How much fuel is consumed on a start up?
 

I drove to a party last week with a friend. The car had just over 1/4 tank. We drove 50 miles total, there and back. When I got home, I noted that the fuel tank was now right at 1/4 tank. The next morning when I started it, it was just under 1/4 tank. So, how much fuel is used at startup? Even if you don't know the exact amount (1/2 cup or whatever), maybe you know it's "the equivalent of 10 minutes of driving or whatever.

Also, I know they say "if you're stopping the car for a few minutes, best to just let it idle". But it doesn't really say how many minutes.

Thanks,

jeff 1991 300d, 117k

Startup on an injected car, gas or diesel uses very little fuel. Probably somewhere on the order of 1-2oz (just off the top of my head) or less.

Someone did calculate it out one time and found that if you're going to sit at idle for more than 10sec, it more efficient with modern injection systems to shut off the engine. Older injection systems like the W123 are still very efficient and probably don't use any more at startup than their younger cousins.

I would just turn off the engine if it looks like I'm going to sit for more than a minute. I perfer not to shift into park, shut down, glow, crank, shift to drive. This might frustrate other drivers at stop lights.

if im in a drive through or something i put it in nuetral, shut it off, turn it on, drive, go

no need for glow plugs if its already warm

How many seconds are you cranking it?

If it starts in one second, you'll use about one teaspoon of fuel to start it (my WAG). Longer crank times use more fuel.

True, I'm just anal.

What more likely happened is that the fuel in the tank warmed up and expanded a bit over those 50+ miles of driving and then cooled off (contracted) overnight, resulting in the lower gauge reading in the morning.

Thats why some fuel stations heat the fuel on the way out the pump so it expands and they sell ever so slightly less per gallon, saving them big $$ over time. I was at a station like that once....the nozzle and hose warmed up right away when the fuel started coming through....I thought it was very odd.

This is an interesting question. If you are just considering starting an engine its not very much. However if you are trying to compare how far you can drive a car that has to be started and not up to operating temperature to one that has be running a while and warmed up on the same amount of fuel I would imagine there would be some measurable difference. Your engine contains a lot of metal, water, and oil to heat up.

On the W123s there are no sensors to determine engine temperature to affect mixture. No "choke" equivalent for a diesel. No cold start injector.

Engine will use as much fuel during the starting process as it will use during the same period of time at idle in other words. Whether it burns it is another question, but it's still injected. Theoretically it's actually less, as cranking rpm is less than idle rpm and injector pump action is directly proportional to rpm.

It takes my fuel guage to wake up in the am. She takes ~3mins to get up to where she left off the day before. She's 180 years old in car years!

Yes, the fuel gauges aren't very precise really. Maybe you park on a slight incline as well?

wait wait... there are car years? how do you figure 180?
I am sure there is a sliding scale on "year computation" and for the MB my cars are only teenagers at best... I would say 2 to 3 years to each "car year" not the other way around like dogs...

I do know this, my car uses about 3.5 ounces of fuel at idle, operating temperature every 10 minutes. You could figure up the startup usage by using an extremely accurate measuring cup as a temporary "gas" tank and marking the before and after of startup.

Isn't the rack held at full fuel while cranking? Those few revolutions at full fuel could equal a good amount of time at idle.

Ara T comes close to the explanation here.

Fuel gauges are thermal devices - they read higher when hot. The temperature within the cabin is important, and a fuel gauge will read less on a cold morning.

Treat pointers on the dash of most cars as a vague indication at best - to call them instruments is a travesty.

Fuel gauges are voltmeters. With a fresh start, the (car/gauge cluster) voltage could be lower or higher than when you shut it down.