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      04-18-2020, 07:08 AM   #1
n00b0rKa
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N52 rev hang on cold startup

Hello guys!

I own an E90 equipped with N52B25 engine (EU spec). Short history about it, I have finally completed the complete engine rebuild (and a proper one, I will post a thread about it later), which was needed because of insane oil consumption (1l/300km).

The engine was since broken in using heavy load procedures, oil changed twice (40 km and 300 km). The engine runs sweet as new when warmed up, however
  • I started noticing a rough idle, only on cold startups
  • Initially, it seemed to be going away by unplugging the MAF, but upon further testing, it does not seem to really matter
  • Tried multiple spark plugs, including 4 prong and 1 prong versions. The updated OEM (brand new) plugs seem to give the best results

And then I noticed an awful rev hang happening only on cold startups. Basically, I start up the car, the RPM shakes up and down a little (but it's a lot better with the new spark plugs), and when the revs have settled, I give a few taps to the throttle to bring the engine up to 4-5k RPM and then instantly let off it. The RPM however seems to hang at around 4k for 1 second, drop to 3k for another second and then proceed to drop to idle as expected from the moment I let off the throttle. Here I have a video to better illustrate the issue.


What's been done already?
  • Proper engine rebuild (Rings, rod bearings, honing, head rework etc.)
  • All adaptations were reset
  • Tested 3 different kind of spark plugs (with little to no effect)
  • Tested MAF response time compared to throttle - seems ok
  • Tested lambda probes, they seem ok
  • Once starting to warm up, the rev hang disappears

So what do you guys think could be?
Cheers, nbK
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      04-18-2020, 07:55 AM   #2
PhaceN52
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Have you tried connecting a laptop with inpa looking for any stored codes? It could be valvetronic electric motor/throttle body/something electrical related if the car wasn't doing it before the teardow.. Did you worked on the head/valves?
How does the car act at driving during warm up?

Last edited by PhaceN52; 04-18-2020 at 10:11 AM..
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      04-18-2020, 10:12 AM   #3
juld0zer
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Carbon buildup on the throat of the throttle body/butterfly flap?
During warm up, valvetronic is at max lift. The throttble body is used instead.
2005 is an original N52 (silver top) so the warm up takes a lot longer than the later models.

If I drive off too soon after a cold start, the idle rpm gets stuck at 850rpm. Restarting the engine doesn't stop it. I have to shut it down and wait for the car to go to sleep and then restart. Finnicky beast. If i park the car to get fuel (for example, or any brief shutdown of the engine) after driving the car until it is fully warmed up (more than 20mins), the idle rpm sometimes gets stuck around 850rpm also. One of the mysteries I am yet to solve, like the fluctuating coolant temps.

Yours is a manual too, like mine.
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      04-18-2020, 03:13 PM   #4
n00b0rKa
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Quote:
Originally Posted by PhaceN52 View Post
Have you tried connecting a laptop with inpa looking for any stored codes? It could be valvetronic electric motor/throttle body/something electrical related if the car wasn't doing it before the teardow.. Did you worked on the head/valves?
How does the car act at driving during warm up?
It was constantly on tester, grapphed a lot of sensors as well. I had some codes related to the adaptations but those were sorted quickly. I'm not sure, however, if it was doing it before. It had other, worse issues, like a huge cloud of blue smoke on startup . We worked on the head, but the car is so healthy once it warmed up that I simply can't believe it could be mechanical related. Once its warm, I could balance a penny on top of the engine and rev it all day, it would stand still. And the new starts are better as well

Quote:
Originally Posted by juld0zer View Post
Carbon buildup on the throat of the throttle body/butterfly flap?
During warm up, valvetronic is at max lift. The throttble body is used instead.
2005 is an original N52 (silver top) so the warm up takes a lot longer than the later models.

If I drive off too soon after a cold start, the idle rpm gets stuck at 850rpm. Restarting the engine doesn't stop it. I have to shut it down and wait for the car to go to sleep and then restart. Finnicky beast. If i park the car to get fuel (for example, or any brief shutdown of the engine) after driving the car until it is fully warmed up (more than 20mins), the idle rpm sometimes gets stuck around 850rpm also. One of the mysteries I am yet to solve, like the fluctuating coolant temps.

Yours is a manual too, like mine.
There is literally no carbon in the engine. Nowhere. We rebuilt the engine with my brother, and made sure everything is cleaned or changed and measured.
Indeed it's an original silver top N52, but related to other cars my family owns (different brands and ages), it warms up quickly.
I did not observe anything like your issues, mine is a complete beast once it's not completely cold. While I was in the early break in period and did not want to leave it idle for too long, even if it was below freezing outside and I drove off almost instantly, I did not notice anything odd.

