Do not overfill the expansion tank as that will cause coolant to puke out of the filler cap .

The expansion tank has a volume of circa 2 litres. When filled to the level of the tank joint there is an air space of 1 litre. As the coolant heats up the volume expands by about half a litre. This expansion causes the air space to compress to a pressure at or close to the relieving pressure of the cap- 15 psig. Thus if the expansion tank is more than half full a pressure sufficient to lift the safety valve will occur and that will blow some coolant and air out of the system

 

Very Beautiful car! Fan spins when hot to help air get through the radiator. Good luck, I hope you find a simple fix. I put one of these in my 87 CSF #8029: CSF x 928 International High-Performance Radiator – CSF Race

 

Not to pile on here, but I will anyway with a note of my own. I won't attempt to second guess any of the great advice you've already received, but rather just to share an anecdote for whatever that is worth. I've had to replace the radiator in my 1983 car, twice, over the decades, and both times I saw a huge improvement in cooling capability. I'm convinced that the radiators just fill up with muck over time. Just before my most recent replacement in about 2019, I also would run normal temps while moving but it would creep up at stoplights. I also had an "invisible" leak in that I was losing coolant through some pinhole leak or leaks that would leave white traces across the radiator and fan blades, but I could never ever actually see any liquid no matter how I tried to catch it red-handed.

I purchased a new CSF radiator from Roger which was a work of art and looks indestructible, and it fit right in. I replaced all coolant hoses, thermostat, reservoir, cap, and timing belt/water pump too at the same time. No more leaks, and the best part is that it runs cooler per the dash gauge - just above the first white line almost all the time, even with AC on, and only on the hottest day standing still with AC have I seen it move up towards the midpoint a little. So, not pointing the finger at your radiator specifically, but just saying generally that when everything is working as intended, the cars have very impressive cooling capabilities!

 

Thanks Burning Breaks. That is all great news and we are wondering if the radiator might be the issue since we've done EVERYTHING else. the CSF radiator looks awesome and see great reviews. Your experience with it seems like a sound testimonial.