Once the engine reaches operating temperatures, I notice a slight odor of coolant when I am under the hood. I see no evidence of an external leak, no coolant in oil and my plugs were spotless when last changed (100 miles or less).

I don't think it's coming from the overflow reservoir. It's got me stymied. Any ideas? Coolant level is normal.

 

water pump might have a slow dribble from weep hole. depending on how tiny the leak is, might evaporate before being noticed. Also check the gasket around the radiator cap, might need to be replaced.

 

I thought of those, but the lack of any visible fluid or vapor has me stumped. Caps are cheap. I'll try one of those first.