This one should be easy... I have a predator tuner for my 2001 SS. I just want to know, when the Predator tells me that my long term fuel trims (or short term fuel trim) are positive, does that mean that the car is running rich or lean.

 

i believe positive means you are running rich

 

Positive LTFTs means that the PCM is adding fuel to the base calulated pulse width. Therefore, you WERE running lean, but now with the +% LTFTs, you are running near 14.7:1...

 

From Diablo Website:

"Determining long term fuel trims:
Long term fuel trims are an accumulated value of fuel added or subtracted during closed loop operation.
To verify and tune for long term fuel trims, drive the vehicle for 30 minutes or until warmed up, monitor long term fuel trims and get an average value. The easiest way to do this is to get on the highway, enable cruise control and sit at 55mph while monitoring the data. Ideal values should be in the 0 to negative 2 range.
Once you get the average long term fuel trim value:
Positive long term fuel trims For a positive value you subtract that value in percent in the predator injector slope. It is also recommended to lean both fuel tables by the same percentage. Negative long term fuel trims For a negative value you add that value in percent in the predator injector slope. It is also recommended to richen both fuel tables by that same percentage.
Once adjustment is done and tune installed, take the vehicle out and verify the long term fuel trims per previously described method. Adjust again if necessary."

EDIT: Found this in one of Diablos forums...

"Going NEGATIVE on slope tells the ECM that your injectors are SMALLER and to spray MORE fuel.

Going POSITIVE on slope tells the ECM that your injectors are BIGGER and to spray LESS fuel."

So, if you are telling the injectors to spray more fuel why then would you decrease the fuel tables, isnt this just negating what you just did?

Thanks!

 

So I guess that you agree with me. A positive LTFT value (+12%) means that the correction that the PCM is doing is to be adding more fuel.

Also, you should not tune LTFTs by changing the fuel injector constant. You should have the correct fuel constant in the program now, unless you swapped out injectors. You should alter the MAF table or the MAP table. When I tuned my nearly stock '99 SS, I had put MAF ends on the factory MAF, this changed the flow charteristics of the MAF. So I altered the MAF table to bring my LTFTs back to where they should be. Personally, I tune so that the LTFTs are between -2% and -5%. In doing this, I will not be getting extra fuel at WOT, and I can more easily tune the PE fuel table to achieve 12.8:1 WOT AFR.

Also, I could not disagree more with Diablo's statement concerning a steady LTFT. You do NOT go on the expressway and run at one speed in cruise control. This will put you into ONLY one fuel trim cell, how stupid! You want to drive around in a normal manner so as to "visit" as many FTCs as possible. It takes maybe anout 1 minute per cell to get completely updated on the LTFTs. After a reasonable drive, you will have several FTC that have been updated correctly for LTFT. Use an average of these LTFTs to guide your tuning efforts.

 

Personally, I think the theory has been backwards for years. I'd rather have slightly positive fuel trims rather than negative and this is why.

Basics:
Fuel trims represent the PCM's "learning" ability around fuel delivery.
Only positive fuel trims are added to WOT.

Assume two scenarios:
A) A car is tuned with +5% LTFT's across the board
B) A car is tuned with -5% LTFT’s across the board

Both car’s will cruise around with 14.7:1 AFR’s and have a nice steady 12.8:1 AFR at WOT the day they’re tuned.

Now, fast forward a month to a totally different climate day (barometric pressure, temperature, humidity are all different). The new environmental conditions call for fuel trims to increase roughly 4% so that car A sees +9% LTFT’s and car B sees -1%. That learned/additional fuel is then added to cruise and subsequently WOT to keep car A close to its originally tuned AFR. But, look at car B. Assuming the 4% increase in fueling requirements is present at WOT, nothing changes to its fuel delivery because WOT doesn’t see negative fuel trims. Therefore, it’s very possible that the car will be running a leaner 13.3:1 AFR.

Going the other direction, say climate changes require a -4% swing in fueling. Car A is still fine with LTFT’s at +1%. This reduction in LTFT’s carries over to WOT and a little fuel is now pulled to keep it close to its originally tuned AFR. Car B on the other hand now sits with –9% LTFT’s and the same amount of fuel delivery at WOT. This time, it could very well be running a richer 12.3:1 AFR.

So tell me, why does everyone say tune for negative fuel trims???

 

You make a very good agrument. I do not have a good response. I have always thought that WOT was not effected as much as the cruising around driving. And therefore, keeping LTFT correction out of WOT was best for keeping a more consistent AFR at WOT.