2001 4.3, with 200k on original engine I believe.
When I bought it, it had too many codes in the history to start diagnosing, so I cleared them out.
p0171(bank 1 lean) and p0300(random misfire) came back on.
It stumbles at idle, hesitates and lacks power while driving, as well as intermittent flashing CEL indicating misfires, usually under heavier load such as accelerating up a hill.
It has been some time since I have had to diagnose this kind of stuff, but I have TorquePro+bluetooth adapter to pull codes and sensor readings.
First thing I tried was spraying hairspray around intake manifold gasket while idling, or tried to at least(mechanical fan made things difficult). Didn't notice a change in engine speed.
I also pulled a spark plug to see how they looked, plug condition is ok, but sooty, indicating rich, not lean. At the time I couldn't find my bluetooth adapter to pull codes.
Then I took a look at my sensor readings after finding my adapter, and bank 2 o2 sensor is cycling up and down like normal, but bank 1 sensor is just reading a constant .1v, however I don't currently have an o2 sensor code. That being said, I think I might have before I cleared them all.
I can not remember fully normal operation of o2 sensor, but if I recall correctly, if the voltage on upstream sensor isn't cycling, then the sensor is bad yes(assuming it's not wiring)? Even though it's not setting a code? Or could some other cause of a lean condition cause the o2 sensor to not cycle?
I can't really think of anything else that explains why only bank 1 is lean and missing, other than intake gasket.
I wish my memory was better and that I could remember how to diagnose the o2 sensor, as I think that is the cause of the issue.
Any help figuring this out is appreciated, I'll continue searching for info/diagnosing in the meanwhile.

 

Welcome to the forum. You're right on target. Bank 1 02 sensor should cycle. The code is calling lean because the constant reading is too low when the throttle gets opened. A delco cap and rotor is the place to start solving the random misfire.

 

I thought about cap and rotor, but I think it would be having problems on both banks if that were the case?? I probably will put a cap and rotor on it at some point, but I am trying not to just throw money at it. I guess what I am not quite sure about is whether the o2 sensor being continually low is the a symptom or the actual problem. It would really help if I had another code to go on, but atm since only bank 1 is lean I am going with the logic of it being an item specific to 1 bank. Which the only 2 things I can think of are LIM gasket vacuum leak or o2 sensor itself. But if it were the symptom and not the cause, shouldn't it still fluctuate at least a LITTLE bit?? I mean it is holding steady at .1v.

 

The 02 sensor is the cause. The sooty spark plugs tell us that it's running too rich (too much fuel). The computer uses the pre cat 02 sensors to adjust fuel, the sensor after the cat is only for triggering cat codes. The computer is adding too much fuel because bank 1 sensor is saying there's not enough. The basic tune up parts are important to the 4.3 so they don't kill us at the pump.

 

Unplug the MAP sensor see if it still does it, if so, get a new one. An MAP sensor is to the left of the spider injector plug and has three wires.

 

Thanks, but I resolved this months ago, lol. It was in fact the o2 sensor.