[BMWerkeUSA]

Hello,

After series of diagnostics and a long research (details can be found here), I have reached a conclusion that I need to replace the catalytic converter (I ran a SCR Functional Check and ISTA said that CAT need to be repalced). I have never done this before and I don't seem to be able to find a DYI. I have several questions:
1) Can anyone point me to a DIY if there is one?
2) I am thinking that I need items 2,4, 6,16 and possibly 7 from the following diagram. Is that the case?

3) Doesn't look like welding is needed, correct?

Is there anything else that I need to be aware of?

Thanks

[Owen81]

You might not need #2,4,6,16,7 unless yours are in poor shape.
You will also need an offset sensor removal socket, I believe 22mm to take the downstream NOx sensor out of the exhaust.
No welding required, just 2 downpipe nuts, the exhaust clamp nuts and the NOx sensor.

[cm335d]

I think the only thing you would need is the SCR itself (#2). The rest could be reused unless the clamp #7 is really corroded.

[BMWerkeUSA]

Thanks Owen81 and cm335d!
I already undid the NOx sensor. #6 and 16 looked a bit corroded but I haven't looked at #7 yet. I will try some liquid wrench before attempting to undo at least #6.

[BMWerkeUSA]

I sprayed #6 and the nuts on #7 with some Liquid Wrench and they came out very easily. At this point, the CAT is out. Hopefully, I will get the replacement by this weekend and install it. I need to buy a replacement for 4 and 6 though.

[BB_cuda]

Man, you could have bought a used one from the many who have deleted it. Good luck with the reinstall.

[BMWerkeUSA]

Thanks!
I did not buy one yet. I am talking to someone who deleted his.

[BMWerkeUSA]

So, I got the used Catalytic Converter (with 40K on it) and installed it. Then, I ran the SCR Functional Check in ISTA and it still says I have a bad Catalytic converter. So far, this is what I did:

1) Replaced both NOx sensors under warranty at the dealer
2) Run a test using ISTA to make sure that the metering unit is functioning correctly. It passed the test
3) I ran another test to measure the amount of urea being metered by the injector. The fluid sprayed during a period of 2 minutes was within the range given in ISTA
4) Both the active and passive tanks were flashed and filled with the genuine BMW AdBlue
5) I replaced the CAT with a good used one as far as I know

I reset the adaptation and will rerun the SCR Functional Check tomorrow. Is there anything else that can potentially be the culprit?

BTW, Nitrogen Oxide sensor before SCR catalytic converter: 111.93ppm but
Nitrogen Oxide sensor downstream of SCR catalytic converter: 228.93

[robnitro]

Did the dealer replace both nox sensors or just the one before?

[dmanb2b]

Can't help with above but perhaps just have a tuner tune out the SCR cat and be done with this.

[BMWerkeUSA]

Originally Posted by robnitro

Did the dealer replace both nox sensors or just the one before?

This what the paperwork from the dealer says: "Replaced the NOx sensors before and after the SCR catalytic converter and deleted fault memory entries"

[BMWerkeUSA]

Originally Posted by dmanb2b

Can't help with above but perhaps just have a tuner tune out the SCR cat and be done with this.

That is my plan B basically if all else fails.

[BMWerkeUSA]

I rerun the SCR system test after resetting the adaptations and ISTA still says the CAT is bad. Here is the report. Is there anything else I can check?

[Owen81]

How could the PPM downstream be greater than upstream? One of the sensors isn't reading correctly, most likely the one after.

Did BMW replace the sensors under warranty? If so get them to do it again because one of them is wrong for sure.

[robnitro]

I'm not sure if having a low coolant temperature could also cause issues. Unfortunately you can't drive it without using up miles so put on defroster, headlights, recirc air on... And run it at 1200rpm to warm it up.

[BMWerkeUSA]

Originally Posted by Owen81

How could the PPM downstream be greater than upstream? One of the sensors isn't reading correctly, most likely the one after.

Exactly! It is as if the CAT is polluting the exhaust gasses!!!

