ABS/POWER PUZZLE

[Z4JOHNBOY]

Right, I have a real head scratcher for you all please.

For context, I will run through recent events. 2008. Z4. E 85 2 L engine. Car has been off the road over the winter and I put it into the garage For a service and MOT. There was a leak on the rocker cover gasket and so that was replaced but the leak didn’t stop so the rocker cover itself was then replaced. Possibly unrelated but there was also a crack found in the coolant reservoir at the same time and so that was replaced too. At the same time I had the wheels refurbished and was driving round on a loan set of wheels for a few days. I have had the car back and it’s fine apart from one thing. When you turn to the left the car jitters as if it’s misfiring. Only when pointing or turning the car to the left though and that no other time whatsoever. It is worse under acceleration. Absolutely fine at idle and fine accelerating in a straight line. When it judges there is a light that flickers on the dash which appears to be a yellow triangle with a yellow circle inside of it which I think is ABS? It doesn’t stay on and there are no fault codes for me to read. I suspected ABS sensors so I changed both front sensors this morning and have also replaced all of the coil packs with brand-new Bosch ones in case any of those had been damaged. But the problem is still there. Does anyone have any idea what this could possibly be short of some sort of glitch with the ABS system itself? Answers on a postcard gratefully received as I am tearing my hair out and don’t particularly want to put the car back into the garage if I can help it.

 

[enuff_zed]

Calibrate the steering angle sensor as that too gives a feed into the traction control/stability system.
Also don't forget there are ABS sensors on the rear wheels too.

 

[Z4JOHNBOY]

No idea how to calibrate the steering angle sensor. Easy job? I know there are sensors on the rear wheels but there is barely any movement from the rear of the car when i am doing these gentle lefts so I just thought that it was unlikely to be a rear ABS issue.

 

[Z4JOHNBOY]

Looks like you might be on to something with the steering angle sensor...

 

[Z4JOHNBOY]

Just an update on this for anyone searching in the future. The problem still rumbles on.

Had a fresh garage look at the problem and he could find no stored fault codes other than the valvetronic - which he had out and cleaned but were fine. No other codes or dash lights other than the traction control flickering on whilst the car was faltering during left hand turns.

One thing he did spot was that the front suspension bushes were very worn, to the point that there was a lot of movement of the wheel laterally. So its booked to go back to him on Saturday to have those changed and the steering recalibrated. The theory is that maybe this excess movement is causing the stability control to kick in, thinking the wheels are out of control. So no fault as such, just the car doing what it should be doing! Although why it only affects left hand turns is a mystery unless one side is worse than the other (other than one single time during a right hand turn).

Really hoping that this is all that it is or the car will have to be scrapped as I am out of options after this. Fingers crossed....

 

[bigwinn]

Have you changed your ABS wheel sensors?

Those symptoms suggest a failing sensor, and if it’s while turning it’s going to be one wheel only

Does the fault occur when you have DTC deactivated (press and hold the DTC button)

 

[Zedebee]

Reading your opening post, the engine restricting power and the yellow light flashing on the dash, indicates a DSC issue. Very unlikely to be a problem with the ABS system, although one of the sensors that feed the DSC or maybe even the module itself is likely to be failing. Have you tried turning on the DTC (press DTC button on the centre console once) or turning the DSC off (press and hold DTC button for five seconds) to see if it makes any difference. N.b. If you don’t know about DSC/DTC, there’s a section in your operating manual.

 

[Zedebee]

Ooops, Bigwinn beat me to it.

 

[enuff_zed]

Having changed lower wishbones and tie rods without the means to accurately track the car I experienced almost identical symptoms on the way to the alignment place. So to me it could well be enough movement in those worn joints to trigger the dsc and cut power.
Of course, you need to get those sorted first anyway or you could be chasing your tail.

 

[Z4JOHNBOY]

Thanks both. I did change the front ABS sensors but not the rears. Mechanic said he was getting no faults from them or the ABS/DTC system. Turning it off does make sense. Just slightly concerned about losing control of the car! But maybe if I took it easy. If I turn it off and the problem goes away though, I still won’t know what the fault or problem is. If the stability control was even beginning to fail though it would surely throw up codes and it wasn’t. Even with the mechanic driving and getting live data. Which is what leads me and my limited knowledge to think its more likely to be excess play in the suspension causing the car to misinterpret things.

 

[Z4JOHNBOY]

Having changed lower wishbones and tie rods without the means to accurately track the car I experienced almost identical symptoms on the way to the alignment place. So to me it could well be enough movement in those worn joints to trigger the dsc and cut power.
Of course, you need to get those sorted first anyway or you could be chasing your tail.

This is really encouraging. Thank you.

 

[Z4JOHNBOY]

Just an update on this mystery. I had the suspension bushes replaced but the problem was still there. The mechanic suggested that the only thing it could be that was left is something to do with the rear ABS sensors even though they were showing no fault codes for him. He said to maybe swap them over as they are not side specific in order to see if the problem of left-hand turns switched to a problem with right hand turns. I swapped them over and gave them a gentle clean in the process and it seems like the problem has completely gone away. One of them was covered with some corrosion which I did clean off so it may well be that one or both of them are on their way out and will need to be changed at some point if the problem returns but for now it is finally fixed it would seem.