07 GMC Sierra: Why is the commanded afr so rich?

[Tick]

I have a truck that was dropped off due to PO172 and PO175 codes, fuel trim system rich banks one and two. After a short drive I noticed that the commanded afr is very rich, and the long term trims are around -20%. Commanded at wot goes into the 8's.

I have seen this on another truck, and I flipped the switch to disable flex fuel which took care of the issue. This truck however is a bone stock truck, and the guy just wants the codes repaired.

Why would it command 11.04:1 afr just cruising along after everything is up to operating temp? And how would a variable stoich for flex fuel ever work in closed loop? I didn't have the wideband hooked up on this truck, but the other truck was still running at 14.6 with the sensors trimming even though it too was commanding a crazy rich value. The fuel trims were just soaking up the difference.

It would seem that anything "flex fuel" would need wideband o2 sensors if it were able to command 11.04 as this truck is doing for example.

I'm not sure what fuel is being used in this truck, but the gas locally is 10% ethanol at most places. A lot of premium is still pure gasoline. The closest e85 is about 60 miles from us and even that shouldn't command 11.04:1?

Maybe the problem is something else rather than flex fuel, but the symptoms are the same as the truck before....

[superwagon]

A commanded afr of 11.04 would be about 65% ethanol. Maybe he filed up with e85 or was running e85 and filled up with e10 gas. I can not look at the your tune or log right now. Just going off your description?

[Area47]

he put e85 in it and didn't drive it for 7 miles for the ecm to figure out it's final number on commanded afr.



put 4 gallons of e85, or 91 octane in it. go drive it for 20 miles.

[Tick]

I talked to the owner and he said that the truck had 87 octane gasoline in it. It was most likely 10 percent ethanol. It was also so low on gas that the "low fuel" message was displayed so I doubt he put any fuel in it recently lol.

Anyway, I changed the stoich value across the board to 14.32 or something like that and the commanded did change. He always runs gasoline which I suppose could be have anywhere from 0 to 10% ethanol. After the change the long terms were within 5%.

He mentioned that a recall had been done previously to fix a similar issue....maybe a dealer reflash due to an incorrectly commanded stoich value far richer than it should be. Anyone ever heard of that? I was kinda shocked that he knew as much about it as he did.

[Area47]

if the recall had anything to do with the fpcm then yes. if they didn't follow the exact procedure, then there lies the issue all together. the flex fuel algorithm is screwed.

involves draining the tank, putting in fuel with less than 3% ethanol and 4 gallons of it. in the tech 2 there is a way to force the ecm in ethanol content relearn mode {did it on my old 2010 6.2 truck}. but does require the flex fuel parameters to be 100% stock.

the stoich the trims are trying to command is 13.24

[Mikehunt]

You should be able to just reset the ethonal content if it is less than 10% in the fuel but the ECM is showing higher, i have seen this many times. If it shows less than 10% ethonal in the ECM but the actual ethonal content is higher then you will have to drain the tank