1989 Other Subaru Models Excessive White Exhaust

Could be a transmission vacuum diaphragm leaking into the intake manifold and being burn- If not it-might mean a blown headgasket cracked head or block

Have it block and pressure tested-do the block first to pinpoint a combustion leaking into the cooling system or a gas analyzer to sniff for hydrocarbons at the radiator fill neck.

Pressure test: do not do a pressure test if there's leakage at the headgasket this might cause coolant into the cylinders and lock it up or bend a connecting rod if cranked thereafter. The headgasket should be repaired before doing the pressure test.