2004 Z4 E85 SMG: shifts engine-off, stalls when braking in gear

[dandzina]

Hi everyone, new here, and hoping someone with E85/E46-style SMG experience sees something I’m missing.

The short version: I have a 2004 Z4 3.0i with the 6-speed SMG/SSG. It has only about 23k miles, but it is still a 23-year-old car. The SMG pressure seems to hold around 49-52 bar and the car shifts through gears with the engine off, but with the engine running it does not seem to release the clutch correctly. If I get it in gear on the lift and press the brake, the engine stalls every time.

That makes me think “clutch not releasing,” but the actuator is clean, dry, and looks good internally, so I’m trying not to throw parts at it blindly.

Here’s what I know so far:

• SMG pump primes and pressure seems to hold around 49-52 bar.
• Pump does not cycle constantly or seem to run excessively.
• With ignition on and engine off, it appears to shift from N to 1st, reverse, and through the gears. Dash confirms gear selection.
• When in gear engine-off, the driveshaft does seem engaged, although with the car on the lift I can still rotate the rear wheels by hand. I may not be interpreting the open differential behavior perfectly.
• With the engine running, it often fails to shift into 1st or reverse while the brake is applied. It bucks or loads the drivetrain.
• If I command 1st or reverse and immediately release the brake, it will often take the gear and begin to move.
• Sometimes after taking 1st, it drops back to flashing N if I do not apply throttle.
• If I get it in gear on the lift and run the rear wheels slowly, then step on the brake, the engine stalls every time.
• It can shift up through the gears on the lift, including higher gears, so the problem seems most obvious at 1st/reverse engagement and when braking while in gear.
• When the failure happens, the rear wheels/driveline feel locked or loaded. Rotating/spinning the right rear wheel or releasing the brake can allow the shift to complete.

I removed the clutch actuator for inspection. Before disassembly there was no evidence of CHF fluid leakage. It had some dry dust, but no oily buildup. I also used a camera to look into the transmission/bellhousing area with the actuator removed, and the clutch area looked very clean with minimal clutch dust.

The actuator was also clean internally. The seals looked good, and I did not see scoring or corrosion in the bore. I’m replacing the seals and boot anyway since it is already out, but visually I did not find an obvious failed actuator.

Before removing the actuator, I drained the SMG hydraulic fluid into a container and replaced it with new fluid. The old fluid appeared to be factory original and darker; the new fluid is brighter green. The reservoir took roughly 20% more fluid than I drained out. I have not yet completed a formal bleed/adaptation process, and the behavior was unchanged after topping off/replacing the fluid.

Foxwell live data showed clutch position around 391 inc and clutch relative position around -554, but those numbers did not seem to move more than a point or two with engine off or engine running. I’m not sure Foxwell is showing the right clutch data for this car. Foxwell also does not seem to provide the full SMG learn/adaptation procedure for the Z4.

Before my next session, I’m setting up BMW Standard Tools 2.12 / INPA instead of relying on Foxwell, so I can hopefully get better SMG live data and access to the proper bleed/adaptation procedures.

My main questions:

1. Does the stall-when-braking-in-gear test pretty much confirm the clutch is not releasing, or can that be misleading because the car is on a lift?
2. Does this still sound like a clutch actuator issue even though the actuator is clean, dry, and not scored?
3. Could a weak accumulator cause this even if pressure appears to hold around 49-52 bar and the pump is not cycling frequently?
4. Could this be a clutch valve/solenoid issue where main pressure is fine, but the clutch actuator is not being commanded or pressurized correctly?
5. Are there common sensor culprits on these cars, such as clutch position sensor, gear/selector position sensor, brake switch, brake pressure input, or wheel speed input?
6. Once I reinstall the actuator, what exact BMW software procedures should I run and in what order? I’m assuming hydraulic bleed, clutch actuator bleed, clutch valve teach-in, transmission adaptation, and clutch bite/slip point adaptation, but I still have a lot to learn about navigating these tools.

I’d really appreciate guidance from anyone who has dealt with the non-M E85/E46-style SMG/SSG system. I’m trying to narrow this down before buying an accumulator, sending the actuator out, or assuming there is a mechanical clutch issue inside the bellhousing.

Unfortunately, the local dealers no longer seem interested in diagnosing the car. They mostly want to talk about buying it from me or converting it to a regular manual, which feels like a huge waste on a low-mileage car with the original SMG still intact.

Any advice is welcome.

Thanks,
Dan