On my XJR the LTFT for bank 1 & 2 are both identical at around 20. Both get equally lower as RPM increases.

This made me suspect the MAF sensor, however the car has a custom huge exhaust fitted by the previous owner, plus lots of other work. Could it be that these numbers are actually normal seeing as the car is using more air? Supercharger has been milled out pulleys changed etc. (Therefore more fuel is required).

Curious.

 

What are the STFTs at the same time? You're checking in park at idle and 2k rpm?

Increased airflow would require more fuel and the PCM would do that by increasing the fuel trims in the positive direction. The problem is once it gets to 25% it sets a too lean code.

 

Just checked and it's quite strange. Banks 1 & 2 LTFT and STFT are almost identical.

At idle, both LTFT are about 20% and both STFT are about 0%

At 2500RPM both LTFT are near 0% but both STFT go up to about 20%

This makes no sense at all. The STFT are basically compensating for the LTFT.

Yes, I do get a P0171 code and Reduced Engine Performance.

What could be going on?

Thanks in advance.

 

Could it be a failing MAF?

 

snapshot will usually tell the story. the first one to cross 20 or so is the one that trips the code so sometimes you only get a 171/174 and not both.

check the maf flow rate against the calibrated value. a maf that underestimates flow at idle will grossly underestimate under load so it’s probably not your problem.

fuel pressure at idle and under load

smoke check precat exhaust leak check, these are your biggest suspects

you can reset trims back to zero by disconnecting the battery and touching the terminals

 

Thanks for the info!

Previous owner fitted a massive Akrapovic exhaust system, there's no leaks in it. The supercharger has been milled out and the pulleys upgraded, plus a lot of other work has been done.

Could that be the issue? I mean as a lot more air is flowing through the car, it therefore needs more fuel, so could the fuel trim readings actually be correct? Or am i thinking about this wrong?

 

no it more or less adapts itself to power adders

you have an issue at idle and it needs to be figured out. reset your trims and start diagnosing

 

I'll do that in the morning, thanks for the info!

 

I don't have specific familiarity with Jaguar ECU tuning but if it's like other systems, there's a lookup table that the ECU checks for the fueling to use at a given airflow (with other inputs playing a role). While your add-ons may increase airflow, as long as the MAF is not maxed out (e.g., once it reads its highest capable voltage) the ECU should just compensate for the additional airflow by selecting the appropriate fueling from the lookup table.

 

Thanks, that's good to know!

I don't have a full list of everything that was done, so was expecting a bit of a nightmare, but you've put my mind at ease somewhat.

Just need to find the bloody leaks now!

Here's a few screenshots from Torque.


Idle RPM

2500 RPM

 

If you can get an inexpensive smoke machine, they can speed your leak finding greatly. I had similar issues in my LR3 (naturally aspired Jaguar AJV8 engine) and the smoke machine found multiple cracks in the plastic intake components that were the source of my issues.

For the XJR, I would use a smoke machine this way (engine not running of course):
1. remove the MAF sensor
2. cover the air filter in plastic wrap and reseal the airbox
3. put the smoke source plug into the hole left by the MAF sensor

Some smoke will leak out of the plastic-wrapped filter, but this approach worked for me with the Land Rover.

 

Thanks! That's the plan, just need to figure out how to make a smoke machine from common household items now haha. They cost around 200 quid so buying one is a last resort, and I don't want to take the car to a mechanic if possible.

I saw someone made one out of a soldering iron in a jar with some baby oil or something...

 

Spray some brake cleaner or carb spray on the throttle body where the power steering vacuum line comes in if the idle changes and the fuel trims level that's your leak causing your lean codes...