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BMW Power Steering System

BMW Power Steering System, Rack, Hoses, and Lines

Aside from a few exceptions, BMW uses power-assisted rack-and-pinion steering on every model. In concept, modern BMWs are no different than the power steering systems from 40 years ago - turning the steering wheel rotates a shaft, which slides a toothed rack with assistance from a hydraulic pump or electric motor, pushing and pulling on the tie rods at each wheel to point the tires in the direction you want to go.

Several major advancements have been made since the original 1970s power rack-and-pinion systems and BMW is now all-electric. But millions of hydraulic power steering systems are still on the road today and they need to be maintained. Steering racks are expensive and replacement can be very invasive. Preventative maintenance such as fresh fluid and replacing leaking hoses will help prolong the life of the rack. However, looking at the power steering system on your car can be intimidating with many hoses crisscrossing the engine bay. We'll explain the layout and function below.

All hydraulic systems will have 4-5 hoses/pipes that carry hydraulic fluid from the reservoir, to the pump, to the rack, and then back to the reservoir. On later cars one of these hoses is actually a metal cooler that sits in the airflow and cools the fluid after it leaves the rack. Factory BMW hoses are reinforced rubber with metal fittings except where the hoses attach to the reservoir with hose clamps. Below is a diagram showing a typical hose layout. This is for an E46 M3 but nearly all BMWs using a belt-driven PS pump will have a similar hose layout.


PS hose layout shown for E46 M3. This is typical for all BMW hydraulic power steering system hoses.
A = Fluid Reservoir | B = Inlet Hose ("Intake Manifold") | C = Power Steering Pump | D = Pressure Hose
E = Steering Rack | F = Return Pipe | G = Fluid Cooling Pipes ("Cooling Coil") | H = Return Hose ("Radiator Return Line")

A. Power Steering Reservoir, aka "Oil Carrier". This sits at the top of your engine bay, usually on the driver's side. You can top off or refill your fluid in this container (make sure you use the correct fluid - see below!). Also inside is a filter that will trap metal and other foreign particles so they do not reenter the system and cause damage to the pump or rack. The filter is not serviceable so it's a great idea to replace the reservoir at some point.
B. Supply Hose, aka "Intake Manifold". A rubber hose running from the reservoir that supplies the power steering pump with fluid. Crimped hose clamps attach it at both ends. These clamps are crimped into place and cannot be re-used so we recommend getting the appropriate premium traditional clamps if you're ordering new hoses. As with any other rubber hose, this one will leak at some point from either dry rotting or swelling from over-saturation.
C. Power Steering Pump, aka "Vane Pump". The internals of a PS pump are quite simple but the bearings can become dry, requiring the complete replacement of the pump itself. The PS pump is usually driven from the same serpentine belt as the alternator and water pump.
D. Pressure Hose. This runs from the pump to the rack. At either end is a hard metal line secured with a bolt and copper crush washer. The hose has extra length so the system can have more fluid capacity. Unfortunately it has a lot fittings where rubber hose meets metal line. These fittings are compression crimp-style swage ("swedge") fittings that are very common in the automotive world. These swage fittings are the most common area for PS leaks on your BMW. It's easier to replace the entire line with a new OEM or BMW part than to try and repair the line.
E. Steering Rack, aka "Hydro Steering Box". Most BMWs use rack-and-pinion steering. The pinion is the steering shaft and it meets the steering rack at an angle. Turning the steering wheel turns the pinion shaft and the pinion teeth mesh with the rack teeth to move the rack side-to-side. In this way, power and manual steering racks function the same. The power assist comes from the hydraulic fluid pushing on a piston which moves the rack. Manual steering is not recommended unless you have a vintage 2002 or E21 with skinny tires.
F. Outlet Pipe, aka "Return Pipe". This hose runs the fluid out of the rack and into the steering cooler. On older models there is no cooler and this hose runs back to the reservoir. Like the Pressure Hose it has a metal pipe with swaged fittings that are prone to leaking.
G. Power Steering Cooler, aka "Cooling Coil". This is a hard metal line that bends back on itself, doubling the surface area. This pipe is usually placed at the front or bottom of the rack so it's in a little bit of airflow. The metal pipe exposed to air has a cooling effect to the fluid inside. Leaks are not very common here but rust and corrosion are. And it's position in the airflow also exposes it to rocks and debris where it can be damaged. Older BMW models like the E30 325i did not have a cooler.
H. Return Hose, aka "Radiator Return Line". BMW has two hoses they call a Return Line. We're going to clarify things and call whichever hose attaches to the reservoir is the Return. "F" in the diagram we are re-naming the Outlet Hose. In this diagram, the Return comes from the cooler and returns the fluid to the reservoir. Like the Inlet Hose (B) it has non-reusable clamping hose clamps where the hose meets the reservoir. Leaks are very common here.

BMW Power Steering Fluid
Your BMW power steering is filled with either ATF or hydraulic oil (CHF) and it is vital that you use the correct fluid as the two cannot be mixed. The cap on your fluid reservoir will indicate which fluid to use*. ATF has been the traditional power steering fluid for decades and you can use any off-the-shelf brand or type (Dexron III seems to be very common at auto parts stores for a cheap top-off). But if you have a green label on the reservoir cap you must use CHF. The two fluids are not miscible and will not mix.

* BMW used the same reservoir cap for either but added a green sticker for CHF cars. They really should have issued a new cap because under the sticker the cap might say ATF! If the CHF label comes unglued and falls off, you could be left with a misleading cap! You can confirm the fluid by checking the color of the fluid - red is ATF, green is CHF.

CHF stands for Central Hydraulic Fluid and it means your vehicle was engineered to draw hydraulic fluid from a single source. The fluid for the power steering system is shared with another system - such as self-leveling suspensions, Dynamic Drive, power convertible tops, or just about any other system that is hydraulically driven. It doesn't matter if your car doesn't have these other systems, BMW supplied your car from the assembly line with CHF and you must continue using it.

Using ATF in a CHF system can deteriorate seals in the hydraulic system, leading to leaks or poor performance from that system. Since some cars rely on the CHF for braking assist you should only be using the correct fluid. There could be other factors as well, such as viscosity differences at various temps.


ATF for Power Steering

Red Line Power Steering Fluid (ATF)

CHF Hydraulic Fluid


BMW Power Steering Leaks
Is the bottom of your engine bay an oily mess? It's not uncommon to see a coating of oil throughout your engine bay and BMW oil leaks can usually be traced to a few problem areas: valve cover, oil filter housing, oil pan, and power steering hoses. Engine oil is always a dark brown or black color. But power steering fluid is red (for ATF) or green (for hydraulic fluid) which makes identifying leaks easier.

Power steering leaks usually come from two sources: the hose connections to the fluid reservoir or the crimped metal fittings used where rubber hose meets metal pipe. The connection to the fluid reservoir is by a crimped hose clamp often referred to as an Oetiker clamp. These are one-time use clamps installed with special crimping pliers. Over time the crimp portion weakens and the clamp is allowed to expand, creating a leak. Since there is no adjustment you can't tighten the clamp to stop the leak. However, you may be able to remove the Oetiker clamp very carefully and install a traditional clamp in its place. As long as the hose itself isn't damaged or swollen you can tighten the clamp and stop the leak.

Power steering hose assemblies are rubber hoses combined with metal pipes. Most of the Pressure or Outlet hoses are built this way so the hose can attach to the steering rack or pump. If your leaks are not coming from the mounting bolts and crush washers (easy fix) it's likely coming from the crimped connection between rubber and metal. This is not an easy fix and a full replacement is typically the best course of action. Though they can be expensive, depending on how complex the hose is, a replacement should give you many years and tens of thousands of miles of leak-free service.



Shop BMW Steering Parts:

BMW Power Steering Hoses

BMW Power Steering Reservoirs

BMW Power Steering Fluid

BMW Steering Tie Rods

BMW Tie Rod Boots

BMW Steering Racks

 

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