E34 m50b25 running rich

[deepblueE34]

I have been, for a couple of weeks now, cracking my head diagnosing a running-rich issue. At this point I'm out of ideas. This is the story.

Last year I swapped a M50B25 into my 520i e34 with the Siemens MS40.1 (yes we have these here in malaysia). I used the old ECU along with all the original 520i sensors. After the swap I drove it for a month and noticed no issues. The power difference from the original engine was noticeable. Then I parked it for 3 months while I was working abroad. When I finally went home, and setup my diagnostic laptop, I read the errors stored in the ECU and found a couple. One of which was easily fixed by checking the sockets that go to the TPS and connecting it to the correct one. The other proved more illusive.

The error code (of the persistent fault) according to INPA is 200 (Lambda regulating limit). At first, I thought since I never changed the O2 sensor before, it could be going south. So I replaced it and deleted the errors and resent the adaptions (using EasyDIS) and took it for a ride... Sadly that didn't help, and the error came back.

At this point I have changed the FPR (thinking that it was clogged causing too much pressure in the fuel rail) and all 6 ignition coils. I have also cleaned the MAF (with MAF cleaner spray), ICV and serviced the spark plugs, after changing the valve cover gasket. Also I tested the injectors spray and cleaned them.

The engine definitely feels smoother when accelerating, especially after changing the coils but the error refuses to disappear. Also my fuel consumption is about 14L/100km, where I use to get 12.

Interestingly, I noticed that the adaption values reported by INPA are always zero (please see attached screen capture) regardless of the O2 sensor voltage, which fluctuates between 0.6 and 0.9. In fact I never saw any change in these values since I started diagnosing this issue. Shouldn't these change in response to the O2 signal? Also my Ignition Angle reading from INPA is always outside the normal range. To be honest, I don't know what the Ignition angle reading means.


Sorry for the long-winded post. I can't think of anything other than that my ECU going nuts. If anyone has any ideas, please help.

[whiskychaser]

Yes, the M50B25 will run with the M50B20 set up. But I'd suggest that the DME you are using should be the Bosch 3.3.1 with the matching sensors.

[deepblueE34]

Yes, the M50B25 will run with the M50B20 set up. But I'd suggest that the DME you are using should be the Bosch 3.3.1 with the matching sensors.

Agreed. I would've used the Bosch DME if it wasn't for the need to get the DME plus the entire wire harness which would be a much costlier swap here.

That said, why does running the original DME and sensors result in a rich mixture and these INPA values? It seems as if the DME is not trying to trim the fuel, yet complaining about it being rich.

[whiskychaser]

Your TPS reading seems to be wrong too. But as far as the fuel trimming is concerned, maybe the answer is that
''Once a certain threshold (ms) value is exceeded the system can no longer offset the problem and a fault is logged''
http://www.bba-reman.com/gb/content/...s_and_problems

[deepblueE34]

That's what I suspected too. But I was expecting it to show some values while it tries to offset the problem. Maybe the ECU is giving up as soon as the o2 sensor comes up to the operating temp. hence the lack of values in INPA.

Also it looks like I have the wrong sensor since the Siemens o2 sensor should generate up to 4.75v according to the ECU's wiring diagram. I'm going to try another sensor to see if that makes a difference.

[whiskychaser]

Good point. IIRC, the Bosch ones only go up to 0.9v

[dgoodsy]

I'm sure running the stock b20 tune that has a timing map for an 80mm bore on a b25 with an 84mm bore doesn't help engine efficiency either.

[whiskychaser]

Probably not. But I do recall someone in Australia doing the same swap and running with the original Siemens sensors and DME. He had issues but IIRC that was because he was using the b20 manifold. I can't find the thread but it may be worth doing a quick search

[deepblueE34]

Yes, turns out that the o2 sensor I bought from the local parts shop had the correct part number but wrong voltage and connector.

The difference between the o2 sensors for the Siemens ECU and those for the Bosch ECU is that the former is a Titania sensor that has a 0-5 voltage range, while the latter is Zirconia sensor that generates 0-1 volts. This is confirmed by INPA.

This could have been prevented if I had my connector on the car harness side was still there, since these two sensors have slightly different connectors that will not allow the wrong one to be installed.

After installing my old sensor, I started getting good values in INPA and the error didn't appear again.

@whiskychaser, you are right you need the b25 intake manifold.

[whiskychaser]

Well, it looks like you found a problem, thought about it, did some research and solved it. Doesn't really get better than that. Always nice to hear the outcome too

[Qasim]

Hi i have same thing i just change the engine from m50b20 to m50b25 and i just have one problem when the car is cold is very good running but when up temperature to normal the engine loss of power and start advanced in low RPM can you help me and thx

[Qasim]

Whiskychaser please can you help me i change the m50b20 to m50b25 my harness is Simens ms 40 when the car warm up loss of power can you help me please