NEED HELP High Negative LTFT / Poor Gas Mileage

[Sparky]

Tires play a huge part. Pretty much low rolling resistance (LRR) tires are almost a necessity.

Tires can and do play a large part but LRR aren't a necessity for decent mpg. I never used LRR tires and stock with Firestone Destination AT I got 20+ mpg highway (3.42 gears) and with a lift and 31.6" tires I would still get 18+.

Terrain plays a big part. Hills will suck more fuel than flat.

 

[gmcman]

Tires can and do play a large part but LRR aren't a necessity for decent mpg.

I agree...but just to clarify, thats why I said "almost".

Our platforms surely weren't built for great fuel economy, they run on the cusp of dismal to somewhat decent for their heft.

I got 20+ mpg highway (3.42 gears) and with a lift and 31.6" tires I would still get 18+.

That's not too bad, all things considered, especially tires and a lift.

I can muster low 20's up to about 70 ish, then it starts to fall off, being hampered with 4:10's though.

 

[TequilaWarrior]

I'm tickled when I break 16. That's on a 40 mile commute over 80% of which is completely flat interstate. Yes, I do upwards of 75 MPH most of the way, but I NEVER get over about 16.6 MPG on stock tires, AWD, and 3.73 locker.

 

[HARDTRAILZ]

I get 15-16 w 35s. I drive conservatively but did get over 20 stock. I always thought these were far improved over my previous SUV mpg experiences.

 

[2advanced]

2002, 4WD.

Tires play a huge part. Pretty much low rolling resistance (LRR) tires are almost a necessity.

Do you have aggressive tread? Is the vehicle lifted?

Yes, the tread pattern on the GoodYear Duratracs is pretty aggressive and the tire diameter is 1" larger than stock. I also have a 2" Lift/Leveling kit on it. Gear ratio is also 3.42.

But the thing is that even with the stock wheels and tires, my MPG's were only 1MPG better at best.

Im going to try to jack the car up both front and back this weekend so I can spin the driveshafts by hand to feel for drag.

I would like to this up again as no one had commented on this. I have also witnessed this and it doesn't seam correct. On warm up I will see it vary downwards of 8.5 but once in CL it stays exactly @ 14.7 no matter the load.

This was observed on a trip I just took with a ScanGuage, I need to check with a scanner app and see if it shows the same. I also noticed it doesn't seem to go into fuel cut on decel often and I believe still showed at 14.7?? but again that could be the ScanGauge or I might of missed it, I need to dig out my adapter.

Does anyone see it vary from 14.7 once warmed up?

@whodwho Are you looking at the Commanded AFR or are you running a Wideband to monitor the AFR? Under WOT, the AFR varies from 11.6 to about 12.25.



You got me thinking, and I pulled up the Tune to see DFCO settings and when in 4th gear, the Cutoff is not active at all. You have to be in the first 3 in order for it to activate. I'm going to try to adjust mine to see what kind of difference it makes.



@HARDTRAILZ What ratio rear-end you have? I'm only running a 30.6" Tire and get the same MPG's you do with 35

 

[HARDTRAILZ]

4.10s
With tires and gearing ratio I end up with an effective ratio of 3.55ish

 

[whodwho]

Are you looking at the Commanded AFR or are you running a Wideband to monitor the AFR? Under WOT, the AFR varies from 11.6 to about 12.25.

Doh! I have been using WB for so long I forgot about narrow band, I actually can't believe these use a narrow band being a 2006 but can see from the graphs it is 8^(

I run standalone on a boosted BMW and not having tuning abilities feels so wrong, seeing your setup I might have to invest 8^)

Thanks for setting me straight

 

[gmcman]

I wanted to update this as I encountered something today that seems like it could play a role in the fuel trims...but time will tell.

Maybe this thread jinxed me..lol, but a few days ago I was seeing high negative LTFT's, a bit higher than what seemed to be regular.

I was getting around -11 to -13ish % LTFT at idle, driving was fine..around -2 to -7. So I wanted to see what happens when I disconnect my MAP sensor so I pulled the connector with the engine idling and the motor stalled. I reconnected the sensor and restarted the engine and it immediately had a misfire on one of the cylinders.

I checked the torque app and it was Cyl #1, I lost the coil. I replaced the coil and now my LTFT's at idle are back down to -7 to -8 with the A/C on.

As of yesterday, my LTFT at idle with AC off was -13, with A/C on was -10. Not with the coil replaced I have -10 with A/C off and -7 to -8 with A/C on.

Could be the coil was on it's last leg as it is the original, I've only replaced the #4 and #2 coil since I bought the Envoy new.

Maybe it was nothing, but maybe the remaining coils are weak and while not causing a misfire, but not a complete combustion..

I'll give it a day or so to see if the PCM readjusts.

 

[2advanced]

…………...

Maybe it was nothing, but maybe the remaining coils are weak and while not causing a misfire, but not a complete combustion..

I'll give it a day or so to see if the PCM readjusts.

If I remember right, your Envoy is 2004, so no MAF? Might it have been a good idea to replace them all (coils) at the same time?

 

[gmcman]

If I remember right, your Envoy is 2004, so no MAF? Might it have been a good idea to replace them all (coils) at the same time?

Actually an 02, but still no MAF.

I did order a set of coils, will keep one as a spare. I'll get those changed out today.

Something else I'm curious about...does the downstream O2 sensor only monitor condition of the cat, or does it also play a role in fuel trims?

When I come to a stop, it almost seems as the steady voltage from the rear O2 sensor starts to slowly creep up, the LTFT drifts more negative. Maybe coincidental, but wondering if the cat affects LTFT's....I didn't think it did.

 

[Mooseman]

AFAIK, no. It just checks if the cat is working. As long as it's showing relatively flat voltages, it means the cat is working. If it's kinda following what the upstream sensor is doing, the cat is dead.

 

[Tankcruiser]

any new info im in same boat wise.....
which torque app to use btw?