I was at an outdoor wedding recently and I could hear a truck idling behind the hedges so I went to investigate. It was an AAA tow truck and the driver was filling out paperwork. I explained that a wedding was going on and asked him if he could turn off his engine. He just said "I'll be leaving in a minute". Sure enough he made noise for another minute and then left. On other occasions I've always gotten similar responses. When I ask drivers if they have some company rule requiring them to keep the engine running, I just get a blank stare. But never once have I been able to get a driver to turn off his engine for me. It's almost always "oh I'm about to leave".

Once I found a tow truck that was idling near my house, and it was empty so I climbed inside and turned off the engine myself. It was pretty easy, just like with a car - just a turn of the key. Then a few minutes later the driver came by and turned the engine on again. Then he sat in the cab checking his smartphone.

If the engine only takes a fraction of a second to turn on and off, why have it idling even for 30 seconds, let alone minutes? It doesn't make any sense to me, but the behavior of truck drivers is so uniform, and often seemingly stubborn, that I feel I must be missing some principle which explains it.

(Originally posted at outdoor stack exchange)

  • 4
    I'm voting to close this question as off-topic because in has nothing to do with maintenance or repair of a vehicle.
    – CharlieRB
    Apr 24, 2017 at 18:20
  • 3
    Diesel engines only use a tiny amount of fuel to idle compared to gasoline engines. Some diesel engines don't even get hot while idling because of how little fuel they use. At the same time diesels are more problematic to start due to the high compression ratios. It is more taxing on the starter and many diesels have two batteries to feed a larger starter.
    – vini_i
    Apr 24, 2017 at 18:43
  • 5
    I'd vote to keep it open, while the story isn't about maintenance the underlying answer is.
    – dlu
    Apr 25, 2017 at 3:18
  • 4
    Regardless, you shouldn't really invite yourself to others vehicles for your own safety
    – method
    Apr 25, 2017 at 10:06
  • 1
    Surely this is about driving technique? A good friend of mine used to have a recovery truck and I can assure you that it was switched off when not in use. Apr 25, 2017 at 10:55

2 Answers 2


Emergency services vehicles are often kept running because they contain lots of electrical equipment that would quickly drain the battery if the engine were off.

Climate control is another reason to keep the engine running: air con requires the engine to be running, and the heater uses engine heat and the engine's water pump.

In full-size trucks with pneumatic brakes, if you switch the engine off for a few hours, when you start again you may have to wait for the air system to come up to operational pressure before you can drive off.

In the US, keeping the engine running is more prevalent than elsewhere: many truck drivers keep the engine running overnight when they stop at a truckstop, where e.g. in Europe it's more common to have a stationary heater instead. Due to low fuel cost, US drivers see no problem using inefficient means to heat the cabin overnight.

Diesels being problematic to start doesn't sound plausible to me. Current engines should have more than enough starter capacity to start more often.

  • I suspect there is another reason: driver inertia/can't be bothered/can't imagine the noise is irritating to some. But that's just speculation on my part.
    – Hobbes
    Apr 25, 2017 at 10:28

One reason is that it could under very specific circumstances damage the engine. Every modern diesel has a turbocharger. Under prolonged heavy load the turbocharger get hot, really hot: Red glowing hot. That is no problem for the turbocharger, he was designed to cope with the heat. The turbocharger is attached to the oil circuit of the engine: The oil cools the turbo and provides lubrication. Diesel drivers are encouraged to let the turbocharger cool down gently by letting the engine run on low load after prolonged high load. Should one suddenly turn off the engine while the turbo is still glowing hot the oil flow stops and the oil in the turbocharger gets burned, depending on the circumstances this could lead to a severe damage to the engine.

Another reason is the auxiliary drive that most trucks use to power something: The winch on a tow truck, the air conditioner on a refrigerator truck, the hydraulic pump on a dump truck etc. The auxiliary drive gets powered by the engine, so drivers are used to let the engine run even if they do not need it at the moment.

On another note: Truck driving is a rough trade, drivers are often subject to burglaries/robberies/attacks. Should the average truck driver detect an intruder in his truck things will escalate quickly.


Not the answer you're looking for? Browse other questions tagged .