Buick 3800 V6 - stumbles, lack of power, need direction

Catalytic converter replaced with a bolt-on Walker unit. Car runs MUCH better and feels close to normal. I also returned the issue air control valve after the old onecs pintle refused to move all. The idle speed is a more normal 750 rpm or so. Much improved power.

I'm still concerned about fuel trims that go below -10% (i.e., rich), high MAF values at idle ( 4.2 or more g/s), and post-cat O2 sensor 2 moving around a lot (as high as 750+ mV).

I'm wondering if the O2 sensor 2 has failed from previous problems. On a new converter, poor catalytic activity seems unlikely. If so, it could be driving the engine to think the engine is running rich, etc.

What about those MAF values?

It's possible but when you graph throttle snaps and idle you want to give it a rest period in-between readings. So snap throttle from an idle then let the o2s get back to normal switching. Which of course means upstream should switch between 750 to 150 mv and down stream should be fairly steady at 450 mv. The only catch to this is when replacing a component like a catalytic converter you will want to reset fuel adapts. Or let it run for a bit to let it relearn the trims. But I would say as a safe bet if o2s have never been changed is to go ahead and change them out. But would say as well to do two quick tests which is create a vac leak and watch fuel Trims and see if it goes lean. Then reconnect you vac leak and after numbers stabilize to spray carb cleaner or another fuel source into throttle body and see if numbers go rich.

Those screenshots aren't horrible. 4.2 grams is acceptable on a 3.8 and +-10% is acceptable fuel trims. As hardtop mentioned fuel tables should be reset to see an immediate change in long term fuel trims (its a learned value).but it doesn't look like short term trims disagree with long term much . You could clean the MAF and see if it makes any difference MAF and 02s are huge inputs to fuel trim calculations

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I'll look into these points.

The graphs were taken from my Diagun V, which recorded data from a test drive. (Recorded so I can focus on driving, not watching a screen.)

Thanks for your patience in looking at that data with me. I wish the Diagun would support landscape mode for graphs. They would be much easier to read that way.

I'm looking at this closely because I don't want to ruin the new catalytic converter. (But it's hard to assess behavior without one thanks to the O2 sensors' roles in fuel trim calculations.)

When I get home I will record the same parameters with my snap on modis edge and save the files to give you a known good to compare to as I have a 2000 Impala with a 3.8 as well. If you want to go to snap-ons website and download the shop connect software you will be able to view the data tables I can email those to you if you like just pm me your email.

Alex

In considering the replacement of the O2 sensors, I'm partial to factory sensors. However the new GM/Delphi sensors look quite a bit different from the factory Delco ones.

The Walker and Bosch ones, though, look much closer. They're also half the price

And experience with aftermarket O2 sensors in these cars?

Bosch are good I have used them in the same engine as the data I sent you.

Thanks. I ordered the Denso ones at about the same price based on generally negative comments about Bosch in this engine elsewhere.

Guess I'll see.