Code P0171, Engine shuts off in traffic?

Hello, you are correct about the carbon build up on the spark plugs and the black smoke, the engine is running rich, but this can be caused by a sensor sending skewed data to the engine computer (ECM). If for example, a sensor is reading lean, the ECM is going to compensate for that condition and enrich the mixture when it really shouldn't. Now here's the tricky part. With an incorrect mixture like this, cylinders will misfire, Misfiring cylinders will cause unburned air to enter the exhaust, causing the Oxygen sensors to read air(lean). It can also be caused by a large vacuum leak, which is the most likely cause of a P0171.

You have a scan tool.
Can you look at any live engine data with your scan tool? Look into Live ECM Engine control module data, and (1) look at the MAP sensor voltage at idle, (2) look at Long Term and Short-Term Fuel Trim (LTFT-STFT) most likely because of the code the fuel trim will be positive. The ECM is reading a lean exhaust so it will add more fuel.
Also check the Oxygen sensor voltage, B1S1 and B1S2(rear oxygen sensor).
If you have the means to do a smoke test on the intake manifold to check for leaks, then do that. If not, here are some guides that can give you some directions. I will sometimes just spray water around the intake manifold gasket and listen for the sound of a vacuum leak.
I don't believe this vehicle has a Mass Air Flow sensor, I don't see one on the wiring diagrams, but if it does it would be on the air intake boot usually near the air filter housing. It measures the air coming into the throttle body. I will post some more information of fuel trims as well.
The MAP sensor voltage should be below 1volt at idle with the throttle plates closed and as you open the throttle the MAP voltage should rise to around 4volts or so.

The 3rd diagram are some possible causes, but disregard the Mass Air Flow sensor, I don't see one on this vehicle. The 4th diagram shows what a MAF looks like.

https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers

https://www.2carpros.com/articles/7-common-symptoms-of-a-vacuum-leak-in-your-car-how-to

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

Its only for the MAP sensor, but the thing to remember is that the Intake manifold, which is where the MAP sensor reads (Manifold Absolute Pressure) is actually in a vacuum, so when its reading low voltage (0.79v) at 2.9psi, this is compared to atmosphere pressure which is about 14.7 PSI. So, 2.9 PSI here is in a vacuum compared to the outside air pressure. Thats how absolute pressure readings work. So, when you open the throttle plates and the intake manifold pressure and outside air pressure equalize, voltage will rise to 4volts at atmospheric pressure of 14.7 psi.
So, if you have higher voltage at idle with the throttle plates closed, you know air is leaking into the Intake manifold through a bad gasket or some other location. Hopefully that makes sense. Manifold vacuum is at its highest with closed throttle plates.

But the Fuel trim numbers will tell the story of what the ECM is doing to compensate for any air leaks. High (Positive) fuel trim numbers mean the ECM is adding fuel to the air/fuel mixture to keep it at a certain amount for emissions purposes, Low (Negative) Fuel trim numbers means the ECM is taking fuel away from the mixture due to an already rich mixture, i.E. Leaking fuel injectors, high fuel pressure, lack of air, for example.

Okay, glad to hear you got it fixed up.