Originally Posted by Owen81

Did BMW replace the sensors under warranty? If so get them to do it again because one of them is wrong for sure.

Yes, they did replace them under warranty and I know they won't touch them again unless and until there is a code. They will blame the CAT and that is the end for them.

I am about the measure the temperature pre-and post-CAT and see if the temperature difference makes sense. I think it is as good of a test as I can think of at this point.

If the temperature difference is good (i.e., post is 100 degree F or higher than the pre) I am going to try to find a good used NOx sensor to swap with my "new" sensor and see what happens.

[BMWerkeUSA]

Originally Posted by robnitro

I'm not sure if having a low coolant temperature could also cause issues. Unfortunately you can't drive it without using up miles so put on defroster, headlights, recirc air on... And run it at 1200rpm to warm it up.

I think that the failed thermostat is the culprit that started all this emission mess. I replaced it very recently and verified the coolant temperature during a
drive with Carly but the problems did not go away. Maybe it is a coincidence but NOx sensors and/or the CAT appeared to have failed simultaneously.

[BMWerkeUSA]

I verified that the temperature after the Catalytic converter is lower than before the Catalytic converter. I measured about 370 deg F upstream and 270 deg F downstream. As far as I understand, it should be the opposite. The temperature should be higher after the CAT due to the chemical reactions. Here is a video explaining it:
https://www.youtube.com/watch?v=tFqodic0MZ0

[BMWerkeUSA]

I wanted to swap the metering unite with another one that I had. To my surprise, I found crystallized urea at the tip of the injector:



It wasn't long ago that I had inspected it and there was not any evidence of crystallization.

I installed the spare metering unit (see below) and took the car for a drive (15min).



Here is the log of NOx readings (PPM, nV) and the exhaust gas temperature for the last couple of minutes during the drive using Carly:





The PPM reading from the downstream sensor is more than the upstream one. I am starting to see some evidence of white dust at the exit of the exhaust pipes. It seems to me that too much urea fluid is being metered? Any ideas?

[robnitro]

There is a point at which too much urea shows up as excessive nox because the sensor picks up nh4 as nox though it's not. But why would the system keep upping the dose?

In your ista post it showed sensors matching ppm before the spray operation. So they are prob good, new replacements.

Maybe the scr adaptations have to be reset? Is there something that has to be run when replacing a scr cat?

[BMWerkeUSA]

Originally Posted by robnitro

There is a point at which too much urea shows up as excessive nox because the sensor picks up nh4 as nox though it's not.

In your ista post it showed sensors matching ppm before the spray operation.

Maybe the scr adaptations have to be reset? Is there something that has to be run when replacing a scr cat?

Here is an extract from the "ST81- Advanced Diesel Technology Workshop" supporting your first point:

"The NOx sensor, however, measures not only nitrogen oxides but
also ammonia but cannot distinguish between them. If too much
urea-water solution is injected, although the nitrogen oxides are
completely reduced so-called "ammonia slip" occurs, i.e. ammonia
emerges from the SCR catalytic converter. This in turn causes a
rise in the value measured by the NOx sensor. The aim, therefore,
is to achieve a minimum of the sensor value." (Page 97)

I know that there is an adaptation for the CAT and I had reset it after the replacement. There is a reset for the metering unit as well. I did the reset right before I logged the above data. I don't know if there is anything else that needs to be done.

[Owen81]

From what I've read, NOx generation takes very high temperatures 2800°F+ so it can't be generated inside the SCR cat. Reading through the FUNCTIONAL DOCUMENTATION, it looks like you could be experiencing "ammonia slip": Page 77. This is from over injection of Urea which would agree with your findings of urea in the tail pipes.

The urea injection is PWM controlled and depends on many factors including urea line pressure and cat temperature. I assume you have good pressure if it is over injecting, and your cat temps are over 200°C so that is good. The other factors that could be off would be the PWM control, sticking/leaking metering valve or another parameter in the process that is requesting additional injection.