I've noticed a strange thing with my gas milage after a recent tune and was wondering if it is a normal thing. The instant economy reading with my foot off the gas at a coast, usually downhill, use to climb to 99 pre tune. After tuning it will climb to 50-60. Initially i wasnt too concerned with this untill I noticed that it only does this in drive. If manually shifted down in third it rises to 99, just like it use to pre tune in all gears. From my experience the instant economy is pretty accurate and now I'm baffled as to why I lose some milage in drive at a coast compared to third. I know this is kinda a non issue because of the little difference this actually makes in my overall milage but it kinda annoys the hell out of me. Anyone know whats up with this? Or should I just get over it?

 

Anything changed with the dfco? Maybe a enable/disable table tweaked?

 

The tune was installed from a sponser. I dont know exactly what all has been modified. I do know the DoD was disabled. Initially thats what I thought was going on when I first noticed the drop but that still doesnt add up to me as to why it gains it back when downshifted. Ive asked the tuner about this and their response was- dont worry about it, you use so little gas while at coast it doesnt effect real world milage. And I agree for the most part. But I just cant get over it as to why it gains when in third. Just seems very fishy to me.

 

A lot of people tweak the dfco to rid annoying backfiring etc. I'm sure it was probably just a enable parameter or something. Also there are tables and multipliers in the background that we don't see that may be letting it work in 3rd vs 4th. It may be related to the dod being deleted. Hard to say.

 

Did you go to bigger injectors? A bigger injector can have issues with the pulsewidth at idle, and low kPa levels. I am going through this, and plan on messing with it today after work. It was not uncommon for my AFR's to hit in the 11's when I am coasting.

 

How did your tuner determine your tune? If you logged it for them with a black box, and did not hit the coasting cells long enough, or with a smooth enough throttle change, it could also make them inaccurate. It would more than likely log a rich value, and the tune would be leaned due to an over correction, but it can have an effect on the final tune.

 

Injectors are stock. Truck is pretty much entirely stock. Cat back, air box and vette servo. Thats it. Was an in person tune. Did some driving around and they installed a new tune.

 

More vacuum in 3rd vs od so the injectors stay open less. Your tuner is right, you use pratically no fuel while in DFCO.

 

I noticed the same thing after a tune with DFCO tables not changed at all. It still enters DFCO because I can see injector duty going to zero on VCM Scanner.