Deceleration Fuel Cut off
07-22-2008, 08:57 PM
#11
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Join Date: 02-10-2007
Location: LBC Ohio
Posts: 2,727
Are we talking about HHRs here? Fuel never cuts off when the engine is running and if it does the engine stops running, so there is a problem. Only a hybrid shuts off all fuel injectors and vehicles with "Active Fuel Management", such as Tahoes and Suburbans, cutoff 4 of the 8 cylinders at times.
07-22-2008, 09:54 PM
#12
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Join Date: 07-22-2008
Location: Philadelphia suburbs
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07-23-2008, 07:46 AM
#14
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Join Date: 11-23-2007
Location: Texas
Posts: 8,210
07-23-2008, 12:23 PM
#15
Senior Member
Join Date: 10-02-2005
Location: VA
Posts: 1,245
To help everyone understand this....
DFCO (Deceleration Fuel Cut Off) is exactly that. During off-throttle deceleration, the ECM cuts off the injectors. I run a wideband O2 sensor and, when conditions are met, can see the AFR go "off scale" lean. This can also be seen by data logging with HPTuners. Basically, as long as the engine is turning, under off-throttle deceleration, no fuel is needed. GM does this to help both emissions and mileage numbers. You need to realize that the ECM is programmed to "turn the injectors back on" below certain threasholds to eliminate any possibility of stalling.
Just for clarification, here are the actual ECM calibration numbers for '06 stock LE5 2.4's for both auto and manual trans (without looking at other year parameters, they should be similar, if not the same):
The primary calibration parameters for either enable or disable are based upon both RPMs and speed in gear, both of which have to be met.
RPMs (applies to both auto and manual):
DFCO enable: RPMs must be above 1,800 RPM
DFCO disable: RPMs drop below 1,600 RPM (this is why it won't stall)
Speed (auto trans):
To enable DFCO speed in gear must exceed X MPH:
1 - 12
2 - 17
3 - 22
4 - 26
To disable DFCO speed in gear (IOW, if speed drops below this value, fuel will return, another reason the car won't stall):
1 - 10
2 - 15
3 - 20
4 - 24
For the manual trans, here's the values:
To enable DFCO, speed must exceed:
1 - 12
2 - 25
3 - 29
4 - 29
5 - 29
The manual disable speeds are (again, why it won't stall):
1 - 10
2 - 21
3 - 27
4 - 27
5 - 27
Does this make sense now? Hope it helps everyone understand it. Hey, it's good for MPG and good for the atmosphere, too!
DFCO (Deceleration Fuel Cut Off) is exactly that. During off-throttle deceleration, the ECM cuts off the injectors. I run a wideband O2 sensor and, when conditions are met, can see the AFR go "off scale" lean. This can also be seen by data logging with HPTuners. Basically, as long as the engine is turning, under off-throttle deceleration, no fuel is needed. GM does this to help both emissions and mileage numbers. You need to realize that the ECM is programmed to "turn the injectors back on" below certain threasholds to eliminate any possibility of stalling.
Just for clarification, here are the actual ECM calibration numbers for '06 stock LE5 2.4's for both auto and manual trans (without looking at other year parameters, they should be similar, if not the same):
The primary calibration parameters for either enable or disable are based upon both RPMs and speed in gear, both of which have to be met.
RPMs (applies to both auto and manual):
DFCO enable: RPMs must be above 1,800 RPM
DFCO disable: RPMs drop below 1,600 RPM (this is why it won't stall)
Speed (auto trans):
To enable DFCO speed in gear must exceed X MPH:
1 - 12
2 - 17
3 - 22
4 - 26
To disable DFCO speed in gear (IOW, if speed drops below this value, fuel will return, another reason the car won't stall):
1 - 10
2 - 15
3 - 20
4 - 24
For the manual trans, here's the values:
To enable DFCO, speed must exceed:
1 - 12
2 - 25
3 - 29
4 - 29
5 - 29
The manual disable speeds are (again, why it won't stall):
1 - 10
2 - 21
3 - 27
4 - 27
5 - 27
Does this make sense now? Hope it helps everyone understand it. Hey, it's good for MPG and good for the atmosphere, too!
