96 Dodge Intrepid 3.5L P0171 Code: Rich Running, Black Smoke, and Fuel Trim Issues

Have you checked the EGR valve and fuel pressure? Vacuum leaks on older Intrepid engines are often hard to detect without using a scan tool to monitor fuel trims while applying brake cleaner under the intake plenum. Fuel rail injector gaskets are known failure points on this engine — if not replaced during the intake gasket work, they may now be leaking. The front intake bolts are also prone to loosening or breaking, which can cause repeated gasket failures. Before blaming a mechanical issue, verify that the oxygen sensors actually respond to rich and lean conditions.

We recently replaced both intake and fuel rail gaskets with the same symptoms. The O2 sensors do not shift from their lean reading under any condition. We performed an EGR leak test using Alldata, which passed. Propane and brake cleaner tests did not reveal vacuum leaks — though given the cost of gaskets, we opted to remove them for inspection. Fuel pressure is within specifications: 42 psi with vacuum, 51 psi without. Compression testing was normal, and exhaust backpressure readings were acceptable.

You should be able to manipulate the O2 sensors by introducing propane into one of the intake ports while the engine is running. If the sensor does not respond, it may indicate a faulty O2 sensor or wiring issue. Also ensure that camshaft timing is correct — improper timing can cause misfires and abnormal fuel trim behavior. Confirm whether the MAP sensor's vacuum reading on the scan tool matches the actual vacuum from a mechanical gauge.

I still cannot manipulate the O2 sensors past a full lean condition. When I remove a large vacuum hose, the engine idles at 2500 RPM and runs worse when fuel is added. Can faulty O2 sensors cause such a rich running condition? Also, the MAP sensor readings match those from a manual vacuum gauge.

Yes — it's uncommon for both O2 sensors to fail simultaneously, but not impossible. Did the TSB for O2 sensor wiring apply before or after symptoms began? Are the signal circuits from the O2 sensors back to the PCM intact? Wiring issues near the bellhousing can cause intermittent faults. Does fuel pressure bleed off when the engine is off? Is compression consistent across all cylinders?

Is the idle fuel pressure actually 42 psi? What is the fuel pressure when the key is off? Have you verified camshaft timing? While rare, a stuck manifold tuning valve (if equipped) could cause air starvation and result in rich running. Can you obtain a fuel trim reading from the scan tool to confirm the actual mixture?

Thank you so much! The issue was found — the crankshaft timing belt was off by one tooth. I never would have guessed the engine would run as smoothly despite this misalignment. This is a huge relief and thank you for your help, Jeff!

You're very welcome! Glad to hear you found the root cause.