Experimental Adjustable Thermostat Radiator

Create folders in your user folder so it looks like this
C:\Users\yourusername\Documents\BeamNG.drive\lua\vehicle\powertrain\
and move the
\Steam\steamapps\common\BeamNG.drive\lua\vehicle\powertrain\combustionEngineThermals.lua
to the above folder to avoid overwrite official content
Open the lua file via notepad (notepad++ is recommended) and scroll to line 517

Code:

local radiatorActive = min((hasCoolantRadiator and M.coolantTemperature > thermostatTemperature and not parentEngine.isDisabled) and M.coolantTemperature - thermostatTemperature or 0, 1)

change it to

Code:

local radiatorActive = min((hasCoolantRadiator and M.coolantTemperature > thermostatTemperature and not parentEngine.isDisabled) and (M.coolantTemperature - thermostatTemperature)/thermostatRange or 0, 1)

Scroll to line 1167 and add the following text between line 1167 and line 1168

Code:

  thermostatRange = jbeamData.thermostatRange or 1

Save it.

Then mod your vehicle, in this example the ETKI.
Create the vehicle folder in your user folder C:\Users\yourusername\Documents\BeamNG.drive\vehicles\etki\
Copy the radiator jbeam flie, in this example, the \Steam\steamapps\common\BeamNG.drive\content\vehicles\etki.zip\vehicles\etki\etki_radiator.jbeam
into the C:\Users\yourusername\Documents\BeamNG.drive\vehicles\etki\ AND RENAME IT
open the jbeam file and rename the "etki_radiator" and "name":"Radiator",
Add the "thermostatRange" below the "thermostatTemperature"

Spoiler: Example

Code:

