Hopefully someone out here can help settle a debate my girlfriend and I are having. The other day while sitting in bumper-to-bumper traffic on the interstate because of an accident, I would turn my engine off to save gas if we hadn't moved for five minutes. She said it takes more gas to start the engine than it does to idle. I know this is true.... to a point. my question is, how long does an engine have to run at idle before it uses more fuel than it would take to re-start it? These are stock production engines if it makes any difference.  

Shutting down a modern EFI engine when stopped in traffic can increase city fuel economy as much as 8%.  That's why "start-stop" or so-called "micro-hybrid" systems are increasingly common in Europe (Where gasoline in some places is selling for over $8/gallon, including taxes.)  The Saturn Aura "Green Line" hybrid does this in the U.S. with a 42-volt starter-generator, for a 2 m.p.g. gain over the standard four-popper.

One problem with doing a manual "start-stop" on a stock-type high performance car is that the 12-volt starter (and possibly the key switch) won't hold up as long under frequent restarting. Battery drain and excessive cranking time are also concerns (factory "start-stop" systems typically crank faster than ordinary 12-volt starters and use automatic restarts based on pedal position). Header heat soak and vapor lock can also plague hot restarts in modified vehicles.   On worn engines with suspect oil pressure, frequent restarts could lead to increased wear because of oil drain back.

Also, in the hotter parts of the country, shutting off the engine means shutting off the AC. Riding with the windows down to compensate can chew up 1-2 m.p.g. if the cruising speeds get over 30-35 m.p.h., which would reduce the gains from "start-stop" somewhat.

Fuel consumption could be higher in a carbureted car if the choke resets (i.e. bad choke pull-off) or the car wastes a lot of fuel getting restarted.  But even then, shutting off the engine when standing over a minute should save a little fuel.


http://speedzzter.blogspot.com  

The vehicle in question is a 93 caprice with a 305 and tbi. Not a modern high performance monster. How long would it have to idle before using more fuel tham it would to start? It is a well maintained vehicle that starts easily.  

I suspect about 10 or 15 seconds.  Easily less than a minute.