The rough idle on cold startup doesn't bother me really, it is noticeable only if you know how smooth an i6 should run, and it's gone in 10 seconds. Which bothers me a lot more is the rev hang... I have not seen anything like this before.
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      05-29-2020, 08:10 AM   #5
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Update: I've replaced my MAF sensor as I finally got a MAF code. The code went away, low-end power improved somewhat. The rev hang has changed somewhat, and now it seems even more software-controlled than before, the revs clearly stopping for a moment on 4k, then 3k, then dropping back again. So after all, this might even be an intended feature to maintain oil pressure while the engine is cold and being revved? At least this is my only guess now.
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      05-29-2020, 10:15 AM   #6
triax37
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Does your car have a 2A94 code? If you do, it could be the crankshaft sensor, located under the alternator. I had some idle issues about 9 months ago. Replaced the sensor, and it's been smooth since. This thread has some helpful info:

https://www.e90post.com/forums/showthread.php?t=1156445
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      05-29-2020, 11:21 AM   #7
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Mines always shifts at 2500-3000 rpm if its cold and i just start driving. But if i pull it out if my carport and come out of the car to close the gate, the car is staying still for 30-50 seconds and when i drive off it doesnt do it anymore.
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      03-22-2023, 02:46 PM   #8
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Hi, i also have insane high oil consumption. Do you know what caused it? Ive done some research and it could be multiple things like valve stem seals, piston rings or ccv. I would appreciate a response.
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      03-24-2023, 09:17 AM   #9
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Quote:
Originally Posted by OilSlurper325i View Post
Hi, i also have insane high oil consumption. Do you know what caused it? Ive done some research and it could be multiple things like valve stem seals, piston rings or ccv. I would appreciate a response.
Mine did burn oil as well. It still does but way more less than before. I didn't jump onto engine internals straight, instead replaced EVAP system and switched to different engine oil viscosity. Manufacturer recommends running 5w30, I switched to 5w50. Frankly, using different spec oil was major reducer in oil burn. That's my personal experience.
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      09-12-2023, 12:16 PM   #10
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Quote:
Originally Posted by n00b0rKa View Post
Update: I've replaced my MAF sensor as I finally got a MAF code. The code went away, low-end power improved somewhat. The rev hang has changed somewhat, and now it seems even more software-controlled than before, the revs clearly stopping for a moment on 4k, then 3k, then dropping back again. So after all, this might even be an intended feature to maintain oil pressure while the engine is cold and being revved? At least this is my only guess now.
I have the same issue with ridiculous rev hang on cold start along with the rough idle. Did you ever end up solving this? Similar to yours, my car runs amazing when its warm and warm starts are perfect which confuse me.
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      09-13-2023, 01:01 AM   #11
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Quote:
Originally Posted by Leucosticte View Post
I have the same issue with ridiculous rev hang on cold start along with the rough idle. Did you ever end up solving this? Similar to yours, my car runs amazing when its warm and warm starts are perfect which confuse me.
Have you scanned for codes? Based on the OPs symptoms, I would have guessed he had a 2A37 fault for the eccentric shaft sensor. The “hunting” of the rpms is most pronounced on cold starts. “Easy” fix of replacing the eccentric shaft sensor, but of course you have to remove the valve cover to do it.
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      09-13-2023, 09:18 AM   #12
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Quote:
Originally Posted by e91Owner View Post
Have you scanned for codes? Based on the OPs symptoms, I would have guessed he had a 2A37 fault for the eccentric shaft sensor. The “hunting” of the rpms is most pronounced on cold starts. “Easy” fix of replacing the eccentric shaft sensor, but of course you have to remove the valve cover to do it.
No codes, although I'm doing MILVs soon so I'll have the valve cover off for that, if it's ESS related I could just replace it then.
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      09-13-2023, 05:45 PM   #13
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A few things to just check over. This engine is super picky about fresh clean oil as it's used so much more for timing lifters etc... So make sure oil change is recent or get one done and run some engine oil treatment before changing.
Someone mentioned that often times the oil filter has a plastic bit that goes in the middle of the filter. Which often gets discarded during an oil change. Thus oil isn't kept in filter housing and drains out overnight. So that's an easy check as well.

Then you have 101 other things to chase down. Does it have a rough idle with AC on..?? Any air leaks intake, PCV valve etc...


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