Code P0171

Hello, actually p0171 is a lean code. What makes you think it's running rich? Lean codes can be triggered by a rich running engine because of a misfiring cylinder there is unburnt fuel and oxygen going into the exhaust and the oxygen sensor is reading this. Are you getting any smoke from the tail pipe?

Okay, there's a few possibilities. A stuck lean oxygen sensor, meaning stuck low voltage .500-.100 millivolts, a non functional oxygen sensor heater circuit, a bad fuel pressure regulator, a sticking open fuel injector, or a bad catalytic converter. Any of these are a possibility with the mileage your car has. The oxygen sensor can be tested with multi-meter, if it's a 4 wire sensor, the 2 white wires are the heater circuit and should read 12volts. The other 2 wires are high and low sensor signal. One of these wires will having a fluctuating voltage signal from .1 to .9 millivolts. A lot of times, and this probably the issue, these sensors get stuck below .5 millivolts telling the ECM that the exhaust is lean causing the engine to go rich. Checking the fuel pressure regulator will require a fuel pressure gauge. But if you have a scan tool that can read live data, it will be much easier to see what the sensors are doing. The car should be scanned for diagnostic trouble codes first.
https://www.2carpros.com/articles/how-to-test-an-oxygen-sensor-02-sensor
https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers
Now if the car is running lean, due to a bad intake manifold gasket, a vacuum leak (broken hose), a clogged fuel filter, a stuck oxygen sensor, the oxygen sensor is reading .5 to .9 millivolts the ecm seeing a rich signal sends the engine lean to compensate.
https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

With the mileage you have, you're car is due for new oxygen sensors anyway at this point. But the car should be diagnosed instead of just replacing parts.

Okay, so if you can actually feel the miss at higher rpm's then there's either an injector leaking, failing coil pack or a vacuum leak. Have you had the ECM scanned for diagnostic codes? There may be a code stored even if the check engine light is not on. That would be my next check, and an injector flow test should be done as well. This car is older and does need all these sensors tested, Do you have access to a scan tool? Your local part store will scan the ECM for free. This will give you direction in diagnosing the rich condition.

The higher rpm off the throttle points to a vacuum leak or a sticking open EGR valve. EGR is only used at higher rpms and sticking open at idle will cause issues. A vacuum leak will allow air into the system and trick the ecm into going rich when it really doesn't need it. I use water to check for vacuum leaks, spraying it around the intake gasket and listen for it getting sucked into the intake manifold. It's a pretty easy test. Were all the spark plugs black or just one or two?

I would go after a vacuum leak first, something is causing the engine to go rich, if the oxygen sensors are okay, I am assuming they are. But check the intake manifold gasket for a leak. I don't think this engine has a mass airflow sensor. So pour some water over the intake manifold while it's running and listen for it being sucked in. This is the safer method to doing this.

Glad to hear, oxygen sensors can be tricky some times. Let us know if we can be more of assistance.