Believe it or not, I recall an episode of the TV series "Emergency!" that addressed the above issue, of all things. Johnny & Roy were having trouble with the Squad vehicle. And, of all people, Dr. Joe Early (played by Bobby Troup) to the rescue. He says to Johnny/Roy, "Sounds like your <devices> are closed. Ya mind if I take a look?" Within a minute or two and the help of a screw driver, he got it fixed and the vechicle started perfectly. Fitting for this episode as this was the one where they diagnosed Dr. Early with significant Coronary Artery blockages and needed surgery (angioplasty) to get them cleaned out. Oddly enough, Nick Nolte played one of the cardiologists.
Question: What actually causes them to "close?"