[Bimmer325i]

Hi Bimmer fans,

This is my first BMW and I've got this weird thing on cold starts.
I've had my mechanic looking at it but it's nothing obvious to him that wouldn't need further diagnosis. Hoping to find some suggestions here.

First, my car details:

2011 BMW E90 325i
160kW 6 cylinder
Automatic
83000km
No modifications

I have uploaded the following video for better visualisation.
Note: I am not touching the accelerator pedal the whole time.

https://youtube.com/shorts/g9H7so91zbk?feature=share

1. I start the engine
2. A few seconds later the motor revs start to jump a fair bit up and down.
3. This repeats a few times.
4. Some shaking of the car can be perceived.
5. After 30s or so the motor idle resumes normally.

Sometimes it is more severe than other times, but almost always I get this fluctuation of the revs. The video shows a medium version of it.

Some other observations:
- This phenomenon only occurs in the morning after a cold start.
- If I start the car the same day after e.g. 9 hours of work, car starts smooth.
- If I put the car into drive and lift the foot of the brake pedal - still no accelerator -, the car jumps forward as soon as the revs go up, as if something pushes the accelerator.
- Once motor is started, the car runs smooth with no rev changes in idle or any other perceived issues.

Maintenance in last 18 months:
- Vanos gears replaced
- PCV valve replaced
- Power steering pump replaced


Any help or suggestions what to check would be much appreciated.
Thanks.

[mapleleaf1773]

A leaking injector could be your issue.

[Bimmer325i]

Thanks.
I looked into this a bit and it could be the case.
Are there other culprits possible?
I sometimes read check or the replace "valves" in that context. What valves are they?

[cheerio]

I've got something similar going on with my 2008 323i. Did a smoke test and found smoke coming out of the PCV nipple, so I suspect the PCV membrane is torn.
Do you hear any whistling noises at idle or when driving?

[Bimmer325i]

Before replacing the PCV valve, I only had very mild symptoms. On occasion, i.e. during idle waiting at a traffic light, my revs would drops below idle revs and then recover. It never stalled but felt like it. This happened only on rare occasions. I thought this might be also linked to the starting issues.

My mechanic suspected the PCV and he replaced it. At first I thought the starting idle issue was gone. Definitely, I'm not getting the intermittent idle rev drops anymore. But the starting issue remained as described above.

I could not hear or perceive any whistling noises at the time, no. But I also had a very whining/noisy power steering pump then.

[Socket]

Could be injectors, coils or fuel pump
Your mechanic should know how to diagnose the fault and fix

[F31B48]

Originally Posted by Bimmer325i

Hi Bimmer fans,

This is my first BMW and I've got this weird thing on cold starts.
I've had my mechanic looking at it but it's nothing obvious to him that wouldn't need further diagnosis. Hoping to find some suggestions here.

First, my car details:

2011 BMW E90 325i
160kW 6 cylinder
Automatic
83000km
No modifications

I have uploaded the following video for better visualisation.
Note: I am not touching the accelerator pedal the whole time.

https://youtube.com/shorts/g9H7so91zbk?feature=share

1. I start the engine
2. A few seconds later the motor revs start to jump a fair bit up and down.
3. This repeats a few times.
4. Some shaking of the car can be perceived.
5. After 30s or so the motor idle resumes normally.

Sometimes it is more severe than other times, but almost always I get this fluctuation of the revs. The video shows a medium version of it.

Some other observations:
- This phenomenon only occurs in the morning after a cold start.
- If I start the car the same day after e.g. 9 hours of work, car starts smooth.
- If I put the car into drive and lift the foot of the brake pedal - still no accelerator -, the car jumps forward as soon as the revs go up, as if something pushes the accelerator.
- Once motor is started, the car runs smooth with no rev changes in idle or any other perceived issues.

Maintenance in last 18 months:
- Vanos gears replaced
- PCV valve replaced
- Power steering pump replaced


Any help or suggestions what to check would be much appreciated.
Thanks.

Have you scanned the car for fault codes? That would be my first port of call. If you don't have a scan tool you can download ISTA which is what BMW dealers use. It's free, you just need a windows laptop and a k+dcan cable. This is the lnk to the software https://www.e90post.com/forums/showthread.php?t=1681573

[Bimmer325i]

Thanks for your replies.

No, I have not scanned for fault codes. Don't have the software or cable but will look into it.

I'll also check back with my mechanic for my next service to go over some of those suggestions.

[cheerio]

Bimmer325i did you end up finding what this was?
I replaced my valve cover and things have improved, but still have a slightly rough idle when cold.

[Bimmer325i]

Originally Posted by cheerio

Bimmer325i did you end up finding what this was?
I replaced my valve cover and things have improved, but still have a slightly rough idle when cold.

Hi,
No, still no progress on my front.

I have however noticed some other symptoms, but I'm not sure if related or not.

