Hi all, I am new here, I have a 2003 Hiace 2RZ-E that has a power problem

my engine is only happy at about 10 - 20% throttle ? which is very slow on acceleration. If I push down the pedal further it makes no difference only alters the engine tone

The engine revs up ok while standing and sounds ok but when asked to go is very poor on power

I do have a crack in my exhaust manifold which is getting changed soon when I locate one

Any ideas anyone its like driving with the handbrake on

Many thanks
Chris