Also I have checked all places for vacuum. As I have noticed this type of engine could have clogged passage from EGR to intake manifold as the result error code:
EGR-3 will be stored

EGR3 - error
 

After all work done, plus spark plugs where replaced with original from GM dealer, I have observed the following:
On highway when accelerating at speed over 120Km, when release gas pedal the check engine lamp goes on. Code reader says: EGR3 error. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>
But EGR valve works fine – I verified all wire, verified valve each solenoid by connecting 12V and observed valves movement, also verified valve solenoids during drive – connecting LED’s to each solenoid wire – all solenoids are operational from engine control module.<o:p></o:p>
EGR passages are not clogged and fine – one from exhaust the second in intake. <o:p></o:p>
As I have mentioned vacuum and fuel pressure are fine. One thing is unusual: on Idle when engine is hot when press and release the gas pedal the engine makes high rpm to low rpm several times like bouncing – trying to find idling point. At cold engine - this is not happens

The high and low RPM movement sounds to me like Hunting at idle.

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Of the following, sometimes just 1, or just 2, and sometimes both can occur:

1) Hunting at idle is usually caused by an O2 sensor becoming Pluged up.

2) A plugged up O2 sensor usually causes to high emissions output from the engine. This is because when it becomes plugged, it reads less, so then the engine is commanded to output more (usually fuel) to compensate with this.

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I suggest checking the O2 sensor readings on the ECM monitor and comparing them with the factory readings it should be getting.

Also, It might be wise to consider replacing the O2 sensor if the hunting continues.

When you changed the plugs, how did they look? Were they dark/black?
Usually a Pluged O2 sensor causes extra fuel to be dumped, then the exas fuel mixture is not completely burning, then coming back through the EGR and thus probably causing an EGR fault code because the EGR is probably open more than usual to try to compensate the exes emissions.

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Now I forget what you have as far as O2 sensors. But being a 1992, I believe you only have one O2 sensor and it is behind the engine on the exhaust manifold.

If you have 2 O2 sensors, then the second one will be down the exhaust pipe some place right after the catalytic converter.

I believe you only have the single wire O2 sensor, these are usually 20-30 dollars a piece and can be found at places like advanced auto parts and auto zone. (I personally buy Bosch when I replace my O2 sensor).

If you have the 3-4 wire O2 sensor, then you will probably have to pay around 40-60 dollars on a replacement. Same stores. (I also buy Bosch brand with these too, but you can buy different if you chose).

The downstream O2 sensor after the catalytic converter will probably be a 3-4 wire version if it does exist.
Being a 1992, like I said above, you might just have one O2 sensor on the exhaust manifold and it is probably just a single wire.

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If you replace the O2 sensor I suggest adding Something like Anti-Seize or some form of thread lube (NOT thread locker) to help it keep from rusting together and so it helps make it easier to remove later. Be careful not to get it inside the sensors breather holes. Usually the come with this, I like to add a little extra myself.

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I hope this helps, I'm interested in seeing the outcome and in helping you get this car working. Seeiing (from my viewpoint) that you put this much money and effort into it, its worth the extra effort, the money parts your domain...lol :P

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EDIT:
The forums were glitched for some reason. They were not properly operating. So I had to attempt to delete and repost a couple times to effective post this reply.

Sorry if this caused any incontinence to anyone, and sorry if the previous posts still show up, if they do I will edit them out. Hopefully they are gone.

I have only 1 heated sensor, 4 wire. Before this emission story happened
I have replaced sensor and it got around 30000 km. Visually looks clean and no deposits. I got it from dealer, but they changed the design - sensor does not look like old types. What I can see from diagnostic and oscilloscope - the sensor voltage is slowly changing during idle, from 400 to 800 millivolts with new type of sensor. I have tried to use original used old sensor AFS-75 and voltage does the same, but a little bit lower 300-600 millivolts. Probabbly current sensor is fine. But spark plugs looked very dark with small gray color spot on the top. Plugs were new from CHAMPION, when replaced it was max 500 km milage. As I have mentioned before I have replaced them with AC DELCO platinum from dealer. Also I have to mention that engine was rebuild - so that all my trials were done with rebuild engine. But what is suprising - that EGR#3 error happened in exactly the same pattern as for old engine.

Sorry this is a long post, but I left everything so you can se my train of thought.

I did not realize it but your fuel pressure should be over 40 PSI at all RPM ranges of the engine. Please read on and tell me what you think.

This is my thought process, I suggest you verify everything else you did first before looking into replacing a fuel pump as they are expensive and its not an easy job. And it involves lots of gas and gas fumes sometimes.


Did you replace the EGR? idk if you did.

I'm at a loss anymore. Sorry I can not be of any more help other than tossing around different thoughts.

Did you get the same code with the old O2 sensor?
If the sensor is different, it might be possible the slight difference in voltage is causing the issue.

Because you only have the one O2 sensor, that means the converter is not causing the problem.

You said all injectors are checked and proper?
all engine sensors are correct? Wires are correct? etc?

I'm going to say this; Maybe the EGR has a sensor built into it to monitor its movement? Its possible. If I'm right, there are 5 wires. Maybe everything working correctly, but the Computer thinks otherwise.

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Before we get into that. What is the error code itself? should be something like P0000 or something like that listed on the scanner.
example: P0314 P0101, etc...

"EGR #3 error code" This is useless to me, sorry. :P
I probably should have asked this sooner.

Also, I will need to know if it is the stored error code, or if it is the real time error code.
and did you disconnect the battery for at least an hour? Did the code show back up? Did any sensor readings look odd when it showed back up?

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looking back on your other posts, I'm seeing 35 PSI.
And you have high NOx. Which right there shows you have to much air, not enough fuel. And Maybe the EGR error is showing up because the EGR is at full open?

Or maybe, the engine thinks the EGR is not proper opening, and causing the high NOx, but really its because of not enough fuel.

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If you have a factory service manual, I suggest you find out what the fuel rail should be reading.

Because I have both the 3.1L MPFI v6 and the 3100 SFI v6, and they both run at around 45 PSI fuel pressure.

The reason I'm coming back to this is because the sensors work, the EGR works, the O2 seems to be reading properly, its not the converter. Ignition system is correct, MAP sensor is correct, etc...

It leads back to fuel & air ratio in my mind.
And knowing what my engines run at, I'm thinking the lack of fuel pressure is causing the engine to get a little less gasoline per injection.
Yes the ECM should compensate, but its running based on a preset that the fuel pressure is in a set range.

here is a link to an Ehow that confirms what my shop manual says for both my V6 engines:
Fuel Pressure Requirements for a GM 3.1L | eHow.com

I suggest checking the regulator, and fuel pump.
But I think you said you checked the regulator. So I'm thinking you might want to test the fuel pump somehow.
When they fail, the fail in two ways: 1) they get weak and then just do not make pressure; 2) they just stop turning.

If anything, I'm still certain that you lack proper fuel pressure.
Because although our vehicles are different, the engines are not. The engines still need the same fuel pressure.

If I was you I would go to the dealership and have them open a shop manual and look at what the secs should be for you.