I noticed that my truck is getting 2 - 3 mpg less than it used to. This wasn't on a signle tank of gas, this has been the average of every tank of gas over the last couple of months and I fill up about once a week. I hooked up my predator and viewed real time live data just to see if anything was different than what it normally is. When I first had RWTD create the custom tune for my truck, the Long Term Fuel Trims were at zero (0) and now they are at negative four (-4). If I'm not mistaken a negative value for the LTFT indicates rich. So, if it is running rich, this could be the reason for the decrease in fuel mileage. I haven't made any changes to my truck in the last couple months, so I'm wondering what caused the change in the LTFT? Should I subtract some fuel to get the LTFT back to zero or should I do further troubleshooting to find out what caused this change?

It has not been to the dealership and it is not generating any codes.

I plan to have it dyno tuned in a few months from now when I buy headers and camshaft. I'd like to get the mpg back where it used to be until then.

 

What are you using to scan? can you post a log?

 

The Predator Programmer has a diagnostic section which allows you to view live data while driving. I think it is capable of recording the live data to a log file, but I will have to check tomorrow to be sure.

Is there any free tools that can be loaded on a laptop to do a scan and log it? I know there was for a friend of mines '95 LT1 Trans AM.

**Edit: You can get a log file from the predator. I will get it tomorrow and post it**

 

Cool. that would help.
First thing that crosses my mind is bad injector. but there are a few other things we should look at too.

 

-4 does not indicate rich necessrily. It means the pcm is subtracting 4 percent fuel to keep the 02's happy.

On any given day your LTFT will probly fluctuate a few points. IAT, humidity, a bunch of stuff is in the equation.

On another note you want them to be anywhere from 0 to -4. That is a pretty good VE table if they are in that range. You want them to be negative anyway, positive trims are dumped on top of a commange PE value making a vehicle overly rich. IE, if at WOT your commanding 12.5 afr and the comp has a positive trim going in to it. It uses the appr injector to get 12.5 then dumps your positive trim on top of it for the safety of the motor.