I've spent a good amount of time and money (oddly enough much more time than money) getting my 96 XJS back into nominal running condition. I've had heavy battles with fuel trims. Always had the ECU read lean conditions until just recently. Starting at an LTFT in the 10 area, I managed to get my car down below 4 on LTFT which was a slow but steady process....and I was satisfied.

Most recently I had my injectors cleaned using SD Faircloth and I figured that would take me all the way home....and when I put in the cleaned injectors, I also replaced spark plugs....and then my LTFT started creeping up. My ride was starting to stall out at stop lights and making slow city turns. My LTFT was 10.2 just this morning. I felt demoralized.

I realized that my transmission was burnt. I had it flushed. The drive got better. Not immediately, but a strange thing happened as I drove home today, I checked my LTFT, which only 8 hours earlier was reading at 10.2, was now down to 1.6. Seriously.

WTF?

How in the world did the car make such a dramatic return to zero? Is there any way that the change in my transmission fluid could affect my fuel trims, that drastically? The stalling is now gone. It took the day, but I don't sense any more hesitation at slow speeds. My transmission fluid was pretty bad, I must admit. I don't know how I let it get to this point.

So, is there any tie between fuel trims and transmission fluid?

 

I'm just making this up but...LTFT is Long Term Fuel Trim, an average of many STFT readings. So... injectors solved the issue and the old bad readings finally fell out of the average.

 

I have heard where one of our club members had a rebuilt trans installed into his XJS and the fuel economy improved, so maybe the trans fluid had a bearing on you LTFT bit of a stretch but who knows?