Thermactor Air Bypass causes backfire!

I believe plugging that vaccum line will now allow your TAB to open, which will cause the smog pump to pump air through the lines that are attached to the rear of your cylinder heads all the time. Normally the air is only pumped through these lines when the engine is cold, pumping it in with the engine at operating temperature would be just like driving around with a bad header leak, hence the backfires.

Edit: I misread your original post, I'll look at my '89 tomorrow to if that valve gets vaccum or not when the engine is cold. I have a similar setup, the tube that originally ran from the valve you mention to the converters has been removed.