with city driving the flow is slower so any air pockets will be more likely to make it run hotter, at freeway speeds the pump is pumping faster so the air pocket flows faster and not likely to be noticed.
raise the front on jack stands , remove the cap and let it idle, you'll be able to see if there was any air trapped in there.
it's kind of like dressing for winter, if you wear layers of clothing and create air pockets you'll retain more body heat :thumb: