Thank you very much guys,
..for your myriad recommendations, I have a lot of catching up to do...
- Today I put the new rubber intake trunk on.... There is no improvement.
- I have a smoke machine for Halloween.. I'll try it on a cold engine tomorrow so the cooling fan(s) doesn't blow the smoke all over..
- I'll find out what the Aux Air Valve is and check it, I haven't heard of it before.
- I'll perform the propane torch test tomorrow and see what voltage I get from the sensor.
- It is one the few things changed during the recent work that has a direct impact on air/fuel ratio.
- The engine runs great with it disconnected.
- If the mechanic kept my old O2 sensor, I'll have him check it (which he might have done when he decide it best to replace it). If it was junk, then the car may have been set up to run without O2 signal and I'll reset mixture and idle without the 'new' O2 sensor. This could be consistent with the mechanic suspecting the ECU has a non-factory map. I'm sure he'd be happy to set up an AFR gauge to check.
...before any kind of fine-tuning of the mixture or idle speed can take place. Also make sure the auxiliary air valve is all the way closed when the engine is warmed up!
- The mechanic went through the settings after installing the 'new' O2 sensor and checked idle. After that I assume the AFM, ECU and L-Jet all cooperated to achieve the correct mixture for the conditions.. (unless it's tricked by bad O2 signal)?
- I'll search for info on the Aux Air Valve, never heard of it.
The oxygen sensor system on the early GTV6 is very basic. The zirconia oxygen sensor needs to be heated to about 500-600 degrees F to generate a voltage and have influence on the air-fuel ratio ('84 and up GTV6s use 3-wire heated oxygen sensors to help them warm up faster).
This is consistent with my problems, which only occur at full temp. That's when the engine starts running rough. My interpretation of this is, it coincides with the moment the O2 sensor comes alive.
Again - the engine runs perfectly under three conditions:
1.) When the engine is cold.
2.) With throttle position at more than 80%.
3.) With the O2 sensor disconnected (although hot idle increases by 1000rpm).
These 3 conditions are when the ECU/L-Jet system calls for "full rich", and the O2 sensor is inactive.
Get your open-loop mixture and idle-speed settings dialed in before worrying about the O2 sensor.
This is what my mechanic worked on. Then I had to come take the car.. He got the basics dialed in, got it to run well enough and then I came and got it. We both assumed the issue was minor and we'd figure it out over the phone..
The mechanic remains involved, happy to take my calls, and I'm cross-checking his ideas with your recommendations..
Thanks again, guys,