DIC Fuel Consumption Bouncing?

[Cordovakid]

Hey - I have a Silverado 6.2 which has about 5500 km (approx. 3500 miles) on it. I've always used 91 octane ethanol-free gas in it, which is available here in Ontario. I travelled down to Chicago on the weekend and filled up with 93 octane, containing ethanol, in Michigan. On my return trip home, I noticed that not only did my actual mileage go down, but the DIC live fuel consumption meter began bouncing up and down most of the trip home. This was on a flat, straight road with a steady speed of approx. 70 mph. This bouncing of the DIC meter has never occurred before - always been within 1 or two mpg of each other. This time, it was bouncing between 15-25 mpg, live reading, during a steady drive.

 

When I got home, the next day I noticed that after my cold start I had a slightly rough idle, also something which has never occurred before. My question is, is this possibly the computer trying to accomodate and adjust for the new gas which was put in the tank, or is it likely something else which I have to be aware of? Has anyone else had this type of problem occur on the DIC meter? Like I said, the fuel consumption has gone up, so it's not just a meter thing. I will be filling up with my usual gas in Ontario tonight, so I'll watch to see if the problem clears up. Any likewise experiences?

[00Silverado4x4]

The ethanol containing fuel will probably cause a decrease in fuel economy but it sounds more like you may have got a bad tank of gas. I'd run the fuel through and get it good and low and then refill at your Ontario station and see if it continues to act this way. If it does then maybe look deeper.

[TxTruckMan]

The instantaneous fuel economy reading should always fluctuate up and down unless you're on PERFECTLY level ground. Every vehicle I've owned does this. Minute adjustments in the throttle cause it to fluctuate.

 

You should notice a slight (~5%) drop in mileage due to the ethanol.

 

Of course, maybe you got some bad gass if you're having other problems.

[fondupot]

If I recall it was quite windy around the Chicago this weekend. These trucks are basically a big sail in the wind. That would be a reason why your mileage went down, along with the "bounciness" of your instant MPG gauge. Also ethanol.

[TxTruckMan]

I've heard that Chicago has a reputation for being a windy city...

[Cordovakid]

Thanks for the replies - yeah, I was figuring it was just bad gas. My other thought, however, was that it may have been a dirty MAF / MAP sensor or something similar, causing the active fuel management to get confused and try to compensate. We'll see what happens after a fresh tank....If it continues, I will be sure to take it in to check for codes and / or dirty sensors, etc....Warranty is a good thing.

[Crf450r420]

I had this happen on my way home just this weekend. I wanted to see if it was the computer, so I connected my phone and as soon as I did it became stable again. I don't know if it was just a reporting issue to the DIC or what.

[HondaHawkGT]

I've had it happen when I mixed 87 octane and e85. Rough idle and some instances of fluctuating RPM's under load. I'm guessing the ECU was struggling with the change in ethanol levels or the e85 had a lot of water in it.

[Cordovakid]

OK, so this is becoming a little more clear now.... I have been back home for about a month now, and have been filling up at the same Shell station since I got back. Always 91 octane, ethanol-free, as advertised on the pumps and on Shell's website. So, it seems like about every other time I start and drive the truck, the DIC instant fuel economy meter will either be more stable and only bounce up and down a couple of points either way while on a flat, straight road at a constant speed, or it will be bouncing all over the place, typically around 3 or 4 MPG on either side of the average - same driving conditions.

 

I am starting to think that there may be a dirty / fouled sensor, a loose connection or a glitch in the programming of the AFM which is either getting properly reset or not, depending on the turn of the key. I guess my question is, what exactly computes the instant fuel economy / consumption readout on the DIC? And, what could affect it bouncing around? I cannot make a determination for sure if the economy is actually going down, or it is just the reading.