Quickest way to find vacuum leaks is start the motor and keep it running by playing with throttle linkage under the hood, while spraying flammable carb or brake cleaner on the inter cooler pipe connections and all areas in the picture above or with vacuum hoses. You will hear engine rev up when cleaner is sucked in where it's leaking. Most common place for leaks is at IC tube connections.
This actually doesn't work real well due to the fact the EEC will compensate instantly by pulling timing. This will kind of work if the vacuum leak is so extreme that retarding the timing won't be enough to compensate.
When the car won't start and idle immediately, the 02 sensors are not the problem because they don't kick in until 30-90 seconds later.
Usually a surging idle is caused by too much fuel. On an old car there are many reasons why it may get too much fuel:
- stuck injectors
- bad fuel pressure regulator
- fouled plugs
- bad cats
- bad 02 sensors
- bad maf
- stuck EGR
If it were me I would start with verifying fuel pressure. This is easy and will rule out injectors and fuel pressure regulator.
Then I would check the MAF - first physically remove it and check/clean the sensor wires. If you have a digital VOM you can check output voltage at idle, it should be around .9v and vary with throttle input.
Then I would pull one plug and look at it. They might be original.
Next I would take all the tubes off, change the plugs and wires while it's apart, and reseal all the tubes. You can be pretty sure that they are leaking after 30 years. If you can do a smoke test, or have one done, this will also help you greatly in finding a vacuum leak.
Now as you know, a vacuum leak won't make the engine run rich, it will make it run lean. But, with your verbal description one can't be 100% confident in your assessment of the symptoms and since vacuum leaks are almost 100% probable on an SC, I still would not rule it out.
One of the side effects of a vacuum leak is that it will cause the computer to learn incorrect fuel trims. Try disconnecting the battery and see if it acts differently. Disconnecting the battery will clear all learned parameters. This will give you some hints as to what is going on.
Bad 02 sensors, or weak ones, will cause the system to learn rich. If it's been running like this for awhile, it can damage the cats. The cats become clogged and excessive back pressure will cause the engine to run rich and struggle if you try to force it to run. Not likely, but something you can't rule out completely.
Stuck EGR - you mentioned seeing smoke around the bypass valve. This seems odd to me, unless perhaps the EGR valve is open when it shouldn't be allowing exhaust into the intake plenum. That is of course if your car even has EGR.