Correct hydroboost plumbing: Use the specified tee and orifice orientation to prevent return turbulence into the reservoir.
• Optimize hose routing: Gentle bends and proper return porting reduce agitation and splash in the reservoir.

Addressing both the mechanical fault and the system’s fluid dynamics—flow, temperature, and air control—provides a durable fix rather than a temporary band-aid.

When a Little Overflow Can Be “Normal”

A brief burp after a fluid change or minor misting on very hot days can occur, especially if the cap venting is sensitive. Persistent foaming, frequent overflow, whining, or a drop in assist is not normal and warrants diagnosis.

Summary

Power steering fluid that pushes out of the reservoir is almost always the result of aeration/foaming or overheating, with common root causes including suction-side air leaks, overfill, restricted return flow or reservoir filters, faulty caps, wrong fluid, or component failures in the pump or rack. Systematic checks—level, aeration signs, hose integrity, cap/vent function, reservoir filter, cooler efficiency, pump pressures, and proper bleeding—will pinpoint the fault. Correct the underlying issue, use the right fluid, and maintain proper routing and cooling to prevent recurrence.

How to tell if a power steering pump is failing?

Symptoms of a failing power steering pump include a stiff or hard-to-turn steering wheel, whining or groaning noises when turning, visible power steering fluid leaks, a foamy or discolored power steering fluid, a burning smell from the engine bay, and unusual steering wheel vibrations or jerking. A slipping or worn drive belt can also cause squealing noises when the steering wheel is turned, which is another indicator of a potential issue with the power steering pump system.
 
Here are the common symptoms in detail:
1. Stiff or Heavy Steering 

• Description: The steering wheel feels heavy, making it difficult to turn, especially at low speeds or when parking.
• Why it happens: The pump isn’t generating enough hydraulic pressure to assist your steering.

2. Whining or Groaning Noises 

• Description: You hear high-pitched whining or low-pitched groaning sounds from the engine bay when turning the steering wheel, particularly at low speeds. 
• Why it happens: This can be due to air in the system (often caused by low fluid or leaks) or worn internal pump components. 

3. Fluid Leaks 

• Description: You see puddles of reddish or amber-colored power steering fluid under your car, especially near the pump.
• Why it happens: Leaks can come from damaged seals or hoses, or cracks in the pump housing.

4. Foamy or Dirty Fluid 

• Description: The power steering fluid in the reservoir looks dark, dirty, or foamy.
• Why it happens: Foamy fluid often indicates air entering the system, which can be caused by leaks or a failing pump.

5. Burning Smell 

• Description: A burning smell emanates from the engine bay.
• Why it happens: This can be a sign that the pump is overheating from excessive friction, often due to insufficient fluid.

6. Steering Wheel Vibrations or Jerking 

• Description: You feel vibrations or a jerky, unstable sensation through the steering wheel when you turn it.
• Why it happens: An unstable steering wheel is often a result of inconsistent fluid pressure from a failing pump.

7. Squealing Noise 

• Description: A loud high-pitched squealing sound, especially when starting the vehicle or turning the wheel.
• Why it happens: This can indicate a slipping or worn drive belt that powers the pump, which hinders its ability to maintain proper pressure.

Why is my power steering pump pushing fluid out of the reservoir?

Power steering fluid being pushed out of the reservoir usually indicates air in the system, caused by a leak on the low-pressure return line between the reservoir and the pump. Other causes include a cracked reservoir, a failing pump, or issues with hoses, seals, or the pump’s pressure relief valve. To fix it, you can try bleeding the air out by repeatedly turning the steering wheel, or find and fix the leak by inspecting the hoses and O-rings, and replace the reservoir cap or clean the pump screen if necessary. 
Common Causes

• Air in the System: This is a very common cause, often indicated by foaming or bubbling in the reservoir. Air can be sucked into the system through a leak on the low-pressure return line from the reservoir to the pump. 
• Damaged or Cracked Reservoir: A physical crack or damage to the reservoir itself can allow fluid to be forced out. 
• Leaky Power Steering Cap: The cap’s seal might be damaged, allowing fluid to spray out. 
• Worn Pump or Internal Damage: A worn or damaged power steering pump can cause pressure issues or aeration that results in overflowing fluid. 
• Faulty Pressure Relief Valve: If the pressure relief valve sticks, it can cause excessive pressure, forcing fluid out of the reservoir’s vent cap. 
• Leaks in Hoses and Seals: Look for leaks or cracks in the power steering hoses and worn-out O-rings where lines connect to components. 

Troubleshooting and Repair Steps

1. Check the Fluid Level: Ensure the power steering fluid level is within the recommended range. 
2. “Burp” the System: With the engine running, slowly turn the steering wheel from lock to lock (all the way left and right) a few times. This helps to work air out of the system. Keep the fluid topped off during this process. 
3. Inspect for Leaks: Visually inspect the power steering pump, reservoir, and all connected hoses for any signs of leaks. Pay attention to connections and seals. 
4. Replace the Reservoir Cap: If the cap’s seal is damaged, replace it with a new one to ensure a proper seal. 
5. Check the Pump Suction Line: A failing O-ring or a crack in the low-pressure hose between the reservoir and the pump can let air in. 
6. Consult a Professional: If the problem persists, or you are unsure about the cause, contact a professional mechanic to diagnose and repair the system. 

What causes power steering fluid to bubble out of the reservoir?

Your power steering fluid is bubbling because there is air in the system, which is a sign of a leak in the power steering system or a low fluid level. The pump can suck air in through worn seals or hoses, or when the fluid level gets low enough. You can try bleeding the air out by filling the fluid and turning the steering wheel lock-to-lock a few times, but you must also address the underlying leak to prevent damage to your power steering system.
 
This video demonstrates how to detect a power steering leak by observing bubbling in the reservoir: 34sBuddy’s DIYYouTube · Aug 20, 2020
Causes of Air in the System

• Low Fluid Level: Opens in new tabA vortex can form at the fluid reservoir intake when the level is low, sucking air into the pump. 
• Worn Seals: Opens in new tabSeals and o-rings in the power steering pump, steering gear, or hose connections can wear out and let air in. 
• Leaking Hoses: Opens in new tabA leak in the low-pressure return hose or the high-pressure hose can allow air to be pulled into the system. 
• Pump Porosity: Opens in new tabThe power steering pump itself can have porous castings that allow air to enter. 
• After a Fluid Change: Opens in new tabIf air was introduced when disconnecting and reconnecting lines, the system needs to be properly bled. 

What to Do

1. Check the Fluid Level: First, make sure the power steering fluid is filled to the correct level, as this is a common cause. 
2. Bleed the System: With the engine off, add the proper power steering fluid. Then, turn the steering wheel from one extreme to the other (lock-to-lock) a few times while the engine is running to help work the air out. 
3. Inspect for Leaks: If the bubbling continues or the fluid level drops again quickly, you have a leak. 
   • Check Hoses and Seals: Look for signs of fluid leaks, especially around the pump and hose connections. 
   • Find the Air Leak: Listen for a whining noise, which can indicate the pump is sucking air. 
4. Fix the Leak: A leak needs to be fixed to prevent further damage. This often requires replacing a faulty hose, pump, or o-ring. 

How do you fix aerated power steering fluid?

Foaming Power Steering Fluid Solutions
If air’s getting in through worn seals, o-rings, or hoses, flush the system and replace those parts before refilling the system with new fluid. You can also try to bleed the power steering system of air.