According to my friend who followed me home when I picked the Sube up,

once in awhile it would puff a bit of white smoke from the exhaust, we think when I would downshift to decelerate. Is this a bad omen?

Intermittent symptoms are hard to diagnose, but I'd say your leaking coolant at you carb spacer or your intake gaskets, it could also be a headgasket getting ready to pop. I'd stop downshifting to slow down, your symptoms will likely go away and you'll stop making it worse if it is a gasket. Downshifting is hard on your powertrain and drivetrain and isn't necessary in most situations. Brakes are cheap and easy to replace or repair and do a much better job.

88 spfi, no spacer...... not a good sign

Cracked head maybe?

 

By pulling the plugs you can narrow it down, look for clean plugs because antifreeze cleans off carbon, also look inside at the top of the piston. If you don't find anything unusual, drop the y-pipe and check the inside for clean spots.

Lol no, I know a cracked head. Once owned an RX.... That would've been giant clouds of white smoke, if it would run at all. This car is running absolutely fine, just that my buddy said he saw a little puff once in awhile while decelerating.

 

Checked over some things just now [again]. Oil is good, no contamination. Can't tell on the coolant as it's low in the surge tank, there is some gunk residue in the tank but it probably just needs a flush. I don't trust that the previous owner did regular maintenance, only failure fixes, like the cv and exhuast. Need to grab a compression test kit sometime this week as part of my full technical inspection work up.

 

I could just be paranoid. But just because I am, doesn't mean they aren't out to get me