If you are seeing black smoke from the exhaust, this indicates a rich fuel mixture (too much gas or too little air) is entering the engine. There are many possible causes for this condition; an organized diagnostic approach is required if you want to accurately determine the cause of the problem.
The first three tests you will need to perform are a fuel pressure test, an engine vacuum measurement, and check for stored OBD-2 service codes and freeze frame data. I assume the check engine light is on; if it is running rich it should be, so there should be service codes stored to take to a troubleshooting manual for relevant diagnostic procedures for the affected systems.
Before getting too deep into engine control diagnostics though, you will need toverify that the base engine is mechanically healthy. No engine control system can make a faulty component operate as designed!
(1) Measure fuel pressure and verify that it is within factory specs; if the fuel pressure is too high due to a return line restriction or a faulty fuel pressure regulator, the engine will run very rich.
(2) Check engine vacuum; it should be around 21 in/hg and steady at idle. If it is too low, or if the gauge needle is bouncing, you will need to perform a compression test and verify correct cam timing and correct the cause before proceeding. Verify that the exhaust is not restricted, as from a failed catalytic converter. If engine vacuum is low, the MAP sensor will read low causing the control module to richen fuel mixture in response to a perceived high engine load condition.
(3) Check coolant temperature and verify that the engine is reaching 195 degrees operating temperature. If it is too cold due to a faulty or missing thermostat or a low coolant level that is not reaching the coolant temperature sensor, then the controller will think the engine is warming up from a cold start and respond by injecting extra fuel.
(4) Verify that there are no vacuum leaks, such as from a faulty plenum fgasket, leaking intake manifold gasket, stuck opern EGR valve, or broken vacuum hose. Low engine vacuum = low MAP reading, resulting in a rich fuel mixture to compensate for a perceived high engine load.
(5) If you are experiencing hesitation on a vehicle with a mass airflow sensor (I believe this vehicle does have a MAF); make sure there are no air leaks in the hose between the MAF and the throttle body allowing unmetered air into the engine. This will not cause a rich mixture, but will cause severe hesitation.
(6) With the engine warm and in closed loop operation, measure front O2 sensor voltage with a high impedance DVOM. Ideally, it should be varying fairly quickly above and velow .45volt, switching over center. It will probably be pegged rich around .90 volt if you are indeed running full rich; what you are mostly checking for here is to make sure it has a reading and is not at zero. A near zero volt reading would cause the computer to think the mixture is too lean, causing it to compensate by injecting more fuel.
If all of the items above are OK, then you will need an OBD-2 compliant scan tool to obtain fault codes and freeze frame data to proceed any further. Iin addition, you will need the factory diagnostic procedures to go with the stored failure records. If you reach this point and do not have access to these items, you should probably take the vehicle in to a shop with the equipment needed to diagnose the failure; otherwise it comes down to guesswork which can become very expensive pretty quickly and may still not fix the vehicle. I would suggest NOT taking it to Autozone or any other parts store for diagnosis; the counter guys there have a code reader tool and little if any training (anyone off the street can get a job there and be out in the lot diagnosing cars the next day). It is thir job to push parts, and they are wrong in their "diagnosis" more often than not. Although taking the vehicle in to a trained technician will cost something for diagnosis, it will be accurrate and therefore cheaper in the long run.
I hope this is helpful; if so an accept would be most appreciated! Thanks!
