Do you know how to bleed the cooling system?
I do, yes. But it was just done at the lube shop. Do I need to do it again?
I would.
That missing plastic piece could indeed be the cause, or at least part of it; it may be changing the airflow velocity and direction in the vicinity of the radiator. That is put there by the car’s design engineers for a reason, to direct windflow, and to prevent road debris from kicking up into the engine. A small rock lodging in one of the pulleys can do serious damage. Suggest you replace that part asap in any event.
@db4690 I was trying cheaper options first.
@GeorgeSanJose The thing has been off for almost a year, so I’d be shocked (but not too shocked) if that is it. Either way, I’ll have to replace it.
@db4690 They want about $30 for a new thermostat. So, it’s the next thing that I’ll try as the only thing after that is the water pump, which I understand to be much more expensive.
Update: Replaced the thermostat and made a mess doing it. Anti-freeze everywhere, in spite of draining it. Not too bad though. Made a rookie mistake and forgot to dilute my antifreeze, but I got it done.
Verdict: Seems to be fixed now. Haven’t had a chance to check the old one by boiling it, but it looks great. Doesn’t look broken by sight. Anyhow, it’s fixed so that is great. Thanks to all of you who advised me. Much appreciated. If you’re ever in SW Kansas, I’ll buy you a beer or something.