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I was sitting outside, eating my lunch the other day, and a small bus (you know, one of the short ones) pulled up to the sandwich shop I was eating at. I noticed that the driver got out of the bus, ordered her food and waited for the roughly 10 minutes until the she received her sandwich and then proceeded back to the bus and ate her sandwich, all the while the bus was still idling the entire time!

My natural assumption is that this is a HUGE waste of gas, and thus prompted the thought "Why?!". So my question is this, are there any distinct advantages to idling a vehicle over shutting it off, and then starting it back up again? If so, is there a general 'time limit' for idling the vehicle?

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Idling the car/bus probably pollutes the air more than it wastes gas. – Dude318is May 24 '11 at 17:19
Diesel engines are very efficient at idle. – Jay Bazuzi Sep 15 '11 at 5:22
up vote 7 down vote accepted

Assuming the bus has a diesel motor, the answer is yes. As an old German Mercedes mechanic told me, "with a gas engine, the primary contributor to wear is hours of operation but with a diesel it is the number of times it is started".

What that really means is not the number of times you engage the starter motor but the number of times the motor is heat cycled. To support that idea, when I lived in Olympia, Washington there was a diesel logomotive that was used for moving cars in a small switch yard. I seldom saw the locomotive in use but it just sat there at idle 24/7.

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I think this is going to be completely personal opinion. If it is less than 40F outside, I will go out 15-20 minutes before i need to leave in the mornings and let my truck idle, to warm up the block but also to warm up the cab so I'm not freezing while driving. When the summer rolls around and its over 110F and I will not be out of the truck for more than 20 minutes, I let it idle and keep the air running. Now does this waste fuel? To some I'm sure it does but it does not to me. The old diesel motors, before all this EPA crap was put on, could idle all day and all night and it wouldn't hurt the motor at all. Gas motors it isn't great to idle it, but also I have never heard of anything major happen that was solely based on idling a gas motor. But again I think its all based on personal preference. I know people that will idle their truck while they are in the grocery store for 2 hours, I also know people that turn their truck off even if their just checking their mailbox for 2 minutes.

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Letting your engine warm up this way in the winter WILL destroy your engine! You don't have enough oil pressure for lubrication which greatly increases wear, the different metals in the cilinder head will expand unevenly leading to a destroyed headgasket and a bend head, your oil will degrade far sooner than expected, the exhaust gas flow will be insufficient to clean the valves leading to damaged valves and valve seats, etc. etc. The problem is, this won't happen right away but gradually over time and you'll most likely sell the truck to someone else who'll have many engine problems. – Alex May 25 '11 at 6:16
I don't understand, when I fire up the truck the oil pressure is the same as when i'm driving the truck, the only thing that changes when I'm driving is the coolant temp. – Zencyl Jun 8 '11 at 21:11

Remember that there may have been other specifics in this case that made it a special case. Maybe the battery had just been replaced or discharged and had to be jump started, and so they were trying to build up a charge? Maybe that particular bus was having mechanical issues that caused it to sometimes be particularly hard to start? Maybe they needed to warm the engine up for a oil change or emissions test?

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