07-23-2008, 01:27 PM
#17
Founding Member
Join Date: 11-23-2007
Location: Texas
Posts: 8,210
Joe,
I'm not sure I understand your table. Are you stating you added something to your HHR to obtain DFCO?
What is your source for these numbers?
As far as GM repair manuals go, fuel is only cut off during extreme conditions and it is not done for fuel economy.
I'm not sure I understand your table. Are you stating you added something to your HHR to obtain DFCO?
What is your source for these numbers?
As far as GM repair manuals go, fuel is only cut off during extreme conditions and it is not done for fuel economy.
07-23-2008, 02:36 PM
#18
Senior Member
Join Date: 10-02-2005
Location: VA
Posts: 1,245
Those numbers are from stock GM calibrations for '06 HHR LE5 2.4 auto and manual trans. I've added "stuff", but have not (yet) messed with the DFCO.
Source? The GM calibration data.
DCFO is commonly implemented for most engines today.Why would we need to burn fuel when slowing down?
07-23-2008, 02:52 PM
#19
Founding Member
Join Date: 11-23-2007
Location: Texas
Posts: 8,210
I still think this could only happen under SEVERE conditions. Here is what the repair manual states:
DECELERATION MODE:
When the driver releases the accelerator pedal, air flow into the engine is reduced. The control module monitors the corresponding changes in the throttle position (TP), the mass air flow (MAF), and the manifold absolute pressure (MAP). The control module shuts OFF fuel completely if the deceleration is very rapid, or for long periods, such as long, closed-throttle coast-down. The fuel shuts OFF in order to prevent damage to the catalytic converters.
FUEL CUTOFF:
The control module cuts OFF fuel from the fuel injectors when the following conditions are met in order to protect the powertrain from damage and improve driveability:
• The ignition is OFF. This prevents engine run-on.
• The ignition is ON but there is no ignition reference signal. This prevents flooding or backfiring.
• The engine speed is too high, above red line.
• The vehicle speed is too high, above rated tire speed.
• During an extended, high speed, closed throttle coast down--This reduces emissions and increases engine braking.
• During extended deceleration, in order to prevent damage to the catalytic converters
DECELERATION MODE:
When the driver releases the accelerator pedal, air flow into the engine is reduced. The control module monitors the corresponding changes in the throttle position (TP), the mass air flow (MAF), and the manifold absolute pressure (MAP). The control module shuts OFF fuel completely if the deceleration is very rapid, or for long periods, such as long, closed-throttle coast-down. The fuel shuts OFF in order to prevent damage to the catalytic converters.
FUEL CUTOFF:
The control module cuts OFF fuel from the fuel injectors when the following conditions are met in order to protect the powertrain from damage and improve driveability:
• The ignition is OFF. This prevents engine run-on.
• The ignition is ON but there is no ignition reference signal. This prevents flooding or backfiring.
• The engine speed is too high, above red line.
• The vehicle speed is too high, above rated tire speed.
• During an extended, high speed, closed throttle coast down--This reduces emissions and increases engine braking.
• During extended deceleration, in order to prevent damage to the catalytic converters
Last edited by ChevyMgr; 07-23-2008 at 02:55 PM. Reason: didn't get it all pasted on first attempt
07-23-2008, 04:26 PM
#20
Senior Member
Join Date: 10-02-2005
Location: VA
Posts: 1,245
I still think this could only happen under SEVERE conditions. Here is what the repair manual states:
Nothing SEVERE about just letting off the gas to slow down. Happens all the time, as in going down a hill, slowing for a stop, etc.
DECELERATION MODE:
When the driver releases the accelerator pedal, air flow into the engine is reduced. The control module monitors the corresponding changes in the throttle position (TP), the mass air flow (MAF), and the manifold absolute pressure (MAP). The control module shuts OFF fuel completely if the deceleration is very rapid, or for long periods, such as long, closed-throttle coast-down. The fuel shuts OFF in order to prevent damage to the catalytic converters.
Other than the RPM and speed parameters mentioned, there are also TPS (which is "stepped" upon throttle release and won't reach the TPS parameter immediately. The MAF parameters are in the very low closed throttle range. Of course MAP will be high (vacuum) closed throttle.
