Going slowly insane with this car that I love but can't really drive. Hoping to get some ideas on what to do next so I'll do a writeup of everything I can remember, sorry for any incoherence.


Issue and symptoms

Car works absolutely fine after starting stone cold. But after reaching normal op temps the car starts to noticeably judder and jerk on partial load, on any speed or gear regardless of revs, also on cruise control.

The judder gets slowly worse with driving, and is downright awful after about 40-45min. Car is still drivable though, but jerky and sluggish. Something feels off.

If the car is left parked for a couple hours, the judder is there when getting back on the road. Less severe when it has cooled down somewhat (not overnight), but it's there from the get-go, before reaching normal temps.

Juddering is possibly more pronounced on warm weather. Maybe.

Alarmingly, there's also a low, bass-like rumble coming from the back when accelerating. Not constant, more like random half-second spikes, but easy to reproduce. Especially driving uphill on low revs. Doesn't correlate with the judder in any obvious way.

Gas consumption is elevated, roughly from 8L/100km to 10L/100km.

No CEL or codes at all. Mechanic has sat twice with me for a trip, monitoring with STAR. Everything seems to be fine according to all available sensor data.

Mechanic says the car seems to be running lean.

The issue has been ongoing for all of 2024, ever so slowly getting worse. Replacement parts have changed the symptoms a tiny bit, but not quantifiably.


Futile attempts to fix and other more or less random notes

- Spark plugs are new
- O2 sensors and MAF are new
- Gas transfer pump (+filter) is new and sounds normal
- Intake leaks have been ruled out with thorough testing
- PCV valve and hoses are tested to be fine (recalled way back)
- Intake manifold was removed and is fine too
- EGR valve is currently bypassed, no effect
- Stock programming, never tuned
- The transmission had some issues last winter, but I had the trans oils changed and they also used some other substance (not sure), it's been fine after though the shift to 2nd is a bit rough sometimes
- Supercharger makes a rattling sound on idle barely audible over other sounds and def. could use a service, but seems otherwise fine
- Starting cold the car seems to sometimes vibrate a bit on idle for a few minutes, but this isn't consistent and barely noticeable
- Timing chain has about 35k miles on it
- There are some electric issues ongoing, the parking assist doesn't work
- Due to the wiring harness oil seep stupidity, there definitely is some oil in the ECU but if it caused something, it probably would not be this
- Compression or gas pressure tests haven't been done, but at least fuel trim values are well within normal limits
- This is an early version M271 and the typical carbon buildup on intake valves has been cleared about 30k miles back, and after that it's 98E in the tank without exception

Also notable is that the post-cat O2 sensor was going crazy before the summer and as a result, the car was probably running rich for a period of months, before both sensors were replaced.

I'm running out of theories but one persists. It has to do with it running rich for months. But not to influence any detectives, it's marked as a spoiler below. Take a guess first.



That's it, thanks to anyone reading this far. Detective work, questions, commiserations etc. duly appreciated. I'm tormenting the mechanic again this weekend.


Good old 2023

 

Supercharger rattling is probably a worn polymer coupler, a while-you're-in-there job not urgent.
A clogged catalytic converter would affect driving on cold engine too and be more pronounced on high rev.
Cannot fully rule out a crankshaft position sensor, usually they have no signal after warm up.
Have you monitored the O2 sensor signals of both?
The ignition coils can go weird when they go to temperature, they wear more than iridium type spark plugs.
Leaking injector can lead to running rich. I remember they revised the injectors.

I would do a log file drive.
PS: After I changed the first O2 sensor I got a rumbling engine noise on high load low rev, but could never verify the sensor as the culprit.
PPS:I remember a problem with the camshaft adjusters, but can't remember exactly what.

 

O2 sensor signals are ok.

I was 100% them / MAF was the issue due to the issues starting seemingly when they have warmed up. But as new ones didn't change a thing, oh well.

And the fact that after the car cooling down for a couple hours, it's jerky as soon as I get going - while the O2 sensors are still offline - points to something else.

I have to agree that the catalytic is probably not the cause, would probably behave differently. And throw the sensors off. Confirmation bias much...

Coils, plugs, crankshaft pos. sensor and injector, that's a good check list to try and rule them out. Not sure which ones would trigger codes but we'll see. With 100k miles approaching soon might make sense to replace all of them.

Thank you.

 

Just throwing it out there, have you tried replacing any of the ignition coils?

 

Welcome from SoCal. Nice reporting. Appreciate it. Definitely give the cat a slight bump with a rubber mallet to listen for broken piece(s) of ceramic material. If the cat is bad you’ll also see a dusty glitter on the back of the car from the ceramic blowing out of the exhaust.

Problems occurring after warm up can also be electrical related. A dried out, previously melty of kinked harness will expand at operating temp gapping the faulty wire to make intermittent conductivity.

This may also explain an oil build up somewhere in the electrics. When cool, a dirty oil blobl will solidify. At engine warm up, depending on what’s mixed in with the oil blob, it will liquify and make or break a contact intermittently.

What brand of o2 sensor was recently replaced? o2 harness & connectors in good nick?

 

maybe pull the injectors and replace the orings while youre at it. Im in agreement with some of the other people that you should at least check the cat, give it a few bonks and see what happens. My car is more rumbly when it starts up before its cold but theres nothing wrong with it so thats just something these cars do. If you hear a lot of engine rumbling you might also want to check the engine mounts, it wouldnt be related to the issue youre having with the car running badly but when I checked mine recently one of them was almost just not there. Also consider checking the diff and the diff mounting points, I know that can cause a lot of rumble and a really messed up diff would make a lot of noise towards the rear and possibly mess with power if it was truly clapped.