running much too rich, RPM much too low, what happened?

Hi all,
My 355 5.2 euro passed its MOT (or Dutch equivalent) last week, but the emissions where rather high. I forgot which one was too high initially (CO2 I believe), but after a spirited test run with high rpm's and white exhaust tips inners, it passed the emissions test.
But as something was not optimal, I planned to look into that while replacing my standard exhaust with a capristo.
I started yesterday evening with the exhaust removal procedure, removing bumper , heat shields , air boxes etc.
Then I thought about the plans to check the sensors.I had replaced a post cat o2 sensor earlier this year because of a damaged wire, and maybe I had installed a pre cat one instead? So I removed it to check (wires got a bit twisted with screwing it out), but it was the correct one.
Then I remembered a posting about the blue ribbon on the pre cat connection, and there it was, connected wrongly! So I switched the connections on both sides, and thought I solved a small problem.
Wrong!
I wanted to start the car to check. I still had the air boxes removed, but the MAF in place, so I did not think that was a problem to start it.
Started the car, but it ran terribly! Cold start normally is like 1500 rpm or so, but now it did not get over 750 rpm. And black clouds out of the exhaust (still the factory one). Running very, very rough, almost stalling.
Thinking that I made a mistake with the O2 connections, I switched them back, but it did not help at all.
I did not get a CEL, and had no codes stored or pending, just the SDL that I already had, which should be unrelated.
So the only thing I could do was checking the plugs, all 8 where black. Cleaned them, no difference, still terrible.
It clearly gets much too much gas, and when I hold my hand in front of the MAF housing, choking it, the RPM goed up to normal 1500 and the car sound normal. So I reinstalled the air boxes etc, hoping it was too much air, upsetting the MAF. But it did not help.
I have reset the ECU a couple of times for each new situation, no effect.
So what could have happened? Did twisting of one sensor's wires cause this? Can o2 sensors influence the rpm and running that much?

Any thoughts?
Any other things I can check?

 

Well, how about this, I cleaned the MAF , just to do something, and I disconnected all 4 sensors to let it run in open loop , just to see what happens.
Well, it started directly at 1300-1400 rpm, sounding much better.
So I stopped the engine, connected both O2's from the side that I never worked on, and started again. Also good, 1300rpm.
Then the side with the exchanged post cat O2.
It started at 1300rpm, and slowed to 1000 in a couple of minutes, sounding like it did before all the problems arose.
But now with the O2 sensor connections swapped to what they should be.
I never got a CEL, and I will check later if I have pending codes from running without sensors at all. But so far I am happy again.

But the bad part is, I messed it up without knowing what I messed up, and I also solved it without knowing what did the trick.
All because I do not have codes at all. I have read somewhere that euro versions do not get codes that often as US versions, any truth in that?

And still I like to hear your thoughts about this, because it may return as I do not have a clue what went wrong in the first place.

 

Nobody any clues?

 

I guess you are right about that wire, it was not wise to disconnect it before. I assumed it had enough slack to even out the winding, but of course the first centimeter after the sensor is not shielded, so turns more easily. guess that there may be a short there.
But never knew that one O2 sensor could cause so much difference in mixture.

About the exhaust, I stopped with working on the car yesterday when it ran ok again, and will continue working on the exhaust switch tomorrow evening and the following evenings (only like an hour of time each evening). Hopefully end of the week I am ready.
As most Capristo 3 users also have test pipes (which I obviously do not have), I am anxious to find out how it sounds as well!

 

When I put my hand in front of the MAF opening, it went up to over 1500 rpm directly (maybe a lot more, I was standing at the back obviously, did not read the gauge), so it received a mixture/gas for that RPM.
What I do not understand is that with a rich condition, choking it helped. That would make it even richer...but it did make the engine run ok on a higher rpm...

Maybe the air sucked in (unfiltered) was contaminated with dust , polluting the MAF sensor? And the cleaning I did helped?
In any case, all plugs were evenly black, so not a coil or plugwire. MAF sounds logical, combined with the perhaps damaged O2 sensor wire...

 

Ok, finished installing the Capristo 3.
Went for a little spin to check it out. (engine works without problems again)
All works well, and what a sound!
I do not have a youtube account I think, so can't post the sound.
But it is very, very loud at 1500-2000 rpm, when driving away and in my neigbourhood with a 30 km/h speed limit.
After that, when the car is driving faster, the sounds becomes bearable.
The F1 shriek is not really there , but that most likely is because I still have the cats.
Or maybe the valve does not open, will have to look into that.