[Eddamoo]

Hi All,

I usually love hanging around these forums trying to solve car problems - now i have one of my own! I'm experiencing occasional hesitation at 1200-1400RPM with light accelerator demand. Ill update this thread with my findings but if anyone has ideas they would be gratefully received!

The problem i've tried to get my head around but here are the symptoms:

• Feels like a sudden flat spot between 1200-1400RPM - not turbo lag
• Can cause the car to surge several times until you drive out of it
• 50%+ accelerator seems to reduce it significantly
• Happens during light throttle only
• Happens also with DTC fully off
• Pretty certain its not the gearbox although it was my first suspicion - happens in-gear in manual mode.
• Seems to happen most when the car is warming up, but not noticeable within the first few minutes of driving. Less noticeable when warm.
• Doesn't feel like transfer box - I've had that before in an F20 and this is more subtle. Tyres are correctly matched and inflated.

Troubleshooting Done:

• Cleaned MAF & MAP sensors
• Full DTC Scan - no codes
• The fault happens before the O2 sensors go closed loop (not hot enough) so suggests that should be ok.
• ISTA Diagnostics - MAF Setpoint vs Control, Boost Pressure Leakage test, EGR Valve opening, O2 sensor tests, Manifold Sooting, Swirl flaps -> All tests OK

Thoughts:
EGR - My first thought was a sticking EGR but that doesn't explain why the car would be OK under 1200RPM. The full EGR system is only 1.5 years old. The car pulls just fine from 1000-1200rpm. ISTA checked out fine, but it does operate the EGR quite aggressively - maybe its a fault only at partial duty. Thats the only thing i have to go on as yet!

Next Steps:
Logging - I intend to take the car out later with some logging enabled - especially around Airflow. I need to flag the exact point of the hestitation which may not work in Bimmerlink but ill see what i can get.

[n00bkiller944]

I will watch this closely. Mine has always done this somewhat, but has gotten progressively worse over the past 3 years or so. 2014, 155k rwd so no transfer case for me... My thoughts have been EGR as well, but nothing to confirm that.

[Watsey]

Given your error testing has proven OK so far I’d focus on the EGR, turbo, and swirl flaps.

The actuation of any of these could have an effect on engine performance.

I’d also run a tank of fuel with a decent full system cleaning agent, i.e. one that will clean the fuel system, injectors, turbo, DPF, etc.

[Eddamoo]

Morning guys, so i logged a drive to work this morning. As expected the car behaved remarkably well but it did exhibit the problem very slightly.

Here is a datazap log of the drive:
https://datazap.me/u/eddamoo/log-171...2&mark=287-392

Markers are on the graph, but what i can see at 21.802, my accelerator position is steadily increasing, but the Airmass Setpoint, Corrected Airflow and Boost demand all drop for a moment.

So the fact the 'Setpoint' is dropping is very confusing. That suggests the ECU is requesting an airflow reduction.

Ill log again on the way home and see if i can get a more obvious log.

[n00bkiller944]

Originally Posted by Eddamoo

Morning guys, so i logged a drive to work this morning. As expected the car behaved remarkably well but it did exhibit the problem very slightly.

Here is a datazap log of the drive:
https://datazap.me/u/eddamoo/log-171...2&mark=287-392

Markers are on the graph, but what i can see at 21.802, my accelerator position is steadily increasing, but the Airmass Setpoint, Corrected Airflow and Boost demand all drop for a moment.

So the fact the 'Setpoint' is dropping is very confusing. That suggests the ECU is requesting an airflow reduction.

Ill log again on the way home and see if i can get a more obvious log.

Keep us looped in. How are you doing the data logging. Maybe I can do it and compare.

[vito140]

I think the EGR is now moving slowly, and maybe the EGR cooler is at least partially clogged, which reduces de air mass comming from EGR.

[Eddamoo]

Originally Posted by n00bkiller944

Keep us looped in. How are you doing the data logging. Maybe I can do it and compare.

Sorry mate i missed this. I am logging using BimmerLink.

Last night i had a smoother commute home from work (thanks to the emptier roads) and managed to get the issue to sustain itself long enough for the car to throw some DTC's - I got 3x O2 sensor codes. It took a good 4-5 seconds of bogging down for the car to realise it and i didn't get an EML.

The only problem is, somehow i managed to delete them when doing some ISTA diagnosis later that evening

The O2 sensor makes more sense to me than the EGR in my case as the logs show that the MAF setpoint is being reduced. Something is 'telling' the ECU to reduce the air demand.

I replaced my O2 sensor for an NGK one only ~20k miles ago. Ill see if i can get the codes again but ill be ordering a Bosch O2 sensor in the meantime.

[Eddamoo]

Update:

2 more commutes and i didnt manage to get the codes to re-appear, but the DPF has been regenerating and it has reduced the hesitation.

Thankfully i found that ISTA stores old logs so i managed to pull the codes:

P2251 - O2 Sensor Negative Current Control Circuit/Open (Bank 1, Sensor 1)
P0132 - O2 Sensor Circuit High Voltage (Bank 1, Sensor 1)
P2297 - O2 Sensor Out of Range (Bank 1, Sensor 1)

So first step is to inspect the wiring of the sensor. If that looks good, ill order another.

Given the problem was worst on warmup and almost completely disappears when the DPF regens, that would confirm a suspected Lambda fault. Apparently they only start working properly from 316C+. If they cannot heat themselves, this would cause fuelling issues.

[Eddamoo]

Issue Resolved:

A new O2 sensor fixed the problem

Quite surprising really as i only replaced it just over 3 years ago and ~25k Miles ago with a a 'high quality' NGK/NTK part.

Replaced with with a Bosch.

[TurboWeasel]

Glad it was a simple fix!

I only use genuine O2 sensors. I've had too many issues with aftermarket ones from AutoDOC type suppliers, even Bosch! There's usually either a calibration discrepancy (strange fuel trims) or something else not quite right with the cheaper alternatives, such as poor service life.

£280 from BMW though

[n00bkiller944]

Just to confirm, you did NOT get a check engine light? You just saw these through data logging?

[Eddamoo]

Originally Posted by n00bkiller944

Just to confirm, you did NOT get a check engine light? You just saw these through data logging?

Yeah correct, no EML was thrown.

I also only got the codes to throw by spending some time to 'force' the problem for several seconds - slight incline, light throttle, cool engine, manual mode. Normal driving didn't throw any codes.

[n00bkiller944]

I need to do this exact testing - What are you feeling now that you replaced the O2s?

[Eddamoo]

The car is now completely smooth. Same road and conditions i cannot reproduce the issue at all.

You could tell on the first drive (<5 miles) after the change however that the fuel trims were still off as the car still wasn't quite 100%. Dont panic if you change your sensor and the car isn't 100% fixed straight away.

I think a good way to tell if its your O2 sensor as opposed to EGR, is to force a DPF regeneration. The stuttering was noticeably reduced with a very hot exhaust in my case. O2 sensors need to be at ~300C to work their best and to get the EGT's that hot, the ECU will reduce the EGR duty, as it needs to burn more fuel.