Seems I am drawn to this forum whenever I have car issues, which is relatively often! Since my Z4 saga I purchased a manual 325i, N52 powered like some of you will find in your Z4 2.5si. Naturally I have issues and I appreciate this isn't an E90 forum, but any help would be highly appreciated! For the record, the car has been to my trusted garage and one other during which they couldn't find the issue. One didn't think there was an issue simply because there were and still are no fault codes.
When I first got the car this stuttering issues was far more pronounced and would appear when flooring the throttle. This action lead to the car almost feeling like a sluggish auto trying to downshift and causing a big jerk. I would then get a jerk at around 3.5krpm. It eventually got really bad to the point where the car stalled and wouldn't start. Turned out to be the exhaust VANOS solenoid, so I replaced both with genuine parts. Since then I have had a more subtle stuttering issue which is evident at around 1500rpm and 1800rpm, sometimes at different points in the rev range too, with the occasional jerk higher up the range and is further complemented with a bit of a rough idle.
Exactly what this feels like it hard to describe, but it feels akin to a misfire. Say I am going 30 in 3rd, the revs are about 1800 rpm, not labouring the car. With a constant throttle the car will surge, judder, stutter (so many words for it) which feels like a faint jerking back and forth. This can also be witnessed sometimes coming off the throttle around that particular rev range, where the car will judder as it slows. Generally in the low rev range there is a distinct lack of torque, at least consistently, with flat spots and hesitation under full throttle.
Attempted remedial actions
- OEM MAF (Hella)
2 Genuine VANOS Solenoids
Checked and cleaned VANOS check/non return valves
Oil and filter change with certified 5w30 fully synth
Checked Large DISA (had quite an amount of oil in the intake as I took it out)
Genuine Valve cover gasket
OEM eccentric shaft sensor and seal
Checked for vacuum leaks, albeit not with a smoke tester etc
Listened for injector function, all sound the same with consistent 'ticking'
I am really at a loss with this and it's making the car such an effort to drive along with really poor fuel consumption (about 20-25mpg mixed). Please someone bestow upon me their wisdom!