Hi,

I was wondering what idling the truck in the morning does to mileage. So here's what I did when I filled up the truck and drove home earlier this week. Engine was up to temp when I filled up and I drove normal. The DIC showed avg mpg as 20.5. This is consistent with my route and old lady like driving style. In the summer it shows 23-25 mpg filling from the same station and going straight home. It is 8.3 miles from the pump to the driveway.

8.3 miles divided by 20.5 miles per gal = 0.405 gal of fuel used.

Next morning it was cold, -1F on the rear view mirror and the windows were frosted over. Truck was plugged in. I started the truck, and went out to scrape the windows. When done with the windows, I let the truck idle for a few more minutes. At exactly 10 minutes from starting the engine, I noted the mileage and the DIC avg mpg. It was 8.3 miles since the fill up and the average was down to 12.3 mpg.

8.3 miles divided by 12.3 miles per gal = 0.675 gal of fuel used.

So, the 10 minutes of idling used 0.270 gal of fuel.

That's 1.62 gal of fuel per hour of idling when the engine starts cold. From what I understand, this is about the expected usage rate.

What's that mean? Well, if you normally drive 20 miles to work in the morning and your driving average is 16 mpg, you would use 1.250 gal of fuel. Add 10 min of idling and you use 0.27 gal more fuel and your mileage drops to 13.1 mpg. If you only drive 10 miles to work, it's worse as your mileage drops from 16 down to 11.2 mpg.


As they say, "Your mileage may vary"

John

i know how to fix that.......just dont let her idle... they say its bad for them anyways

I was told that an engine in a tractor-trailer uses 1.25 gal/hr idling. My calculations according to my scanner come to around .25 to .30 gal/hr idling for the Dmax. Diesels use WAY less fuel idling than do gassers, probably less than a third. In 10 minutes, it should have been only a few ounces. Here are the numbers: 8 uL/injection, 4 injections/revolution, 650 rev/minute. In one hour, that comes to 1.25L/hr, or 1.32 quarts/hr.

Marc,

Tell you what, you are probably correct for a HOT engine. The cloud of unburnt fuel coming from the exhaust, makes me think a cold engine uses more fuel than that. Do you have the same information for an engine started when it is 0 F?

jb

Fuel usage is a minor issue. Idling a light diesel engine at low RPM is quite detrimental because a diesel engine does not generate much more than enough heat to sustain combustion in cold weather. This allows raw fuel to wash down the cylinder walls, dilute motor oil, create many times more soot, and that white smoke you seen at start-up is actually unburned fuel blowing out the exhaust. If you want a quick warm-up, using the block/coolant heater is your best option, leaving it plugged in overnight costs less than the fuel you waste with that 10min idle. But if you really want a ready to run-N-gun diesel in 0* weather check this out: http://www.ventechlhg.com/ Even that doesn't matter, you still need to warm-up the transmission and differiental oils with moderate driving.

...which is why I high-idle. Maybe I'm stupid, but I say that high-idling a diesel is better for it than making several cold starts per day in really cold weather.

For example: When it was 15 and 20 below here for about a week, I was driving to school instead of walking. Sure I bundled up a few times and walked, but why fight the cold if you don't have to? When I was in class for an hour, I'd let my truck high-idle in the lot. My logic that idling at 1200 RPM for several hours per day was easier on the truck than barely getting it up to temp on a short commute, shutting down, and then making a cold start again after letting it sit for a couple hours.

Am I way off track here?

Cougar,
You are right, that fuel usage was with a warm engine. I will have to use the scanner sometime on a cold start to see what it is like. There aren't any great answers to this problem, it is hard on a diesel to run cold. I added an idle bleed on my lp system to supplement at idle to help clean up the combustion when cold. I do know that some company makes a propane fired block heater that does something like 30K btus, that ought to get it up to temp!

not sure but with mine when i would let it idle for about 20 minute in the drive before driving through one tank.. my milage went down a good bit..but i stopped doing that now, and milage has gown back up.
robert