OBD2 scan reveals unexpected engine problems?

Yes, I would change all of the spark plugs and wires at once. Also, on the EGR I would try cleaning it like you have suggested and if that doesn't work replace the valve.

Here is a guide to help walk you through doing the spark plugs:

https://www.2carpros.com/articles/how-to-change-spark-plugs

AC Type .................... 41-921

here is the location of the EGR and I can tell you with this type of valve you will probably end up replacing it, but you never know. Check out the images (below). Please let us know what happens.

Ken, thanks for your response. In researching the problem, I found several additional parts to the EGR valve-- presumably bought separately.

(1) EGR solenoid
(2) EGR sensor

This information comes from a phone chat with an auto parts rep, so I am not sure how much is accurate. In general, should I look further into these items? Ordering them locally may be more difficult, but Rock Auto may stock them.

I like Rock Auto; I would get the parts from them. I am not sure about buying the EGR parts separately, I think it is all inclusive. It will show an image on rockauto to confirm.

How much of a hazard is posed by driving the car for a few days' of local trips with the old (and suspected defective) EGR valve still in place, pending arrival and installation of the new EGR valve?

The P0404 and P1404 codes appeared when I asked my local O'Reilly's to scan the system for errors, if any. Unfortunately, I had not done regular scans before that date, so the first (but unnoticed) code appearance can be only guessed to have been two months ago.

Although I would like to clean the original EGR valve, and actually loosened the valve to invert it for overnight "soaking" in brake cleaner spray, the next step is to pry the silver EGR mating gasket loose from the throttle body. If that gasket breaks or crumbles, that makes the situation worse, since I cannot use the original but broken gasket.

The EGR valve in place but not working will cause the engine to emit KNOX gases which is not good FYI.

In this car engine, the EGR valve appears stuck closed-- when I start and run the engine to warm up, exhaust smell enters the cabin. Trying to understand the EGR system, the oxygen sensor tells the BCM computer the engine is ready to receive partially-burned exhaust gases, and the computer diverts some of the gases into the EGR. Which is stuck in the closed position, despite the computer's order to open for exhaust gases. That closed position explains the exhaust smell, but doesn't it also introduce the possibility the computer has switched to a leaner mixture, to accommodate the exhaust gas it "believes" are being diverted into the cylinders? And that means my stuck EGR is actually running my engine hotter and leaner. What damage could this introduce?

More information.
This afternoon, I attempted to remove, disassemble and scrub-clean the EGR, hoping clogged passages are the only problem. Unfortunately, I could not devise a way to take apart the EGR for a more thorough cleaning than simply squirting throttle body cleaner into both passages. Is it possible to disassemble an EGR for this GM engine? I found one TORX-type screw on the bottom-center, which seemed to be spring-loaded, but which simply rotated, and did not unscrew. And where two bolts appeared to pass through the EGR body, there was no visible opening for a nut or bolt head on either side of the body.
This EGR is the electronic type, with only a five-pin connector inserted into its top portion, and no other lines or connectors.

Here is a multi-view graphic for the part:

https://www.rockauto.com/en/moreinfo.php?pk=6698424&cc=1303287&pt=4968&jsn=9

.

For better or worse, I have decided to do what most people do-- buy a new EGR valve. There are too many factors to control, especially since the car has an undocumented history, to spend more time experimenting with a used, questionable valve.

Thanks for the discussion of options !

You are welcome, please use 2CarPros anytime we are here to help.

Ken, in attempting to understand more of the EGR system, does this explanation of my EGR symptoms make sense?

In this car engine, the oxygen sensor tells the PCM computer the engine is ready to receive partially burned exhaust gases, and the computer diverts some of the gases toward the EGR. The EGR, however, seems stuck in its closed position, despite the computer's order to open for exhaust gases. That closed position explains an exhaust smell in the cabin, but it also means the computer has switched to a leaner mixture, to accommodate the exhaust gas volume it "believes" is being diverted into the cylinders.
And that means my stuck EGR is actually running my engine hotter and leaner.

Nothing explains "an exhaust smell in the cabin". there should be no smell in the cabin.

This afternoon, I got the EGR, and plan to install it tomorrow.
Need your opinion on whether to use the stamped metal gasket which came with the part, or use an exact-fit, layered asbestos fabric (?) gasket for the same part, which I ordered separately, in case the EGR did not come with a gasket. The layered, fabric gasket is the more conventional kind of gasket, and may provide a better seal under extreme heat.

The gasket it came with should work just fine.

Ken,
After installation of the EGR, an OBD2 system scan (O'Reilly's) showed no primary diagnostic trouble codes, which is good news. .
Now, I am puzzled-- if I have not installed the spark plugs and wires, yet, how could the P0304 code for "misfire on cylinder 4" simply disappear?

Because if the exhaust gases are leaking from the EGR it can affect individual cylinders.

If exhaust gases can leak from the EGR, could that explain occasional whiffs of exhaust gas in the cabin, too?

(After EGR replacement, I have not detected any exhaust gas smells-- so far.)

Ken, now I understand what you meant by "leaking from the EGR". The EGR, itself, does not leak to the outside, but fails properly to open or close on computer signal. That introduces an improper fuel/air volume into the nearest cylinder.

Hi,

Thank you for the update. Have you been able to resolve the issue? We're interested in knowing.

Take care,

Joe

Yes, Joe, I have been able to resolve the codes issue for my Buick 98.

Earlier, I had received two EBD2 warning codes-- P0404 for an EGR
valve "open position performance" and P1404 "closed position
performance" and one EBD2 code P0304 for "cylinder 4 misfire
detected".

After installing a new GM-brand EGR from RockAuto, all codes simply
disappeared, including the P0304 "cylinder 4 misfire" Of course, I have
not operated the car much since the EGR installation, so the "Check
Engine" light might reappear.

My problems with this car arrive dramatically, and more or less
unexpectedly, because the wife of the person who sold the car to me
simply had tossed out all the services documentation over the years,
so, I have to guess what items will malfunction next.

The car has 252k miles, but was well-maintained by a professional
shop, so that offers some relief.

The car was received without the passenger side wheel well inner
sleeve, which protects the accessory belt and pulleys from splashed
water and road debris. I know the number of the OEM part for the
sleeve, but no one' has it. (I have not searched all the junkyards.)
To resolve that problem at least, for now I am searching for a
robust, waterproof material to use for a hand-made sleeve.

Any ideas?

Hi,

It's good to hear you have resolved the problem. As far as the part you can't find, is it the inner fender liner? GM made other vehicles that may use the same one but have a different part number. It's just a thought.

Let me know.

Joe

Yes, Joe, it makes sense that a widely used GM engine like the v6 3.8L would have a similar fender access panel. Between a junkyard find and showing a veteran mechanic a photo of the gaping hole in the inner fender, I might find something.

On the EGR cleaning procedure, I now have the speedometer cable, and probably can do it this week. The "cable" I bought from eBay is only the cable housing, not the internal cable.
1. Do I need to clip off the little knob at cable's end? I would hate to lose anything in the EGR passage.
2. Running the cable like a Roto-Rooter will produce loose soot, and that should not enter the new EGR valve. To blow out the soot before the EGR is bolted back into place, should I run the engine at idle, or try to operate the engine briefly to force out the carbon before reinstalling the "EGR? (The EGR is electrical, not pneumatic).