We put a new radiator into my sons 92 240 last night, and also replaced the thermostat. Today it was heating up to the red zone on the dash gauge. We did not flush the system as time did not allow last night. And there was a layer of crud that settled out in the container of the old antifreeze. The temp did drop back to normal after a few minutes of idling in the driveway, but he said it was running hot while on the highway, and I saw it was hot when he pulled in, but again, it did drop back down to normal. I guess I figure some crud could be preventing the thermostat from opening? I never had an issue before with the b230's after a radiator change, anyone have any ideas what could be the issue?

It is not an oem Volvo thermostat, but I have not had problems with the generic thermostats before. I know there is an outside chance the thermostat is brand new and bad, but I am thinking through what else could be the issue before throwing more new parts at it.