Neil only attempt the rad blanking if you are 100% certain that your stat is staying closed when you are traveling but opening normally when stationary. Your last test indicated that the stat was opening but you didn't confirm it was stationary?
The blanking plate in my pic is on runners/sliders so I could experiment with it in differnt positions. Mine work better at the bottom on the portion of the rad that poked out from under the car rather than at the top as in the pic. Also whilst I saw an improvement it could have been better. I had planned a larger one but never got around to it before moving to my current +4. If you are going experiment by blanking the external grill, go for a much bigger area maybe say 50% ?
When you do the test monitor the temp VERY carefully with the OBD rather than the gauge. If you see it approaching 120 stop immediately and remove the the blanking as obviously its not working. Don't go beyond the temps that you have already seen before as you are into potential head/gasket damage territory. I never actually saw 120 with mine, I think 115 was about the highest I ever caught on the OBD, my gauge was useless in that car.
Richard reiterates a valid point. Yes its bonkers to blank a radiator in an attempt to cool a car so yes of course do this at your own peril. Regarding the thermostat, that might be one difference between your car and mine. The first things that I did after checking my orig 90C stat was working in a pan of boiling water was to replace it with an alternative 82C stat, the lowest I could find. I didn't notice any difference with that stat but left it in anyway, so bare in mind that my car had a different stat to yours and the blanking plate.
Good luck
