"Check Engine" rough idle after engine Shampoo??

Ok, This is it, never again am I going to FUkn' do an engine shampoo, done it three times in my life...yes I cover the Alternator, but I have done it on various cars and buggered, sensors, switches, have lights turn off and on...lol...damn
So anyway, did the E34 so I could find an oil leak....found that, but now the car, only sometimes at idle will stumble/sputter like the car is about to stall, in fact once even backfired when I revved it. The check engine light comes off and on, when cruising it comes on, if I tromp it, the light comes off. Not much rhyme or reason however...
What should I start looking for?

In a terminal/plug somewhere.

Try a stomp test. it may help narrow which plug/terminal to dry out.

Or wait it out, and hopefully it will dry out on it's own.

I vote for water in between the Crank Position Sensor/Flywheel Sensor for Ignition and Injection

Not for a 535 but on my M50 I had water trapped in the plug channel after a engine wash. the ignitor helped to seal the water in.. It never got dry until I pulled all the ignitors out to dry off the plug channel with compress air and rags..