See, here's why I thought the O2 sensor might need replacing. In 2002, when I got the 1985 Fiero, its O2 sensor looked like the left half of this image:
When I went to remove it, I put a screwdriver in the crack and gave a very slight twist and the thing came apart as you can see on the right. And yet the computer thought everything was hunky-dory, at least until I sprayed the thing with WD-40.
Well, replacing the O2 sensor on the Escort took about ten minutes and it didn't fix a damn thing. *sigh*
On the plus side, I didn't have to put the car up on jack stands to fix the exhaust rattle. I lay on the driveway on the passenger side and could reach the flex coupling with a wrench; the rear U-clamp had been snugged up but not fully tightened when I put the engine in. I tightened it down, and the rattle is gone.
Since I've eliminated the O2 sensor as the source of my closed-loop stumble, now I have to look elsewhere for the source of the trouble. I'm not sure what that means yet.
I put another jug of the Techron fuel system cleaner in and filled the tank. I got 29.9 MPG in mostly city driving, which is not very good fuel economy for that car. I previously got 33-36 with the various Escorts I've had; with the old engine that car's personal best was 34.
But the most recent figure is, after all, almost entirely city driving. To avoid the stumble, I have to keep the engine speed over 1,800 RPM, which means avoiding 5th gear most of the time. It seems as if I've been doing a lot of standing in the car, too--like the drive through at McDonald's--and that doesn't help. Also, if I have a leaking fuel injector (which is a possibility I have not yet explored) that's certainly not going to help, either.
So the next thing to do is to pull the spark plugs and check the carbon deposits on them. If one is black, that's the fuel injector to replace. If all are the same, I'm going to suspect the fuel pressure regulator and try swapping it with the one from the other engine.
But not today. I'm still feverish and all I really want to do is go back to bed. I don't even have the energy for WoW or anime.
Yeah, I'm sick, all right.