Problems with AFM numbers jumping around?!?

[Whiskey Bent]

So I was driving down the highway the other day and noticed my mpg numbers were jumping around like crazy. I was in cruise control on flat terrain and it would jump from 20 to 55 to 30 and just all over. It got kind of annoying so at a stop light I shut the truck off and restarted and it seemed to fix it until today it started again. Anyone else experience this? What's going on with my truck? I just tuned 1000 miles on it and have only had it for a little over a month? Help!

[peejaykay]

Sounds normal to me. It's instant MPG.

[14SLTZ71]

So I was driving down the highway the other day and noticed my mpg numbers were jumping around like crazy. I was in cruise control on flat terrain and it would jump from 20 to 55 to 30 and just all over. It got kind of annoying so at a stop light I shut the truck off and restarted and it seemed to fix it until today it started again. Anyone else experience this? What's going on with my truck? I just tuned 1000 miles on it and have only had it for a little over a month? Help!

Did you feel anything unusual when the readouts were jumping around or was it just a visual thing?

[Whiskey Bent]

No didn't feel anything different at all. And I always keep my dash screen on the fuel mileage page so I am used to it fluctuating up and down. It's usually gradual. This was with no change in speed, terrain, or jumping in and out of V4 & V8 mode. And the severity of how far apart the numbers were jumping around had me kind of worried.

[fondupot]

normal

[pm26]

I would not call this normal if fluctuations are unusually severe given steady speed and load conditions. Maybe there is something wrong with the system. I know the numbers jump around in my 2009 Impala if I press on the accelerator and then let go of it suddenly, but under steady speed, the fluctuations are slight.

[fondupot]

I would not call this normal if fluctuations are unusually severe given steady speed and load conditions. Maybe there is something wrong with the system. I know the numbers jump around in my 2009 Impala if I press on the accelerator and then let go of it suddenly, but under steady speed, the fluctuations are slight.

 

Your 2009 impala didn't have direct injection. DI is much more efficient and will show more fluctuations in the instant mpg, because it can turn off fuel supply and turn it back on faster than older standard fuel injection. Not to mention the fuel shut off feature under coasting. The injectors will actually stop injecting fuel when coasting, and the wheels are driving the motor, this results in the instant screen showing 99mpg, but as soon as you hit the gas it will drop back down.

 

what OP is describing is 100% normal. Don't get obsessed with the instant MPG screen. It will only drive you nuts.

[pm26]

 

Your 2009 impala didn't have direct injection. DI is much more efficient and will show more fluctuations in the instant mpg, because it can turn off fuel supply and turn it back on faster than older standard fuel injection. Not to mention the fuel shut off feature under coasting. The injectors will actually stop injecting fuel when coasting, and the wheels are driving the motor, this results in the instant screen showing 99mpg, but as soon as you hit the gas it will drop back down.

 

what OP is describing is 100% normal. Don't get obsessed with the instant MPG screen. It will only drive you nuts.

He said that it was under steady state conditions, so there should not be any shutting off of fuel , jumping in and out of the V8 mode, etc. Yes, I believe that what you mentioned will cause greater variation in instant MPG under different conditions, but at steady speed the fluctuations should not be huge even with a direct injection engine. I believe he mentioned this as a problem because the truck was not doing it before.

 

Personally I never pay attention to instant MPG readouts. I have that even on my Kawasaki Concours bike. I like the real MPG numbers on that one - close to 40 MPG.

[Whiskey Bent]

He said that it was under steady state conditions, so there should not be any shutting off of fuel , jumping in and out of the V8 mode, etc. Yes, I believe that what you mentioned will cause greater variation in instant MPG under different conditions, but at steady speed the fluctuations should not be huge even with a direct injection engine. I believe he mentioned this as a problem because the truck was not doing it before.

 

Personally I never pay attention to instant MPG readouts. I have that even on my Kawasaki Concours bike. I like the real MPG numbers on that one - close to 40 MPG.

 

Correct! It never jumped around like that before.

[Ventilator]

So I was driving down the highway the other day and noticed my mpg numbers were jumping around like crazy. I was in cruise control on flat terrain and it would jump from 20 to 55 to 30 and just all over. It got kind of annoying so at a stop light I shut the truck off and restarted and it seemed to fix it until today it started again. Anyone else experience this? What's going on with my truck? I just tuned 1000 miles on it and have only had it for a little over a month? Help!

 

 

Yep, after they done the Peek download.

 

My gas mileage went down 2-3 MPG too.

[CAtaylor]

So I was driving down the highway the other day and noticed my mpg numbers were jumping around like crazy. I was in cruise control on flat terrain and it would jump from 20 to 55 to 30 and just all over. It got kind of annoying so at a stop light I shut the truck off and restarted and it seemed to fix it until today it started again. Anyone else experience this? What's going on with my truck? I just tuned 1000 miles on it and have only had it for a little over a month? Help!

If your overall fuel mileage hasn't been affected I wouldn't worry about it. Mine has done that periodically since day one, seems to do it more depending on where i get fuel. 19K miles later she is still doing fine.