With the car running in idle and in P position, whenever I briefly tap the accelerator pedal for a second, the car shakes a little, same as during those cold starts. Feels like something is moving in the engine bay? I have the same when in idle waiting at the lights and then accelerate from standstill - feels like a little shake and hesitation. Any ideas?

[Socket]

Where are you located?

[cheerio]

Originally Posted by Bimmer325i

Hi,
No, still no progress on my front.

I have however noticed some other symptoms, but I'm not sure if related or not.

With the car running in idle and in P position, whenever I briefly tap the accelerator pedal for a second, the car shakes a little, same as during those cold starts. Feels like something is moving in the engine bay? I have the same when in idle waiting at the lights and then accelerate from standstill - feels like a little shake and hesitation. Any ideas?

I think I might have exactly the same symptom! Are you me? Occurs only when tapping the accelerator very quickly, pressing it down slower makes the engine rev as expected. The tap makes the revs 'hiccup' and drop by 50-100rpm momentarily before they recover. Does that match your issue? Would be great if we could diagnose this together if so.

I'm 99% sure this isn't caused by any vacuum leaks, as I've replaced the valve cover, PCV breather hose, and all intake manifold seals. DIY smoke test revealed no noticeable leaks.

Injectors all looked good when I took the manifold off, and fuel pressure was tested to be a healthy 5bar, so doubt it's related to fuel system.

Installed brand new NGK spark plugs 2000km ago. Coils are original, which probably isn't great, but checking operating smoothness values in ISTA, everything looks consistent across all 6 cylinders, so I doubt it's a coil/plug/injector issue. Something interesting I noticed - watching the live data for spark advance when the 'hiccup' happens, spark advance for both banks goes to -10% momentarily. At idle this returns back to ~0%.

I pulled the VANOS solenoids and VANOS return check valves. All were pretty clean. Washed them out with brake cleaner and reinstalled, which had absolutely no effect.

No fault codes, and the issue disappears after the car has warmed up, which makes it a nightmare to diagnose. Car runs great at WOT. Before I learnt to do this stuff myself, took it to a couple of local mechanics who were hopeless... I guess I can live with this but I'd rather not.

Only clue I've got is the spark advance figures. Seems unlikely to be coil/ignition related, as it's consistent across all cylinders, and I don't see how the engine warming up would make the coils work better. Could be an air intake issue, I guess? Large DISA sounds a bit anemic compared to the new small one I put in, and I haven't tested the throttle body. Last option is it's VANOS playing up. This seems the most logical to me, as VANOS runs on oil pressure, and oil pressure could certainly change as the engine warms up.

There were a number of revisions made to the N52 engine over the years (oil passages in head, camshaft bearing ledge seals, amongst others). Bimmer325i what month/year is your car? How many km? Mine is 09/2008, 136k.

Edit: just realised you said the shake still happens when tapping throttle waiting at lights. Can't remember if this also applies to me, or if mine is only when cold, as it's pretty slight. I'll test tomorrow and get back to you.
Edit 2: just re-read the first post, your car is 2011 and 83k. Likely rules out any of the issues that BMW fixed with engine revisions.

[Bimmer325i]

Originally Posted by Socket

Where are you located?

Hi Socket,
I'm in Perth.

[Bimmer325i]

Originally Posted by cheerio

I think I might have exactly the same symptom! Are you me? Occurs only when tapping the accelerator very quickly, pressing it down slower makes the engine rev as expected. The tap makes the revs 'hiccup' and drop by 50-100rpm momentarily before they recover. Does that match your issue? Would be great if we could diagnose this together if so.

I'm 99% sure this isn't caused by any vacuum leaks, as I've replaced the valve cover, PCV breather hose, and all intake manifold seals. DIY smoke test revealed no noticeable leaks.

Injectors all looked good when I took the manifold off, and fuel pressure was tested to be a healthy 5bar, so doubt it's related to fuel system.

Installed brand new NGK spark plugs 2000km ago. Coils are original, which probably isn't great, but checking operating smoothness values in ISTA, everything looks consistent across all 6 cylinders, so I doubt it's a coil/plug/injector issue. Something interesting I noticed - watching the live data for spark advance when the 'hiccup' happens, spark advance for both banks goes to -10% momentarily. At idle this returns back to ~0%.

I pulled the VANOS solenoids and VANOS return check valves. All were pretty clean. Washed them out with brake cleaner and reinstalled, which had absolutely no effect.

No fault codes, and the issue disappears after the car has warmed up, which makes it a nightmare to diagnose. Car runs great at WOT. Before I learnt to do this stuff myself, took it to a couple of local mechanics who were hopeless... I guess I can live with this but I'd rather not.

Only clue I've got is the spark advance figures. Seems unlikely to be coil/ignition related, as it's consistent across all cylinders, and I don't see how the engine warming up would make the coils work better. Could be an air intake issue, I guess? Large DISA sounds a bit anemic compared to the new small one I put in, and I haven't tested the throttle body. Last option is it's VANOS playing up. This seems the most logical to me, as VANOS runs on oil pressure, and oil pressure could certainly change as the engine warms up.

