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View Full Version : Injector Fuel Cutoff While Coasting?


scullerboy
09-17-2013, 12:09 AM
I just installed a Scangauge II and while coasting in Drive it goes up to 9999 mpg, with a couple of intermediate readings. All the Scan-G manual says is that "if ScanGauge improperly detects fuel cutoff, then you may see fuel economy being reported as 9999 mpg or 0.0 LHK."

So, is my '06 actually getting 9999 mpg (essentially infinity, since you are moving and no fuel is being burned) or is this a ScanGauge SNAFU?
If I have fuel cutoff, is the default SG cutoff setting of 24 normally a good one?

thanks,
Tim

sailquik
09-17-2013, 12:16 AM
scullerboy,
Nothing wrong with the Scan Gauge II.... that's just how they work in Sprinters.
Much better information (a lot steadier too) is available with the PID GPH (Gallons per hour).
Yes, the fuel system shuts off all fuel when you are coasting.
There are only 3 modes
Increasing Power
Cruising
Trailing throttle (coasting).
Have you done the initial "fillup" percentage yet?
Your Scan Gauge II will really not be very accurate for fuel mileage (MPG or GPH) until you get close to the correct percentage.
My '06 used to run around 35% as I recall.
Hope this helps,
Roger

MillionMileSprinter
09-17-2013, 12:20 AM
Like Roger said, you really HAVE to go through a couple of fill up cycles to get it properly calibrated to your Sprinter.
That said, I also get the 9999mpg when I'm coasting for more than a second. I'm always happy to see that number show up!

scullerboy
09-17-2013, 12:40 AM
Thanks Roger. I haven't done the initial fill up percentage yet, with the large tank and diesel here 20-40 cents per gallon more than in any sizable town, I'll wait until I have the T1N somewhere cheaper to fill her up.
I mostly wanted to know if the engine did in fact cut off fuel, which means it's time to rethink my long-standing habit of coasting in neutral unless I knew I'd have to brake anyway further down the hill. Could shifting into neutral do any harm to the Sprinter transmission? I already learned to be careful shifting back into D, since the spring-load can cause an inadvertent downshift. Ouch.
Only had the '06 for 1k of its 6k odometer miles. (!) Never thought I'd get my dream retirement van.
Tim

sailquik
09-17-2013, 01:04 AM
Hi Tim,
The engine just continues to spin, with the fuel shut off completely.
You might get some tiny advantage by shifting to neutral, but I do not think it would be measureable and the fuel it takes to bring the engine back up
to the current RPM at the transmission may be significantly more that anything you save in neutral.
There is not "engine braking" effect per se, but the engine will tend to hold back slightly as the RPMs drop very slowly.
You will get really good mileage leaving it in gear, and lightly touching the pedal to keep the % Engine LOD between 0% (coasting) and 30%.
Shifting to neutral is not recommended (check your owners manual) but I do not believe it will hurt anything other than the engine needs to come up from idle
speed to match the revs at the transmission.
You will save far more fuel by shifting it out of gear (maybe even shut it off) when sitting at long traffic lights at intersections.
An inadvertent downshift won't hurt a thing either.
The ECM (main engine management computer/electronic-engine control module) and the TCM (Transmission Control Module) will not allow any downshift that
will over rev or hurt your engine or transmission.
If you wish to get the best overall mileage and performance, you will soon be down shifting manually to keep the %LOAD down and the RPM up where your
OM-647 makes it's most efficient and best horsepower. (Hint...it's in the 2700-3200 RPM range).
Hope this helps,
Roger

Aqua Puttana
09-17-2013, 03:31 AM
...I mostly wanted to know if the engine did in fact cut off fuel, ...Tim

54588

NelsonSprinter
06-16-2014, 04:16 AM
I was worried that only the fuel was cut off and the injectors would be getting no lubrication during deceleration, but now I know the injectors are actually deactivated so they're not pumping dry

Boater
06-16-2014, 12:58 PM
One source (BBC Top Gear?) suggested that for modern common rail injection systems you will use more fuel coasting in neutral because the engine management will fuel it to tick over at idle, whilst if you have lifted off and gravity is already driving the engine through the transmission faster than idle it can shut the injectors right off until the engine speed drops, and of course you do get some benefit from engine braking (although less than a gas engine) and reduce the risk of wheel spin or engine shock if you try to re-engage at the wrong RPMs (although I'm used to manual transmission, not sure if auto allows that?).

chromisdesigns
06-16-2014, 09:05 PM
Why would compression
braking be LESS on a diesel? Seems like with higher compression ratio engine has to work harder to compress the cylinder charge, which should result in more braking power, not less.

owner
06-16-2014, 10:05 PM
Because pretty much all of that energy used to compress the air is then returned to the engine on the downward cycle. There is relatively little engine braking effect on light diesels because there is no throttle butterfly for the pistons to pull against.

sailquik
06-16-2014, 10:24 PM
Actually, since the turbo is pretty much not making any boost, and there is no throttle valve for vacuum to build behind there is basically ambient atmospheric pressure, corrected for your current altitude being pulled into the cylinders.
Very little if any turbo boost pressure to force air into the cylinders, no throttle valve, no fuel to make power....the engine is just spinning, but it does develop
a little more resistance when you spin it faster in a lower gear!
Roger

chromisdesigns
06-16-2014, 10:30 PM
Actually, since the turbo is pretty much not making any boost, and there is no throttle valve for vacuum to build behind there is basically ambient atmospheric pressure, corrected for your current altitude being pulled into the cylinders.
Very little if any turbo boost pressure to force air into the cylinders, no throttle valve, no fuel to make power....the engine is just spinning, but it does develop
a little more resistance when you spin it faster in a lower gear!
Roger

Ah, ok. A shame they don't take advantage of computerized valve operation to improve braking. Or is that mot possible on this engine?

owner
06-17-2014, 03:53 AM
You can do it with the VG turbo. Adjusting the turbo vanes to make a restriction in the exhaust that increases engine braking. MB don't appear to be doing this even though everything is already there to be able to do it. I'm sure there is a reason why they don't, but it would have been handy on the sprinter.