I've set the tier 1 to 100%, but all cars are spawning with tier 3  parts.

​{
  "Engine parts": {
    "Tier 1 chance": 100,
    "Tier 2 chance": 0,
    "Tier 3 chance": 0,
    "Min condition percent": 100.0,
    "Max condition percent": 100.0
  },
  "Min fuel amount": 0,
  "Max fuel amount": 0,
  "Min health percent": 15.0,
  "Max health percent": 50.0,
  "Module presets": {
    "2 sockets": {
      "Use vanilla presets": true,
      "Custom presets": []
    },
    "3 sockets": {
      "Use vanilla presets": true,
      "Custom presets": []
    },
    "4 sockets": {
      "Use vanilla presets": true,
      "Custom presets": []
    }
  }
}

Do you have any other plugins installed which could do this? Such as Auto Engine Parts.

WhiteThunder

Do you have any other plugins installed which could do this? Such as Auto Engine Parts.

No other plugin. 

I'm flying around the map, looking at the engine compartment for each car, some cars have low tier parts, some have none, some have tier 3 parts. Manually spawning a car spawns it with tier 3 parts. I don't know what I'm doing wrong

What exact command do you use to spawn the cars? Asking because some of the prefabs come with parts.

Also, what happens when the plugin is not loaded and you spawn cars?

WhiteThunder

What exact command do you use to spawn the cars? Asking because some of the prefabs come with parts.

Also, what happens when the plugin is not loaded and you spawn cars?

That solved the issue. I was possibly spawning a car with parts. ''spawn car_1mod_storage''. 

I spawned ''2module_car'' and they spawn with low tier parts.