Looks like you've only touched 2 of the many potential causes for those results.
result from ignition misfire or misfire from excessively lean or rich air/fuel mixtures. Known causes: Engine Misfires, Ignition System Failures, Excessively lean air/fuel mixture, Excessive EGR dilution, Timing Issues, Catalytic Converter, Worn piston rings and cylinder walls, Carbon deposits on intake valves. NOx
is produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperature and pressure. Causes: Cooling system problems, Fuel Mixture excessively lean, EGR System Problems, Overheating Issues, Faulty O2 Sensor(s), Excess Carbon on Valves, Catalytic Converter, Timing Issues, Carbon deposits on intake valves.
Start with what is common. Timing problems, Catalytic Converter, O2 Sensors, injector(s) causing a lean condition... Then compression check on all 6 cylinders. Then branch off from there if all was good at that point.
You really need a way to "scan" the PCM for live sensor data, otherwise you're guessing. If you could see what O2, timing, and the injectors were doing "live", that would help you a lot. You'd either see the issue in the numbers, or see there is no issue with these systems and know to move on. If YOU are going to maintain the vehicle, you need to spend the $300 on a good scan tool. Otherwise, pay a mechanic their diagnostic rates which add up to $300 after only one or two trips.
I do not know if the Factory Service Manual addresses Emissions as I've never had to be concerned with that, but you may wish to scan through it and see. Here's a link to download it. Just make sure you are reading from the v6 section at all times. Also, Adobe Reader will let you keyword search the manual so I highly recommend using it. The manual is several thousand
One thing I notice is there was not an OBD test performed. It is my understanding this is always
done and if there are any codes, instant FAIL.
When you first turn the key from OFF to RUN and the instrument panel bulb/systems check runs, do the Check Engine AND Service Engine Soon lamps illuminate? If either doesn't, scan the PCM for codes.
Here's the thing... Most mechanic's don't have the tools or knowledge to work on these cars. You're going to be hard pressed to find a shop that can scan an OBD-1 ECM/PCM, and it's even harder to find an experienced someone who was actually alive and working on these engines 30 years ago. Sure, any good mechanic can muddle their way through, but they are going to be what you're doing; GUESSING! There is experience and education behind their guesses, but it's still guesswork.
Being able to scan for codes and see live sensor data eliminates almost all the guesswork and usually points a finger at something somewhere. Even if the numbers don't point to anything specific, that in itself tell you things. Scan-tools are very powerful assets and well worth the money even if they reveal no problems, EVER; only get used once or twice, or only successfully diagnose 1 problem. It's always there when you need it. I go to mine several times a year just to lay an eye on all the sensors and systems. I get a feel for what is normal for my engine in my environment, and get the peace of mind that "all is well". Sometimes you can catch a problem BEFORE a code sets. ...but mostly, being able to read and clear codes is where it's at. In the Factory Service Manual I linked for you, there is a well thought out diagnostic flow-chart for every
code the ECM/PCM can set. When you go to AutoZone or wherever and they pull the codes, their printout only lists the most common solution across all makes and models and it isn't always correct for you specifically. The flow-charts will walk you through diagnosis from most common cause to least common in a logical manner. NO GUESSING!
The Factory Service Manual, a good scan-tool that reads and logs live sensor data, a multi-meter, and some basic "mechanics" tools are the very minimum of what you need at your disposal to keep these cars going. ..and these simple things carry you pretty far. If YOU are not interested in turning your own wrenches, at least pass along the FSM to your mechanic.
Good Luck, and welcome aboard!