{
"etki_radiator_thermod": {
    "information":{
        "authors":"BeamNG",
        "name":"Radiator(thermostat mod)",
        "value":130,
    }
    "slotType" : "etki_radiator",
    "flexbodies": [
         ["mesh", "[group]:", "nonFlexMaterials"],
         ["etki_radtube", ["etki_engine","etki_radiator"]],
         ["etki_radiator", ["etki_radiator"]],
    ],
    "mainEngine":{
        "radiatorArea":0.34,
        "radiatorEffectiveness":7500,
        "coolantVolume":8,
        "radiatorFanType":"electric",
        "radiatorFanTemperature":100,
        "radiatorFanMaxAirSpeed":2,
        "thermostatTemperature":80,
        "thermostatRange":20,
        },
    "nodes": [
         ["id", "posX", "posY", "posZ"],
         //radiator
         {"frictionCoef":0.5},
         {"nodeMaterial":"|NM_METAL"},
         {"selfCollision":false},
         {"collision":true}
         {"group":"etki_radiator"},
         {"nodeWeight":2.7},
         {"engineGroup":"radiator"}
         {"chemEnergy":200,"burnRate":0.4"flashPoint":250,"specHeat":0.8,"smokePoint":150,"selfIgnitionCoef":false},
         ["rad1", 0.0, -1.8, 0.7],
         {"chemEnergy":false,"burnRate":false,"flashPoint":false,"specHeat":false,"smokePoint":false,"selfIgnitionCoef":false},
         ["rad2", 0.0, -1.8, 0.3],
         {"engineGroup":""}
         {"group":""},
    ],
    "beams": [
          ["id1:", "id2:"],
          {"beamType":"|NORMAL", "beamLongBound":1.0, "beamShortBound":1.0},
          {"beamSpring":1000000,"beamDamp":150},
          {"beamDeform":7500,"beamStrength":"FLT_MAX"},
          {"deformGroup":"radiator_damage","deformationTriggerRatio":0.05}
          //radiator
          {"deformLimitExpansion":""},
          ["rad2", "rad1"],
          ["rad1", "f15"],
          ["rad1", "f15r"],
          ["rad1", "f13rr"],
          ["rad1", "f11rr"],
          ["rad1", "f15l"],
          ["rad1", "f13ll"],
          ["rad1", "f11ll"],
          ["rad2", "f11"],
          ["rad2", "f11ll"],
          ["rad2", "f11rr"],
          ["rad2", "f13ll"],
          ["rad2", "f13rr"],
          //to engine
          {"beamSpring":14000,"beamDamp":300},
          {"beamDeform":1450,"beamStrength":3600},
          {"optional":true}
          //duplicate beams for better flexbody breaking
          ["rad1","e4r"],
          ["rad1","e4l"],
          ["rad1","e4r"],
          ["rad1","e4l"],
          //tube
          ["rad2","e4l"],
          {"optional":false}
          {"deformGroup":""}
          {"beamPrecompression":1, "beamType":"|NORMAL", "beamLongBound":1.0, "beamShortBound":1.0},
    ],
}
"etki_radiator_race_thermod": {
    "information":{
        "authors":"BeamNG",
        "name":"High Performance Radiator(thermostat mod)",
        "value":270,
    }
    "slotType" : "etki_radiator",
    "flexbodies": [
         ["mesh", "[group]:", "nonFlexMaterials"],
         ["etki_radtube", ["etki_engine","etki_radiator"]],
         ["etki_radiator", ["etki_radiator"]],
    ],
    "mainEngine":{
        "radiatorArea":0.34,
        "radiatorEffectiveness":13000,
        "coolantVolume":9,
        "radiatorFanType":"electric",
        "radiatorFanTemperature":100,
        "radiatorFanMaxAirSpeed":3,
        "thermostatTemperature":80,
        "thermostatRange":20,
        },
    "nodes": [
         ["id", "posX", "posY", "posZ"],
         //radiator
         {"frictionCoef":0.5},
         {"nodeMaterial":"|NM_METAL"},
         {"selfCollision":false},
         {"collision":true}
         {"group":"etki_radiator"},
         {"nodeWeight":2.7},
         {"engineGroup":"radiator"}
         {"chemEnergy":200,"burnRate":0.4"flashPoint":250,"specHeat":0.8,"smokePoint":150,"selfIgnitionCoef":false},
         ["rad1", 0.0, -1.8, 0.7],
         {"chemEnergy":false,"burnRate":false,"flashPoint":false,"specHeat":false,"smokePoint":false,"selfIgnitionCoef":false},
         ["rad2", 0.0, -1.8, 0.3],
         {"engineGroup":""}
         {"group":""},
    ],
    "beams": [
          ["id1:", "id2:"],
          {"beamType":"|NORMAL", "beamLongBound":1.0, "beamShortBound":1.0},
          {"beamSpring":1000000,"beamDamp":150},
          {"beamDeform":7500,"beamStrength":"FLT_MAX"},
          {"deformGroup":"radiator_damage","deformationTriggerRatio":0.05}
          //radiator
          {"deformLimitExpansion":""},
          ["rad2", "rad1"],
          ["rad1", "f15"],
          ["rad1", "f15r"],
          ["rad1", "f13rr"],
          ["rad1", "f11rr"],
          ["rad1", "f15l"],
          ["rad1", "f13ll"],
          ["rad1", "f11ll"],
          ["rad2", "f11"],
          ["rad2", "f11ll"],
          ["rad2", "f11rr"],
          ["rad2", "f13ll"],
          ["rad2", "f13rr"],
          //to engine
          {"beamSpring":14000,"beamDamp":300},
          {"beamDeform":1450,"beamStrength":3600},
          {"optional":true}
          //duplicate beams for better flexbody breaking
          ["rad1","e4r"],
          ["rad1","e4l"],
          ["rad1","e4r"],
          ["rad1","e4l"],
          //tube
          ["rad2","e4l"],
          {"optional":false}
          {"deformGroup":""}
          {"beamPrecompression":1, "beamType":"|NORMAL", "beamLongBound":1.0, "beamShortBound":1.0},
    ],
}
}

Then select the mod radiator


And it works as shown