My car is a Toyota Mark II, Model is GX 81, Year of manufacture1989, a nice straight 6 cyl engine,?

automatic transmission. Sweet machine. 3days ago a mechanic told me one engine nozzle is not supplying gas at all or not fine. I know nothin abt this. He said he corrected it. The results are that the automatic transmission appears to be stuck on OVERDRIVE OFF mode. When i hit the on on the gearshift 2,3 times, the dashboard O/D OFF light responds on & off but NO response at all on the engine performance.The car is still heavy and sort of wants u to press hard on the gas paddle for gears 3,4,5 to kick in. Thats not usual on the O/D ON mode. It does not change gears as usual. Number 1,2,3 takes a lot mo speed to kick in than previously. I dont like to drive that fast. I dont get No.5, which i used to get easily at around 60km/hr. I think it takes guzzles mo fuel too. Does it? Normally fuel consumption is around 8km/litre in town. Can someone please help? I need to travel to Arusha, 1400km round trip,but am worried on the fuel consumption. I am in Dar es Salaam,Tanzania, East Africa.