I always heard it was a bad thing anyway. However, I was also told that you could let diesel powered vehicles idle as long as you want. Indeed, I hear that truckers sometimes let the Big Rigs idle all night long.
I went to a drive-in last weekend with my wife and daughter. I let my gas powered SUV idle for about and hour and a half at one point. I didn’t seem to hurt anything.
Is it bad to let gasoline engines idle for a long time? What about diesels? Why?
I don’t know about the effects on the engine and other systems, but two reasons you not to let your car idle: 1) you’re using gas and 2) you’re emiting exhaust.
I meant to add that diesel fuels thickens at low temperatures (not sure how low), making it harder to start. That, plus providing heat, is one reason truckers will let their engines idle overnight when they’re sleeping in them.
The catalytic converter on the underside of your car gets hot. In fact, it can get really hot. If you’re stopped over dried leaves, papers, etc., for a long period of time, they can actually catch on fire.
As long as you don’t overheat, there’s really no problem other than wasting gas, and putting out noise and fumes, not to mention wear on the engine.
Wasting gas is the big one - you’re getting zero MPG.
You’ll also tend to get gasoline contamination in your motor oil, but it usually doesn’t get bad enough to matter.
Another consideration is that, according to Ford, you should count an hour spent idling as 33 miles towards your next oil maintenance procedure. You’ll notice “hour meters” in a number of work trucks by different makers, and even in very new police cruisers made by a couple of makers.
With gas at nearly $3/gallon, I shut my car off when the drive-through restaurant I’m at is slow.
Forgot to mention, I saw an experiment one time done with Mercedes passenger car diesels (3 liter engine). They burned a little over a gallon per day of diesel fuel.
Anecdotal evidence from a guy operating gas-powered work trucks (approximately 6 liter engine in Department of Transportation highway maintenance applications) indicates a gas usage rate close to a gallon per hour.
I’m not sure what the physics of all that are, and why the consumption varies so much.
Mr. Slant, depending on how slow the drive in is, you are much more likely to be wasting gasoline by starting and stopping the engine. Starting an engine uses quite a bit of gas, and I wouldn’t be surprised if it was significantly more than the amount of gas burned at idle for a minute. Now, if you’re stopped for 5-10 minutes at a time, that’s a different story.
I question this. If starting a (warm) engine uses a lot of gas, where does that go? Smoke out the exhaust pipe? I see no detectable amount of this.
I note that one scheme hybrid vehicles use in their search for efficiency is to stop the engine when its output is not needed, even if only for a short time.
In the days of carburated cars this was true. Modern fuel injected cars with their high pressure fuel systems don’t waste much gas, if any at all, when starting a vehicle, especially if it is already warmed up.
I’ve been told by several mechanics that a fuel-injected car uses about 10 seconds’ worth of idling time when started.
But don’t forget to think about wear and tear on the starter, solonoid, pinion, flywheel etc… If you’re only planning to have the car for a few years, it probably won’t be an issue, but if you’re planning on having it around for a while, it’s something to think about.
Quite the opposite.
When idling there’s not enough exhaust gas flow to keep the catalyst hot, so it starts to get cold. And when the catalyst gets cold, it can’t break down the aromatic carbon molecules or whatever it is supposed to break down.
The danger of dried leaves catching fire was true for the older cars. Nowadays most cars (in order to increase efficiency) have the catalyst attached directly after the exhaust manifold, so it is a lot higher than the rest of the exhaust pipe.
I have never seen a cite for this, but I would tend to agree with it. Starting a warm fuel injected engine burns little to no extra fuel.
Some cars would drop out of closed loop if idled for extended periods of time (exhaust got too cool to run the converter) When doing smog checks back in the 80’s I knew that some cars had to be driven right before the tailpipe test and not idled or they would fail. 
It has been 14 years since I did any smog tests, so I don’t know if this is still a problem.
The biggest reason why people say you shouldn’t let your gas engine idle is because of fuel burn. When a gasoline engine idles, a vacuum is created in the cylinders and this severely limits efficiency. The same phenomenon doesn’t happen in diesels. That is why diesels will use a fraction of the fuel during idle as a similar gas engine.
That is according to an article I read the other day but I can’t remember where. I can’t find anything through a search so if someone who knows what they are talking about could expand on this in more detail that would be great.