Transmission Fluid and Fuel Trims?

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?

Valde, No doubt two unrelated issues. Just an anomaly that both occurred around the same time. I know you will recall, there was an issue with one of your injectors not operating…resolved about three weeks ago. You replaced it with an operating unit. LTFT takes a number of engine runs for the ECU to monitor STFT and adjust LTFT. More than likely, it took that period of time before the LTFT returned to a normal reading. How long…depends on how often you drive the vehicle, and how far per trip. So, injectors good, new plugs, and clean tranny oil. Car runs well. All is good. Pat yourself on the back…grab a cold beer and go cut the grass. SD

Veekay - What are you using to read the fuel trims?

OBD Fusion is the program I use that plugs it to the OBD2 port.

Thanks - I have Dash Command which has a gauge to show trims but I like the graph Fusion gives you. Think I’ll check it out.

Finally got to the bottom of this!

After driving for too long of a time with the LTFT at 10.something, it was the TPS that was the root of the problem!

The car drove fine, it just had really bad fuel mileage. No check engine lights, no smoke, not really any other symptom, other than poor fuel economy.

I replaced the TPS, with a brand new one, a week ago and my LTFT is at 0.0! I had a spare old TPS that I inherited as part of a used bin of XJS parts and it did not help. I will take this TPS apart and see whats doing.

i was not happy about my V12 MPG!
so i replaced ECU with adjustable ECU,SDS, now i can adjust fuel trim while driving, monitored with A/F gage!center%20consol%20J%20-bond%20system%20002 .
i have when driving a steady 70mph achieved 16/18mpg, its a PreHE and they were noted for around 12/13mpg average!
ron