Turtle holds lean burn pretty much indefinitely, except for when it drops out occasionally to clear the cats for a couple of seconds, no matter where I buy my gas. That's one thing the (sort of irritating) throttle smoothing circuit seems to help with. It doesn't really stumble at all in lean burn.
I cleaned the EGR plate and plugs when I first bought the car, and they weren't really even bad. I usually only get a little stumbling if I'm cruising in fifth at like 35-40 mph, then I hit the gas. If I downshift, no stumbling at all.
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