There were a number of revisions made to the N52 engine over the years (oil passages in head, camshaft bearing ledge seals, amongst others). Bimmer325i what month/year is your car? How many km? Mine is 09/2008, 136k.

Edit: just realised you said the shake still happens when tapping throttle waiting at lights. Can't remember if this also applies to me, or if mine is only when cold, as it's pretty slight. I'll test tomorrow and get back to you.
Edit 2: just re-read the first post, your car is 2011 and 83k. Likely rules out any of the issues that BMW fixed with engine revisions.

Thanks for your post.

I don't think my revs drop when tapping the gas pedal. I'll observe next time I'm driving.

Just drove my partner's VW Tiguan 2013. This is so smooth in comparison. Driving is different, but when tapping the accelerator, pretty much nothing happens. No shaking or wobble.

My spark plugs are due at the next service. And reading more and more on different sites, the injectors could be a culprit. Not always, but certainly there is a pattern. It's hard to compare symptoms not seeing them first hand.

Unfortunately, I'm not in a position to do much diagnosing or even rectifying myself. Easy DIY yes, so I'm much reliant on my mechanic.
Cheers

[Socket]

Originally Posted by Bimmer325i

Hi Socket,
I'm in Perth.

Oh.... bit too far for me to take a look

[cheerio]

Small update - once the car has warmed up, the rev drop/shake still happens when tapping the accelerator pedal.

At a cold start, revving the engine to 3000rpm for a few seconds before starting off seemed to get rid of the bouncing rpm.

Not sure if it'll help this, but I think I'll replace my coils anyway - they're >12 years old and a couple have torn rubber sheaths. Will let you know if this improves things at all.

[Bimmer325i]

Originally Posted by cheerio

I think I might have exactly the same symptom! Are you me? Occurs only when tapping the accelerator very quickly, pressing it down slower makes the engine rev as expected. The tap makes the revs 'hiccup' and drop by 50-100rpm momentarily before they recover. Does that match your issue? Would be great if we could diagnose this together if so.

Just a quick feedback, that I checked today and definitely the revs do not drop when tapping.

[Bimmer325i]

I have been to the mechanic today.
The car does not throw a single fault. All good.
He checked some other parameters, not entirely sure what they were. Cylinder check with all six values showing around 1.00 (?). Anyhow, not sure what to do next. Injectors don't seem to have an issue.

[cheerio]

My new coils haven’t arrived yet, but I still have a sneaking suspicion that it’s VANOS or oil pressure related. I dropped my oil pan last week, as well as cleaning the VANOS solenoids, and after doing that job, the bouncing RPM was much worse than usual on first start. Slowly reduced over a minute or so, and since then it’s behaved as it did before.

I believe there’s a test program in ISTA for checking accuracy of the VANOS timing control. Might look into this soon. The solenoids are supposedly a common failure, and will cause issues at idle and low RPM before they completely die, but at $400+ to replace the pair, I don’t want to throw parts at the problem.

[cheerio]

Found the VANOS programs in ISTA, ran them for both intake and exhaust. The program waits for the engine to reach 80C before running the tests - is cold start idle controlled with VANOS or with the throttle body?

Both intake and exhaust programs seemed to get to the desired setpoints just fine...
I'm back to being stumped. Coils arrive next week so we'll see if those help.

[cheerio]

New (Eldor) coils are in. Startup seems slightly more even, and there seems to be a touch more torque at WOT. Could just be placebo though :P

I still had the bouncing RPM happen on a cold start last night, so evidently the problem isn’t coils.

I’m really focusing now on a high pitched whistling noise, most audible around 1.5-2k RPM, which occurs both under gentle acceleration and coasting. I suppose it could be aerodynamic noise, but the whistle sounds like a very even, pure tone, and doesn’t change pitch with engine RPM or vehicle speed. The strangest part is that I’m noticing the noise a lot more after having replaced the oil pan gasket a few weeks ago. Currently leaning towards it being a front crank seal leak. I’m not quite sure if this would cause fluctuating RPM, though.

[Socket]

Doubt it is the front crank seal, it would not cause the symptoms you describe.
Other things that can cause the revs to surge/bounce around include

Faulty throttle position sensor
Failing HPFP
Vacuum leaks
Failing transmission
Faulty vanos actuator
Excessive carbon build up
Out of date DME and control module software

DME software was upgraded in 2012 to fix a known surging issue with the N55 engine

If you are still on the original version, best to get it updated using ISTA to the latest versions (better still get your local dealer to do it)

If the car is not throwing codes it makes it difficult to diagnose but also is an indication of a software issue


The whistle could be a failing diff or a million other things

You can adjust the window gap to reduce wind noise by adjusting the catch in marked in the pic below

Hope this helps