AMM adjustment does nothing

87 740 Turbo lh2.2. I recently replaced my o2 sensor and was able to tune my AFR in the ballpark by observing the sweep. However I recently discovered an exhaust leak which has since been fixed and I decided to retune. So I started again and at the start everything went well numbers was rich as expected so I leaned it out a bit, but then almost right after my car began to misfire, cylinder 3. So I stopped the carvan restarted it thinking it might help. Misfire stopped, but I look and now my o2 sensor is staying below 0.5v (0.2-0.4) indicating lean. So I try to richen it out but The voltage won't change. I would hate to have to replace my o2 sensor again after literally 2 days but I don't know what else it could be. I'm pulling 18 mm HG vacuum cold at idle so I doubt it's a vacuum leak.

Since finishing my build about 3 weeks ago and only starting it and letting it idle in the garage occasionally since then, I've killed a narrow band and a wideband sensor and now maybe a brand new replacement narrowband. WHAT IS HAPPENING!?

Anybody? I added some more details.

What does the spark plug on #3 look like?

Not sure, can't imagine it looks very different from brand new though considering I replaced the spark plugs during the build and it hadn't been run much. Wondering if maybe my car was staying in open loop for some reason.

The O2 sensor makes voltage.

If you are actually killing sensors then contamination is likely. That, or MAYBE a short in the wiring.

Or a bad coolant temperature sensor. Had an LH 2.0 B23F car that ignored both the AMM and the O2 sensor inputs, due to a bad new-in-package Bosch coolant temperature sensor. I made sure that it was the correct sensor, btw (blue, instead of the black one used on LH 2.4 cars). When the head gasket popped, I reused the sensor that was in the head I swapped on. Ran normally.

-J

Examining the spark plug tips will give you an idea of what's going on. Black and fouled will be too rich, white and burnt too lean.

Hmm that's interesting about the coolant temp sensor might have to check that out. As for the spark plugs I'm almost certain they'd be black to an extent from all the idling I did before realizing my previous o2 sensor was toast. Unless that would "burn off" or something like that right after you start running lean to show the white burnt colour you speak of.

Note: If you have a boost leak with LH 2.2 it will mess up your AMM baseline adjustment. I recently replaced turbo outlet end of the boost hose going from the turbo to the intercooler on my 88 765T. I had to turn the AMM adjusting POT screw CCW (lean) more than 3 turns to get the LED tester to blink. It was way rich. The last time I messed with the AMM adjustment I found a boost leak in mentioned boost hose.

Sorry, forgot to update. I checked the resistance the next day and has essentially maxed out the pot at around 1000ohms and was simply turning it the wrong way. I set it back to 380 and retuned and all is now well. Aside from my spool issues but hopefully I'll fix those soon as well. While chasing my spool issue I have run into leaks a couple times and have retuned after every time I repair one.