Old plugs were black and brown with carbon deposits built up on them. All four of them. The new ones are undoubtedly getting to be just as bad since the system too lean code came back after I changed them. It does idle on 3 cylinders. Also, wouldn't a compression test indicate a valve problem? Thats the main thing I was testing for when I did a compression test. I couldn't imagine any problems with the engine internals really considering this is a rebuilt block with a little under 40k on it so far. Even if something did go wrong like lifter or something, you think it would affect the air/fuel mixture?
the thing is that you have engine lean code but you say is blowing black smoke which is usually a sign of too rich, so somehow you get a feedback from the O2 that is too lean and the ECU will try to compensate by giving it more fuel (based on the code description) - now that might be the reason you are getting black smoke. You are right, the compression test should tell you if you have a bad valve. If it idles in 3 then find out which cylinder is and focus on that one (injector + spark related) - if it is not a constant one then it might be the O2 feedback that is bad, could be the sensor ground, the sensor itself or who knows - speculating here but it could also be that the spark is off randomly and you get unburned fuel in the exhaust which burns in your cat and there's the black smoke (you also said is dripping raw fuel and that your cat was burned inside so there was my thinking behind that). If you find the problem post an update as this is an interesting one.
Even if something did go wrong like lifter or something, you think it would affect the air/fuel mixture?
if for example 1 cylinder does not burn the air/fuel mixture you could get both more oxygen and raw fuel in the exhaust (should also have backfire though) - i didn't know if you eliminated the spark as a cause so that's why i suggested that, but your logic is correct, if an intake valve was stuck open or was bent or not open at all it should show on the compression test (given that all 4 cylinders show close to the same reading) - now if it is an exhaust valve that has a rod bent or rocker broken and it does not open you could still get a good compression reading but that cylinder won't participate at all - this was just a suggestion if you were running out of ideas and eliminated all the other possibilities
EDIT: also thinking of what could happen if an injector does not open at all, might just pump air in the exhaust and affect the o2 reading which could result in the ecu/pcm compensating and you would get more fuel in the other 3 cylinders and out in the exhaust as a result, but from what you described is quite extreme so again, just a thought along with all the other ones
