My solution is to make a copy (clone) of the car, and change the parameters of the queues to only allow it to be full or empty. You should only do this if you are having a problem with a car acting as it should when comming out of a portal, and you have checked to make sure you have all products installed, and aren't missing any dependencies.
 
Below is the "queues" section of that car. I would change the initial count to the max...in this case 90000...and eliminate all the products from the allowed products list that aren't what you intend for it to carry. Then I change the username to reflect the car's purpose, such as "Blah blah blah (whatever the current name is) _corn_full"
 
queues {
 load0 {
  size     90000
  initial-count    0
  animated-mesh    "default"
  allowed-products {
   0    <kuid2:151900:100039:1>
   1    <kuid2:30671:9070990:1>
   2    <kuid2:30671:9110100:1>
   3    <kuid:46162:60001>
   4    <kuid2:30671:9130213:1>
   5    <kuid2:30671:9110412:1>
   6    <kuid2:30671:9100620:1>
   7    <kuid2:30671:9070810:1>
   8    <kuid2:30671:9120100:1>
   9    <kuid2:30671:9170199:1>
   10    <kuid2:30671:9100810:1>
   11    <kuid2:30671:9110411:1>
  }
  conflicts-with-queues {
   0    "load1"
  }
 }
 load1 {
  size     90000
  initial-count    0
  animated-mesh    "default"
  allowed-products {
   0    <kuid2:30671:9110710:2>
   1    <kuid2:30671:9252101:1>
   2    <kuid2:30671:92601122:2>
   3    <kuid2:30671:925059001:1>
   4    <kuid2:30671:925059002:1>
   5    <kuid2:30671:925171003:1>
   6    <kuid2:30671:9252390:1>
   7    <kuid2:30671:9250510:1>
   8    <kuid2:30671:9252020:1>
   9    <kuid2:30671:9283620:1>
   10    <kuid2:30671:9390110:1>
   11    <kuid:-3:10002>
  }
  conflicts-with-queues {
   0    "load0"
  }
 }
}
 
I'm sure someone else will be along to tell you how to get your portal to behave, as I said, I do this a a last resort, and because I find it makes my session building much easier for my own use. Obviously, if you intend to upload the seesion, you wouldn't want to do this.