Inconsistent Idle

[georgebest]

My recently rebuilt engine is working fine..with one exception. Idle is a bit inconsistent, not to a level where I'd call it rough but you can hear constant adjusting in every second. Did the followings on the engine during rebuilt:
- New timing chains and guides and tensioners
- Rebuilt VANOS units
- New VANOS solenoids
- New spark plugs and ignition coils
- New MAF
- New CCV & OSV
- New Camshaft sensors, new knock sensors, new crankshaft sensor

Everything is either genuine or Bosch, put in only high quality parts.

Engine has no errors.

RPM moves back and forth between 710-795.


As far as I can tell, everything looking good here:


The "lambda-sensor voltage" bar seems all over the place, I wonder if this could be the issue:


"Roughness" looks fine to me:


Obviously this is not a major issue, but after spending so much time and money on the engine, I want it to be perfect.

Any ideas?

[georgebest]

This is how the RPM fluctuation looks like:

[TheAngryBear]

The RPM gif isn’t working for me...
Did you happen to reset the throttle adaptation?
Check out your throttle position while it is doing this. I was amplified by a massive vacuum leak but mine behaved the same and even went to engine failsafe.


2001 750iL DD74441
Stock

RIP: 2003 540iA Sport GS56111
H&R front springs, Ultimate Cup Holder, Euro Dash & Armrest, Grom, BavSound Stage1

[georgebest]

The RPM gif isn’t working for me...
Did you happen to reset the throttle adaptation?
Check out your throttle position while it is doing this. I was amplified by a massive vacuum leak but mine behaved the same and even went to engine failsafe.


2001 750iL DD74441
Stock

RIP: 2003 540iA Sport GS56111
H&R front springs, Ultimate Cup Holder, Euro Dash & Armrest, Grom, BavSound Stage1

yes, adaptations were reset. I dont think I have any vacuum leaks.

[740ilDuke]

Try a different throttle body.

[georgebest]

Agreed, so far I think either throttle body or O2 sensors.

[Timm]

It's not the O2 sensors - they are working perfectly - the adaptations are absolutely spot-on as are the rest of the readings. So, don't go changing anything until you do a video (with sound) that shows us exactly what your problems is - because it doesn't sound like an engine problem!

[georgebest]

Hard to hear it on video, cause it's not a rough idle. Only fluctuating in a small range. Anyway, here you go:



Thanks Timm!

[georgebest]

Here is again the RPM gif cause not always appearing above



RPM.gif

[Timm]

There's a quick 'squeak-squeak' that can be heard a few times - that's either the A/C cutting in and out or something (steering pump/alternator/water-pump) that is binding. Try turning the A/C OFF and try again - if the squeak is still there it is time to check the ancillaries for bearing wear.

[georgebest]

There's a quick 'squeak-squeak' that can be heard a few times - that's either the A/C cutting in and out or something (steering pump/alternator/water-pump) that is binding. Try turning the A/C OFF and try again - if the squeak is still there it is time to check the ancillaries for bearing wear.

I think that's just the sound of the new belt, video and INPA was done with A/C OFF.

Belt tensioners & water pump are brand new, perhaps alternator could be an issue? Something forcing the engine to constant adjustment, I guess throttle valve could be it if not O2 sensor.

[Timm]

Yes, having listened to the video again, something is putting load on the belt causing the squeak and the change in revs - the simplest way to make sure that this is the case is to remove the belt and run the engine from cold for a minute or so.

[georgebest]

Yes, having listened to the video again, something is putting load on the belt causing the squeak and the change in revs - the simplest way to make sure that this is the case is to remove the belt and run the engine from cold for a minute or so.

That's a great idea, will definitely try!

[georgebest]

SOLVED!

First I have replaced the O2 sensors, it was a long shot but I wanted to replace them anyway, put in new Bosch sensors.

Old O2 sensor:


New O2 sensor:


Not much change, idle remained inconsistent, so it wasnt the O2 sensors.

I figured the other likely culprit could be the throttle body, so I got a new Siemens one from FCP and put it in.

See the RPM fluctuation with old throttle body:


Idle was too high and fluctuation is rather high too, going between 718-792

Now let's put in new throttle body and also clear adaptations:


BINGO!! Going nicely between 597-606, so it was the throttle body all along!


Engine also sounds much better, though I still have a vacum leak at the secondary air pipe that I havent fixed yet, will have better sound when that is done:


Thank you guys for all the inputs!

[georgebest]

And a bonus roughness with new throttle body, this has improved a lot as well:

[georgebest]

After installing new throttle body fuel consumption went down, now doing aroun 15.5 MPG vs previous 13.7 MPG (mostly city), might pay for itself

[Timm]

This is the second time we have seen this - the throttle body just doesn't close correctly, it seems to get hung-up just at the point where it would normally sit to get the idle at the right speed......

[georgebest]

It has also solved the belt squeal sound, I have no idea how, I was ready to put in a new tensioner. It feels like having a new engine.