[hbs18]

Hi everyone, I've been trying to diagnose this issue with my car for a while now and could use some help.

When applying light throttle, the car seems to accelerate in bunny hops, rather than smoothly increasing the RPMs, as you can see in the video below. In the video I'm accelerating with a constant light pedal application - the revs start stumbling in second gear and seem to smooth out past 2100RPM. When it's stumbling along, it also often does not shift smoothly, and when I release the throttle it tends to jolt the car (unless I'm very slow and smooth with releasing the throttle, which I've already trained myself to do all the time). When heavier throttle is applied, the car accelerates fine and shifts perfectly smooth. My car is a 2007 320d with the ZF 6HP19 transmission.

https://youtube.com/shorts/1C-VIRhx1_o?feature=share

I'm led to believe that the car only does this when the torque converter is in lockup. It does not do this (at least, I can't notice it) in first gear, and when I accelerate harder. It also drives perfectly smooth on very cold days when transmission doesn't lock up the TC until it's warmer than 35C.

I've taken a data log of the TC turbine speed and engine speed. I'm accelerating with light throttle, and you can see the engine RPM bouncing while turbine speed accelerates smoothly. What this means I'm not sure, but I think there is a chance of the TC lockup clutch slipping (or something like that). At the end of the graph, I'm accelerating harder, which I guess allows for some torque converter slip and ends up in engine RPMs rising smoothly as well. There you can also see how engine RPMs behave oddly when I release the throttle.



The reason why I'm very confused about all of this is because the transmission has never had any issues where RPMs slip upward like a failing torque converter would typically act. There are no codes at all thrown by the transmission. I have also recently had a rough idle and a check engine light for low rail pressure, but I've replaced the fuel quantity control valve which made the idle better. I was convinced that this issue was also caused by the same thing that was making the engine idle roughly, but it seems this was not the case. The transmission has had its fluid replaced twice so far, and 200k kms is a bit early for the TC to fail.

[RonnieP]

Hey,

After spending one year trying to solve my car's issue, your description fits best what is actually happening. Could you meanwhile solve the issue?

Currently I am trying to setup a live recording of the same signals you plotted. What tool did you used for live monitoring and recording, did you had to manually add the signals PIDs?

Engine is working fully OK (confirmed by bimmerprofs). Mileage is 92000km. Now planning to visit transmission expert. Not sure whether it is the torque converter, some internal sealing, leaking solenoid, other? Also transmission fluid got flushed recently including the reset of adaptation values and re-run adaptation sequence ZF provided, but your described phenomena remained.

[sonofsun]

To a casual observer, doesn't appear to be a torque converter issue since the problem manifests itself only at the 4th gear. May have to do something with solenoids and controls for the 4th gear.

Transmissions are more complex than anything else in the car. Hard to find experts who know what they are doing with these ZF transmissions.

[RonnieP]

The bunny hops in 2nd en 3rd gear is the real problem that hbs18 is describing. What should be smooth acceleration (see pedal) is now giving harmonic torque swings. I can tell it also happens in 5th and 6th where locked torque converter should happen. I hope somebody can give some more insights