I did a test run today, and I think I have two problems.

First, I think craby is onto something in thinking that one or more cylinders aren't firing well. In open loop before it warms up, the car is almost decently drivable. Not great though, as if firing on seven cylinders. Tomorrow when the car is cold I'm going to try the suggestion to wet down the manifold and see if any areas dry more slowly than others.

I also think something else is wrong, probably O2 sensors. Craby mentioned this possibility early on, and I should have listened more closely. According to BertBanditReynolds' excellent "Scanner readings - What do they mean" PDF, O2 sensors are the only thing come into play when you hit closed loop. From the TunerPro data I can see that the O2 sensors are behaving, I think, very strangely. I would expect left and right to follow a similar pattern but they often diverge, and they cause the INT and BLM values (left and right) to go absolutely haywire. Keep in mind that I had to replace the cat because it was completely clogged with soot, so the O2 sensors being mucked up is entirely plausible.

AC Delco O2 sensors are $40/pop, so I'm thinking it might be worth just replacing them. Is AC Delco the best choice, and how hard is it to replace the O2 sensors? (I'll go look for a tutorial, but I wouldn't mind a real-life opinion on the effort required.)

As always, many thanks for your great advice!

P.S. I did the test run with a fuel pressure gauge taped to the windshield just in case, and fuel pressure is solid. I also have the full Tuner Pro data from the run if anyone is curious to look at it.

I like to use a non-contract thermometer to check the header temps. Very handy tool that can be used to check the temps of heads, coolant AND things like the rear end and bearings for overheating problems leading to failure.

Not sure if it can happen to an OEM wiring harness but insure the O2 sensors are plugged into the correct sides. With aftermarket harnesses it's been known to happen and you can guess what havoc it causes.

No vacuum leaks?

Thanks, I do have a non-contact thermometer, that'll be even easier. As far as I can tell I have no vacuum leaks, but as I said, I am a total amateur. :-)

Hey all, a final update on my rough running and I want to start with huge thanks to the people here and especially craby, who was dead right at every turn. The first shop ran the new sparkplug wires incorrectly and they were melting on the header and arcing and managed to mess up the new Optispark that had been installed. I took it to a different shop (F&F Auto Clinic in Bothell, absolutely fantastic people) and they fixed everything. Another new optispark, wires and plugs, everything correctly routed and the car is running like an absolute DREAM. Pulls smoothly from any RPM in any gear, hell I put it in 6th at 45 mph and even then it accelerated perfectly smoothly.

So two conclusions... First, this is an incredible community and I really appreciate the help and guidance you all (and especially craby) have provided. And second, if you are north of Seattle and need a good shop for these cars, F&F Auto Clinic is your place.

Cheers!

Great news and professional of you to share it with us and close the thread

Now get out and enjoy your sweet ride :)

kool and thank you. always ask for the old parts any shop replaces beforehand. they will then know they cant say they replaced something when all they did was clean it up. glad you got her going. also a way to check plug wires for bleeding off spark is to find a very dark place like a garage or alley and start up the car and have a look under the hood. if plug wires are putting on a light show for ya you know wires are toast. be careful easy to put hands where they dont belong in the dark. lol or just grab plug wires in daylight and if you light up wires are questionable. some low quality wires will bleed off a bit new.