[othello]

>Even with the motor installed, I can push the car by hand and it rolls freely.

That seems not right. There should be a noticeable drag when a Neu 1512 is connected to the drivetrain. Remove the pinion and turn the shaft by hand. If it turns freely like a feigao/Lehner your rotor might be (partialy) demagnetised or the shaft is not connected properly to the shaft. If so the motor will compensate the loss in torque by taking far more amps (leading to higher temps of your components) when accelerating but you might also experience a higher revving motor -> more top speed. This might also lead to cogging while starting up as amp draw is much higher then usualy.

Around 4000mAh and a runtime of 25 mins calculates to an average amp draw of only 9.6A (with 18.5V -> 177W average). This shouldn't lead to such high temps. You did not mention outside temperatures btw.

Your tires don't seem to be able to provide super high grip (Amps while accelerating shouldn't be very high). I would expect much tire slip leading to high motor rpms to keep your car moving at a decent speed. This also heats up a motor.

Running on a grass field is challenging for electric components. I can empty my batteries faster on this type of surface as drag is higher and more power is needed -> leading to higher temps of all components.

I would expect your motor to be the root of your higher temps.