Idles great, severe loss of power under load

The car is over twenty years old and I don't know the condition or miles on it. It's likely to have had this problem before it sat unrun. A CEL doesn't come on until the computer tells it to. and the computer has to see a problem persist for a while before it assesses it as problematic enough to illuminate that CEL. So my guess is that you'll see that light pretty soon..... I agree that it's likely that the clogged exhaust is the problem and that's probably a bad converter. You can test for that.

A vacuum leak will make the idle high on this type pf system so I doubt you have one. You can get a back pressure test kit from most parts stores for the exhaust, it can't run right if it can't breathe, also make sure the fresh air intake is not restricted.

Also, after sitting for 18 months you might have water condensation in the gas tank. Put a bottle of Drygas or similar product in the gas tank and fill it up. Hopefully that's all it is. Hope that helps! Jim