Dont use any additive products,
Before doing anything, check the source of the p0171 code (lean). Its usually an air leak, once that is resolved, then check tappet gaps and replace valve cover gasket with center plug seals too.
The evap purge valve has hoses going to back of car, it usually has a blue 2 pin connector on toyota.
The lean code can be due to a bad pcv valve too, the idea is to isolate each vacuum user and check short trim, if it starts to repair itself, that item that you isolated was the culprit
If everything checks out ok, and you dont own a scope meter, swap out the maf with a good known part and check, if it goes ok after a new MAF buy a brand new toyota boxed MAF sensor (about Rs 20k). Or a denso (rs 12k). Scrap parts are useless.
Also do a small test, while at idle check the trims and then accelerate and hold at 3500 rpm, if trims go negative, your maf maybe dirty, try cleaning it (beware to only clean it when the engine has rested for atleast 20 minutes) or you can fry it.
