Common mini van evap problems are the LDP (leak detection pump) or lines. Evap canister site right under the steering rack on the cradle. Rack leaks fluid on top of it. Connections to canister are rubber & crack. Solenoid mounted to right front core support & has two lines coming off & need to be checked for cracks. One goes down towards the canister & other to the intake. Smoke & scanner are the best ways to find evap problems, but you checking all lines & connections are a good start. Also look at gas cap seal/o-ring. If dry & cracked could be the source as well.