Something wrong with my DBW or ECM? Or normal?

[Howard]

2005 CTS, 2.8L

When I'm coasting in gear at speed, the mileage reads 0L/100km (or infinite mpg for you imps) as it should, but below certain speeds, something happens to make the indicator show that the engine is actually using gas. For example, the readout shows 5L/100km after speed drops below ~45km/h in 4th gear, and 5-6L/100km after speed drops below 55km/h in 3rd gear, with more gas being used earlier in the taller gears. Didn't bother checking 5th and 6th but I would imagine they cut fuel to lower speeds (as per trend).

Why does it show that gas is being used?

Might be related to this: http://forums.anandtech.com/showthread.php?t=2070355

 

[boomerang]

Downshifting would be my guess. The ECM is raising the RPM to make the shifts smoother.

 

[Zenmervolt]

There is an RPM cutoff point where the injectors will begin firing again even when the car is coasting in gear. This cutoff point varies between cars and even between transmission options (theoretically it can also be varied by which specific gear is selected as well). Generally this is done to prevent jerkiness when fuel flow is re-introduced.

For example, on my S70 with the automatic, the injectors are re-activated if engine speed is below 2,400 RPM. According to factory manuals, the manual transmission version is programmed not to re-start the injectors until engine speed drops below 1,800 RPM.

For my 951, which is much older, the fuel injectors are not re-started until the engine drops below 900 RPM. This is just barely above idle and it definitely makes the throttle something of an on/off switch when I'm creeping along in 1st gear. Very touchy. For drivability reasons, most new cars re-start fuel flow nearer to where the Volvo does.

So, the short answer is that it shows gas being used because gas is being used.

ZV

 

[SJP0tato]

Agreed 100%. With a wideband monitoring the A:F ratio you can easily tell when the ecu/ecm starts firing the injectors while engine braking because it goes from extremely lean to something around stoich.

There is an RPM cutoff point where the injectors will begin firing again even when the car is coasting in gear. This cutoff point varies between cars and even between transmission options (theoretically it can also be varied by which specific gear is selected as well). Generally this is done to prevent jerkiness when fuel flow is re-introduced.

For example, on my S70 with the automatic, the injectors are re-activated if engine speed is below 2,400 RPM. According to factory manuals, the manual transmission version is programmed not to re-start the injectors until engine speed drops below 1,800 RPM.

For my 951, which is much older, the fuel injectors are not re-started until the engine drops below 900 RPM. This is just barely above idle and it definitely makes the throttle something of an on/off switch when I'm creeping along in 1st gear. Very touchy. For drivability reasons, most new cars re-start fuel flow nearer to where the Volvo does.

So, the short answer is that it shows gas being used because gas is being used.

ZV