When you take a shit so bad and so hot, that it feels like the pot is boiling beneath your ass!
Hey Jim, I just had to run to the bowl. My ass hit the seat and I just did a five star shit! It was a real pot boiler! I thought the porcelain would melt, it was that hot!
Turd-boiler refers to a car that is of advanced age and runs like shit. It doesn't run smoothly, hiccupping along and spewing clouds of noxious fumes as it labors along. Also, it is an embarrassment to be seen in, either owning it or riding in it.
Dude, did you the car Dave was driving, that thing is a turd-boiler?!!