ConnorsStepside

hey yall, ive got a 2000 sierra with the 4.8 and 4l60e. i live in the hill country so i am always going up and down hills. something that i notice the truck does only as i go down hills, only going over 45 mph, is the engine will change tones, and the exhaust will change tones with it, and will slightly start to almost auto brake is what it feels like. if i tap the gas pedal it will go away, but will come back either within a few seconds or on the next hill. Im not sure what this is, i wouldn't think itd be a good thing. tow haul is not on either. any ideas on what it could be? thanks for the help

FFDP

iTrader: (2)

In technical terms it's called de-acceleration fuel cut off.

The engine will shut off the fuel injectors and turn itself into just an air pump with no actual firing of the cylinders. The engine tone changes because there is no longer an explosion of air/fuel inside the cylinders and now it's just air being pumped in a out.

This practice has been done for a long long time. It helps with fuel economy and also engine braking to help save your brakes if you are going up and down many hills.

ConnorsStepside

was this only done on some of these systems? i know our suburban didnt do this. ive never heard of this system but i guess it totally makes sense

FFDP

iTrader: (2)

Anything LS based since 1997 has used this. I'm sure the vortec computers did this as well in the mid 90's.

Also the GM V6 line up has this, the 3.1, 3.8, 3.4, 3.5, 3.9, 4.3 etc etc all use DFCO. Other car makers use this same feature as well.

Whether or not you can hear it or sense that it's happening is up in the air on some vehicles. On some it's very seamless because the exhaust is really quiet or it's in a out of it so fast that you can't tell. Certain variables have to be triggered to enter DFCO. Like vehicle speed, engine rpm and what gear the transmission is in for example.

After market exhaust will make it far more noticeable.