[GM91]

I had an E90 320i N46B20BF with a blown head gasket, bad piston rings and bad valve stem seals, so I went ahead and replaced the short engine to avoid rebuilding the old engine. The short engine I found available was for an X1, it had almost the similar model and engine features, the engine model is N46B20BD which I currently have installed. After installing the new short engine, the car runs fine most of the time, but sometimes I hear misfire sounds especially when going uphill. Sometimes I would hear it when going on a straight road; however, it always happens while accelerating at the start of the gear and once the car begins to shift to the next gear (i.e when almost shifted to 2nd/3rd gear, the engine will commonly misfire multiple times until it shifts to the 3rd/4th gear, after which it starts misfiring again while pressing the gas pedal then it stops misfiring when at the end of the 4th gear, afterwards no or very rare misfires once car shifts to 5th/6th gear).

It is worth noting that my older engine never misfired like the new engine.

Things I have done:

- Replaced coils, replaced spark plugs
- Tried with new injectors but the problem was still there
- Tried with a new ECU (for E90 320i, but didn't try an X1 ECU)
- Did a compression test - there was no difference between the cylinders
- Replaced intake air boot
- Replaced VANOS solenoids
- Replaced oxygen sensors and cat converter (since they were recorded as faulty on OBD - no longer recorded at the moment but misfires still happen)
- Did a smoke machine test into the intake air - no noticeable leaks (I have not done a proper leakdown test with compressed air yet where the engine is placed in TDC)

The car has no error codes on OBD except sometimes it would show the following when going for long trips:

- Pending: P2272 (Heated oxygen sensor HO2S 2, bank 2 - signal stuck lean)
- Pending: 2A70 (valvetronic)

The OBD error 2A70 is never confirmed (always pending), the P2272 code is sometimes set to confirmed. I cleared the codes multiple times and it always stores these after long trips (of 60-100km).

My question is:

- Could it be possible that the ECU for my E90 car, which is for N46B20BF, has different ignition timings than the standard for N46B20BD, which is why random misfires happen? Should I find someone with an X1 ECU (for similar engine as above) and try their ECU against my car?
- If not, could there be a problem with intake air/my fuel pump (even though I do not get any errors) that prevents fuel from reaching the cylinders on time? (i.e hence the lean condition reported by the OBD)

I have been trying to tackle this issue for a few months and would really appreciate any guidance. I took it to multiple mechanics but they do not know what the issue is. I have INPA and can get ISTA+ if it helps track down the issue.

Thanks a lot!

[GM91]

Update: took it to a few mechanics who suggested there might be an internal fault in the engine since there are no error codes on the OBD, but wouldn't it always misfire if that was the case (and not only under heavy load)? I would really appreciate any guidance or suggestions

Thanks!

[pete9903]

Any update on your issue?

You only described hearing the misfires, but are you feeling hesitation or lugging as well when it happens?

[GM91]

Originally Posted by pete9903

Any update on your issue?

You only described hearing the misfires, but are you feeling hesitation or lugging as well when it happens?

Yes, I do encounter hesitation/lugging when the noises are present as the car feels like it does not want to move when pressing the gas pedal. It also feels like the car is shaking: once this happens, I lift my feet off the gas pedal and then press it again after 2-3 seconds, after which the noises are gone and the car is moving again.

Unfortunately, I'm still living with this issue - here is what I have done so far:

1. Replaced spark plugs
2. Replaced ignition coils
3. Replaced injectors
4. Replaced knock sensor
5. Replaced fuel pump and fuel filter

After doing the above, I had 0 misfires on the DTC and non-continuous monitors through the OBD; however, I'm still able to hear popping sounds when pressing the gas pedal at low RPM under load. I cannot hear the sounds once the RPM is over 2.5K. The noise is only heard when the car is on "D" and the gas pedal is pressed: this never happens on "P" or "N" even when pressing the gas pedal.

Here are the symptoms:

1. On cold start, the noise is most prominent when putting the car on D and pressing the gas pedal. I always wait a few minutes, usually around 5 minutes, before I start moving the car to reduce the noises (no garage, I park the car outside).
2. When going uphill or under heavy load, pressing the gas pedal around the 1.5K RPM mark causes the noise. If I hear the noise, I lift my feet of the gas for 2 seconds and then press the pedal again, after which the noise is gone.
3. The noise can be clearly heard when putting the car on D and then lightly pressing both the brakes/gas pedal.

The DTC has no confirmed errors; however, I see the following errors frequently as pending (MEV17N46):

2ACB: O2 Sensor Exhaust Sample Error (Bank 1 Sensor 1)
2AC7: O2 Sensor Exhaust Sample Error (Bank 2 Sensor 1)
2ABE: Post Catalyst Fuel Trim System Too Lean (Bank 2)

It is worth noting that the noise is only present when pressing the gas pedal. If it helps, the engine is at ~70k miles.

I suspected a carbon buildup/pre-ignition or a bad valvetrain.

- I used a MAX44 to reduce carbon buildup; however, the noise is still present.
- I took the car to a mechanic to investigate as I suspected bad valvetrain; however, they said that everything was installed correctly and the valvetrain had no issues.

I'm thinking about doing a walnut blast, but not sure if this would solve the problem. I'm trying to find a mechanic with tools that can diagnose this better or confirm the issue before I do anything else.

Also, not sure if this is related, but at 120 km/h or above, I start smelling a weird smell in the cabin (feels like something is burnt or heated - does not smell like oil or rubber, smells like exhaust fumes): I checked the valve cover and under the hood, there was no oil leaks. Could it be that there's an exhaust leak somewhere under the hood, which also causes these popping sounds (which are most prominent at low RPM and speeds of 20-40km/h)?

I would appreciate any suggestions or guidance.

[Raghza]

I see that you have already changed those, but judging by the latest error codes, I would replace the oxygen sensors to OEM equivalent.