ecohauler
New member
I finally got my ScanGauge II hooked up to my Sonic and went for a short drive with it. As sx sonic pointed out in another thread, the behavior of the ECU in terms of deceleration fuel cut-off (DFCO) is somewhat erratic. This thread is for discussion of how to most reliably enter DFCO in order to maximize fuel economy. Hopefully, we can also use it to determine whether the Trifecta performance and eco tunes make any changes in this behavior, and this may help us determine whether a tune will actually save fuel on an individual basis.
A word of caution for anyone new to this concept: DFCO is not by any means the only tool you should be using to maximize fuel economy. This is doubly true in our cars, as it is more difficult for us than for most to hit the fuel cut-off. Despite the fact that DFCO uses no fuel, there are many cases in which a better overall fuel strategy means clutch-in coasting, maintaining a steady speed, or maintaining a steady throttle position. The goal of this thread is to pinpoint the times when this is something we are even capable of doing, but be advised that just because you can do something doesn't mean it's the best option.
Don't be shy about your findings. If you're not running any additional telemetry, you probably won't be able to find much, but otherwise if you've noticed something that either confirms or refutes something in this thread, please speak up. Let's figure out the best way to drive our cars, whether they're manual or auto, 1.4T or 1.8, tuned or stock.
My findings are on a 1.4T 6MT with the stock tune:
Ambient temp: ~60°F, humidity: 86%, nighttime, clear.
The engine does not appear to enter DFCO unless it is up to operating temperature (above ~200°F). This makes sense, as DFCO does have the effect of cooling an engine. I actually got the temperature gauge on my Saturn to read completely cold coming down a mountain one time, when it was previously warm. Without fuel, an engine is just pumping cool air with relatively little friction, so it will run cold rather quickly in a prolonged DFCO state. It makes sense for a more advanced ECU like the Sonic's to add a little fuel to ensure that the engine will be able to run efficiently when you start using power again.
Either the ScanGauge is slow to read or the ECU is slow to send the signal for an open injector loop. I found under engine braking conditions that DFCO strongly corresponded to an ignition retard signal, but the IGN reading would swing negative sometimes several seconds before the MPG would read 9999. My hunch is that DFCO is already active when the ignition signal retards and the ECU is just slow to report it, but I could very well be wrong.
It appears that more engine braking than necessary does not help in our case. I found that, at 60 MPH I was able to somewhat reliably engage DFCO in 6th gear, less reliably in 5th, and virtually never (once, briefly) in 4th. I believe this to be in line with the first finding, that the ECU prefers to keep the engine warm. Pumping more air through the engine without fuel will cool it faster, so the ECU accounts for this by blowing up some fossil fuels to keep warm. It seems that engine braking above 2k RPM does us no good at all, especially since drum brakes (for everyone except RS) mean our brakes have a strong front bias anyway. That's going to be a hard habit for me to break.
Once DFCO is engaged, it cancels around 1200-1400. It's hard to tell exactly because of the lag on the ScanGauge. This is a comparatively low engine speed. Potentially, this could mean longer deceleration, resulting in better fuel economy. I could not get the Sonic to enter DFCO, however, at an engine speed below 1500.
It seems that, more than with the cars I grew up with, the Sonic is programmed to foster a symbiotic relationship. If we keep the revs reasonable, it rewards us, (if it feels like it). If we use the engine as our primary brake (which will cause premature wear if done too often), it punishes us at the pump.
A word of caution for anyone new to this concept: DFCO is not by any means the only tool you should be using to maximize fuel economy. This is doubly true in our cars, as it is more difficult for us than for most to hit the fuel cut-off. Despite the fact that DFCO uses no fuel, there are many cases in which a better overall fuel strategy means clutch-in coasting, maintaining a steady speed, or maintaining a steady throttle position. The goal of this thread is to pinpoint the times when this is something we are even capable of doing, but be advised that just because you can do something doesn't mean it's the best option.
Don't be shy about your findings. If you're not running any additional telemetry, you probably won't be able to find much, but otherwise if you've noticed something that either confirms or refutes something in this thread, please speak up. Let's figure out the best way to drive our cars, whether they're manual or auto, 1.4T or 1.8, tuned or stock.
My findings are on a 1.4T 6MT with the stock tune:
Ambient temp: ~60°F, humidity: 86%, nighttime, clear.
The engine does not appear to enter DFCO unless it is up to operating temperature (above ~200°F). This makes sense, as DFCO does have the effect of cooling an engine. I actually got the temperature gauge on my Saturn to read completely cold coming down a mountain one time, when it was previously warm. Without fuel, an engine is just pumping cool air with relatively little friction, so it will run cold rather quickly in a prolonged DFCO state. It makes sense for a more advanced ECU like the Sonic's to add a little fuel to ensure that the engine will be able to run efficiently when you start using power again.
Either the ScanGauge is slow to read or the ECU is slow to send the signal for an open injector loop. I found under engine braking conditions that DFCO strongly corresponded to an ignition retard signal, but the IGN reading would swing negative sometimes several seconds before the MPG would read 9999. My hunch is that DFCO is already active when the ignition signal retards and the ECU is just slow to report it, but I could very well be wrong.
It appears that more engine braking than necessary does not help in our case. I found that, at 60 MPH I was able to somewhat reliably engage DFCO in 6th gear, less reliably in 5th, and virtually never (once, briefly) in 4th. I believe this to be in line with the first finding, that the ECU prefers to keep the engine warm. Pumping more air through the engine without fuel will cool it faster, so the ECU accounts for this by blowing up some fossil fuels to keep warm. It seems that engine braking above 2k RPM does us no good at all, especially since drum brakes (for everyone except RS) mean our brakes have a strong front bias anyway. That's going to be a hard habit for me to break.
Once DFCO is engaged, it cancels around 1200-1400. It's hard to tell exactly because of the lag on the ScanGauge. This is a comparatively low engine speed. Potentially, this could mean longer deceleration, resulting in better fuel economy. I could not get the Sonic to enter DFCO, however, at an engine speed below 1500.
It seems that, more than with the cars I grew up with, the Sonic is programmed to foster a symbiotic relationship. If we keep the revs reasonable, it rewards us, (if it feels like it). If we use the engine as our primary brake (which will cause premature wear if done too often), it punishes us at the pump.
Last edited:
. 
