My 1990 Toyoto seems to loose power, just notice it is smoking from underneath on drivers side rear

Could be a brake wheel cylinder seizing up, effectively causing you to slow down because brakes are grabbing. You might want to have this checked out. The muffler on these came out on the pass. side but you may be seeing oil/antifreeze in the exhaust burning up due to a head gasket leak or bad rings/valve stem seals.

My guess is that the Air Flow Sensor has gone bad. It's connected to the top half of the box that contains the air filter. If you know how to run the internal diagnostics, the trouble code should point you in that direction if that is the problem. If you're handy with an Ohm meter, take the readings outlined in the Haynes manual or other manual. If it's bad, take your Ohm meter to Pull-A-Part or a junk yard and buy one that's got good readings. These parts are right pricey in a parts store and out of sight at a dealership.