The air was steaming? Yeah it was hot as hell but the firetruck didn't mind.
Why you may ask? Well that's cause he's being f*cked. Literally the prime is "Spreading his seed" in the firebot.

Heatwaves p.o.v
I was a moaning mess! It was too much but in the same time I loved it! I needed it! I needed him!!  I moaned as the prime went faster and harder. He grabbed my waist and fragged me hard against the wall while whispering naughty things to my auto receptor making me blush a dark blue.

A hour has past and the two bots came out the closet tired but as other bots stand there quiet the two leaders quickly make their way to heatwaves berthroom to rest but are unsure of what to say to the bots in the morning.