Of course it leaked.

Of course it leaked.

"In this Special Series about the Mechanical Restoration, I document the repairs, the replacement parts and the mechanical upgrades to the E46 M3, including the Performance Power Steering Reservoir.

Of course, my E46's original power steering reservoir was leaking. In this entry, I document why they all leak across my M cars, and I touch on the common solutions." 

They all leak. 

In 2022, my M5 V10's power steering pump was giving out, making all sorts of atrocious noises on start up. It was quickly replaced.

In the diagnosis process, I had been keeping a CHF-11S Pentosin pint in the trunk and having to fill it up every other weeks of Winter driving. It isn't unexpected - yet frequency was high. It kept requiring new fluid after the pump was replaced.  

Of course it leaked. 

Weeks later, we noticed my reservoir was leaking itself to the chagrin of every other component in the steering system. Recently, my E60's steering rack developed noises, and we discovered it was leaking during the S85-002 engine swap process.

A used power steering pump isn't that extensive, but a rack & pinion? Ouch! 

They're all the same. 

Much is made about the % of parts that aren't re-used - but unfortunately, BMW M cars are built off BMW AG standard chassis and will use some of its parts: the power steering reservoir is such an example.

This generic design has been in use since the small of engines in an E30 all the way up to a V10 in the E60. There are very small design differences in the angles of the lower fittings: the E8x/E9x units have an angle, all the other chassis have fitting coming out straight. 

Of course, it also leaked in my E46 M3. It had been slow dripping fluid for months. 

The heat generated by these engines and their systems are worlds of differences that are not fully contained by their varying cooling systems. My V10 cooks. 


There are various culprits to the leaks. Unlike popular thoughts, it isn't merely a question of capacity. BMW M specs CHF fluid as it is less harsh of seal, if the previous owner used a petroleum or synthetic based fluid, the seals have degraded faster.

My reservoir had the sweaty feel of leaks. 

The seal under the cap is where most of the leakage stems from. They will deteriorate over time.

The hoses and clamps that connect to the reservoir may be old, brittle, and degraded as well. They are another source of leaks. 

Most importantly, the filter within the reservoir cannot be serviced. It traps any debris from damaging components downstream such as the rack, cooler, and pump. 

The solutions.

Below is a variety of solutions that exist on the market. You can mix and match some of them for a comprehensive fix according to your goals. 

1. Increase viscosity. 

You can ditch the CHF and opt for a higher viscosity ATF fluid. You will degrade seals faster, cold temperature performance will be atrocious but you won't boil the fluid. 

This solution optimizes for performance first at the detriment of everything else. It wasn't suitable for my use. 

2. Increasing cooling. 

The CHF boils as it moves beyond its operating range - stop it from boiling with an improved power steering cooler then! 

A coolant is expensive: the vaunted DO88 unit is 600 USD + installation. It doesn't address the degrading seals. 

3. Improve the seal. 

Track oriented owners have used the "rag" solution forever. Grab a microfiber cloth, wrap it around the cap, and use zip ties - I salute the simple geniuses. My M cars are street cars, as such I can't accept the rag solution as a temporary fix.

Other solutions on the market are CNC'ed caps that thread on the original plastic reservoir. That should be considered a dress-up part. 

4. Increase capacity. 

This is where most aftermarket companies have focused their efforts. There are various CNC'ed reservoirs along with partial solutions such as increased capacity CNC'ed cap that claim to solve the issues entirely. 

#1 was a no-go, #2 was needlessly expensive for my planned.

I chose to focus on finding an encompassing solution to #3 and #4, with the added requirement of a serviceable filter, and proper aesthetics. 

Up next: I unbox the Performance Power Steering Reservoir and document its features. 


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