I don't know where they get their definition for "vary rapid", other than just let off the gas. No braking is necessary to trigger DFCO. "Long" is just the time for the ETC to step down to the ETC threashold, about 3 sec. from a quick look at a scan.
FUEL CUTOFF:
The control module cuts OFF fuel from the fuel injectors when the following conditions are met in order to protect the powertrain from damage and improve driveability:
• The ignition is OFF. This prevents engine run-on.
Unrelated to DFCO, but true.
• The ignition is ON but there is no ignition reference signal. This prevents flooding or backfiring.
Unrelated to DFCO, but true.
• The engine speed is too high, above red line.
Unrelated to DFCO, but true. The RPM limiter on the 2.4 LE5 is 6,750. Both fuel and throttle are cut.
• The vehicle speed is too high, above rated tire speed.
Unrelated to DFCO, but true. Not necessarily true to rated tire speed. For example on a 2LT, the OEM 17" tire carries a "T" speed rating (=118 MPH), yet the calibration is set for 108. That cuts both fuel and throttle also.
• During an extended, high speed, closed throttle coast down--This reduces emissions and increases engine braking.
Doesn't have to be that long (3 sec. noted above), nor high speed. Yes, it does reduce emissions (no gas = no emissions). Engine braking is enhanced. On a manual trans you can actually feel when the DFCO kicks in. Helps ecomomy, too, since if you aren't putting any fuel into the engine...
• During extended deceleration, in order to prevent damage to the catalytic converters
I don't see how the cat would be damaged if DFCO wasn't implemented, since the engine would continue at stoic just like normal. However just pumping air will cool it some, which should help life, but that condition doesn't happen as a large percentage of the operating cycle.
Nothing SEVERE about just letting off the gas to slow down. Happens all the time, as in going down a hill, slowing for a stop, etc.
DECELERATION MODE:
When the driver releases the accelerator pedal, air flow into the engine is reduced. The control module monitors the corresponding changes in the throttle position (TP), the mass air flow (MAF), and the manifold absolute pressure (MAP). The control module shuts OFF fuel completely if the deceleration is very rapid, or for long periods, such as long, closed-throttle coast-down. The fuel shuts OFF in order to prevent damage to the catalytic converters.
Other than the RPM and speed parameters mentioned, there are also TPS (which is "stepped" upon throttle release and won't reach the TPS parameter immediately. The MAF parameters are in the very low closed throttle range. Of course MAP will be high (vacuum) closed throttle.
I don't know where they get their definition for "vary rapid", other than just let off the gas. No braking is necessary to trigger DFCO. "Long" is just the time for the ETC to step down to the ETC threashold, about 3 sec. from a quick look at a scan.
FUEL CUTOFF:
The control module cuts OFF fuel from the fuel injectors when the following conditions are met in order to protect the powertrain from damage and improve driveability:
• The ignition is OFF. This prevents engine run-on.
Unrelated to DFCO, but true.
• The ignition is ON but there is no ignition reference signal. This prevents flooding or backfiring.
Unrelated to DFCO, but true.
• The engine speed is too high, above red line.
Unrelated to DFCO, but true. The RPM limiter on the 2.4 LE5 is 6,750. Both fuel and throttle are cut.
• The vehicle speed is too high, above rated tire speed.
Unrelated to DFCO, but true. Not necessarily true to rated tire speed. For example on a 2LT, the OEM 17" tire carries a "T" speed rating (=118 MPH), yet the calibration is set for 108. That cuts both fuel and throttle also.
• During an extended, high speed, closed throttle coast down--This reduces emissions and increases engine braking.
Doesn't have to be that long (3 sec. noted above), nor high speed. Yes, it does reduce emissions (no gas = no emissions). Engine braking is enhanced. On a manual trans you can actually feel when the DFCO kicks in. Helps ecomomy, too, since if you aren't putting any fuel into the engine...
• During extended deceleration, in order to prevent damage to the catalytic converters
I don't see how the cat would be damaged if DFCO wasn't implemented, since the engine would continue at stoic just like normal. However just pumping air will cool it some, which should help life, but that condition doesn't happen as a large percentage of the operating cycle.