Those block heaters loosen up over time and need to be retightened. Just look for the coolant crust forming around the perimeter. I usually do that at every coolant change and use a little plumbers grease on the O ring. Use a Volvo cap (NAPA p/n 703-1396) on your Dorman bottle or the original French Cap (Lyon France Renault spec) if you still have it. Eventually you will probably go to the Macs aluminum can, and with that you can run open or closed. I prefer closed as it runs a little cooler and its a Euro thing.