Finally got rid of the problem. Just reved hard while driving in the morning(becouse theres not much vehicles on the road.). Saw a massive black smoke comming out. After got back to home, put the van in idle (neutral) and rev hard few more times. Black smoke reduced gradually. now it's just alil bit of smoke. Thanks guyz. As i think the problem couse by driving the vehicle under speed for a long time(in months). Carbon get diposite inside the exhaust(becouse of the low exhaust temprature, carbon deposites won't get burn).
Anyway thanks everyone....!!! :D