I appreciate this is a question that can't be answered in facts and figures, so I'm after a general idea. I drive a 1986 Supra, which has a modest 2.8-litre engine. I often shut the engine off while waiting at traffic lights to save fuel, but I'm wondering how much it saves when I have to crank the engine before pulling away.
Somewhere along the line, I heard that it took about 30 seconds of idling worth of fuel to start a cold engine (i.e. the same amount of fuel as the engine would burn idling for 30 seconds), and for a hot engine, that drops to around 10 seconds, so I try to shut the engine off if I'm going to be waiting for more than 10 seconds.
I'm wondering if this "rule" only applies to more modern cars, which is why I mention the age of my Supra. Obviously all EFI engines "flare" after being started with the key to ensure the engine catches; if more sophisticated fuel injection systems have finer control over the extra fuel used during the flare, they might use less fuel to start.
The perfect situation is waiting facing downhill where I can let the car roll, then bump-start it using its own momentum instead of the electric starter (since it doesn't flare), but those are few and far between.

  • While I appreciate the nature of the question, I'm not sure it can be answered definitively. It seems to me all answers will be subjective and opinion based, versus factual. Every vehicle is going to be different. If you are asking specifically for your car, then you need to take into account the state of tune for the vehicle itself and also how worn out is the engine. I think your 10 sec wait time is probably not a bad measure for engine shutdown, but again, that is JMHO. – Pᴀᴜʟsᴛᴇʀ2 Jun 16 '15 at 16:24
  • @Paulster2 : I can take a stab at answering this question objectively – Zaid Jun 16 '15 at 16:28
  • @Zaid ... AB-SO-STINK-IN-LUTE-LY!!! It is my opinion someone cannot. I have no control over whether some else can. If you can, I'll upvote in a moment and encourage others to do so as well. I look forward to reading (and upvoting) your answer! Please, please, please, prove me wrong ... I need that every once in a while. Helps keep me humble, which is no easy task ;-) – Pᴀᴜʟsᴛᴇʀ2 Jun 16 '15 at 16:32
  • I think it's not just an a little richer mixture, which costs more fuel after starting. The battery has to supply electrical devices while the motor is off, and also the starter. So, it needs some charging when the motor is running again. The motor has to handle that additional load, which costs some extra fuel. – sweber Aug 30 '15 at 14:49

This question is very multi faceted because fuel economy is such a complex topic. When a car is starting first the computer check the coolant temperature sensor to see how hot the engine is. In the memory of the computer there is a look up table that dictates how much fuel it should add for a specific temperature. This table is very conservative and the computer will add more fuel than is actually needed just to make sure the engine starts. Then the computer will set the idle speed motor to the max value as per look up table as well this is why that flare up happens. Whey you start cranking the engine the computer starts adding fuel and spark, but it will take at least 3 to four revolutions before something happens and then at least a half second for everything to stabilize. A this point the computer takes control of the idle and brings the flare down. Finally, this is the point that is often forgotten, the computer has to bring the engine under closed loop control. In closed loop control the engine is most efficient as compared to open loop control that uses a similar table mentioned up above. The biggest obstacle to getting under closed loop control is the temperature of the oxygen sensor. If it is too cool it will not read and will start reading when it is sufficiently warm. Most vehicles are equipped with oxygen sensor warmers. With the age of your car i'm not sure if it has one and that would cause it to take longer to warm up.

With all of this being said i'm not sure how much fuel you are actually saving. The extra fuel needed to start the car and to bring it under closed loop control may actually waste more fuel then just idling for that 30 seconds. Also your starter and ignition switch both have a rated life, an average number of times that they can preform their job. That amount may be 1000 times or 10000 times but turning the car on and off like you are is greatly increasing the rate at witch you will reach failure. Ultimately the extra wear you put on your engine components may out weigh the fuel savings that you might be getting.

  • You're right about the closed-loop aspect - I hadn't considered that. Supras only got oxygen sensors late in the mk2's life though - mine doesn't have one, so it runs open-loop permanently. However, I have heard that cars with them deliberately run rich from cold to warm up the sensor with hotter exhaust. I also appreciate that heated oxygen sensors use electrical power which has to be replenished by the alternator. I'm looking to fit an aftermarket ECU as part of some performance modifications, so maybe I'll be able to finely tune the startup parameters that way – Gargravarr Aug 31 '15 at 16:13
  • Have you read this? Way less than 30 seconds of idling to pay back the energy for the starter when the car is warm. physics.stackexchange.com/questions/57794/… – FarO Nov 27 '15 at 13:37

If your driving is consistent, you could test it out by measuring your gas mileage, with shutting your vehicle off at lights, and not shutting it off. You would want to average at least 3 tanks of each to make sure there aren't any weird outliers (stuck in traffic, muggy weather).

If you want an exact answer for your vehicle, you would need some way to measure the amount of fuel passed into the engine. If you have datalogging access to your ECU, you might be able to calculate fuel consumption based off of injector pulses.

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