https://www.jaguarforums.com/forum/xj-x308-xj8-xjr-27/fuel-trims-help-110289/

Some of you may remember I've been having lots of problems with high fuel trims and emissions, above is my previous post if any-one wants to look back.

In brief the bulk of it was both banks both LTFT and STFT where very high, making no difference at idle or at 2500k RPM (engine always hot when testing). LTFT's where 14.8 both banks and STFT's where 19.0 both banks

Firstly I replaced the passenger side (UK) bank 2 upstream/upper lambda sensor, as a local garage told me it wasn't working.

Unfortunately this made no difference, so I took the plunge and booked it in with a popular Jaguar specialist who is far from cheap.

This guy went to town, found no fault codes with his special "jaguar ECU testing computer", checked both lambdas where plugged in correctly and I think he said they were working. Also he performed a smoke test to see if there were any induction leaks, he found a couple of very smalls ones which he has since fixed but said they weren't the cause of my problems and didn't fix the fuel trims in return.

I picked the car up and he was scratching his head, he didn't even charge me, the only thing left he could think of was a faulty MAF sensor which he reckoned wouldn't always throw a code up. He advised me to replace so I did.

Fitted a second hand MAF sensor today I got some results although not that they make much sense.

Bank 2 now works perfectly, both LTFT and STFT stick around "0" moving -/+ no more than 3 either way on both trims. However Bank 1 drivers side (UK) is still reading high LTFT 14.8 and STFT 19.0.

I also noticed bank 2 O2 sensor reading 1.0 volts, whereas bank 1 02 sensor shows 0.1 volts - I double checked with the Jag man and he's pretty sure both lambda/02 sensors are ok.

I replaced bank 1 02 sensor around 4/5 years ago, could be it this is my remaining problem? Where both my MAF and 02 sensors faulty and replacing the MAF has got me half way there? Obviously if it was just the MAF sensor it would have completely fixed my issues, so there must be one last problem sitting somewhere...bank 1 suffered from HGF a year ago, O2 sensor could have got wet, or running the car with a faulty MAF for some amount of time could have killed the older O2 sensor through black carbon deposits....just thoughts crossing my mind.

Opinions most welcome....in other words help!!

 

I believe you are squarely on the right track!
One thing, try a hard boot reset. Also, your readings are supicious in that a correctly working sensor, even a wideband ensor, should vary in voltage. what is the range and period of your variation?

 

Hard reset done before posting my findings. The volts seem to be stuck not moving at all, I suppose I could swap over the O2 sensors and see if the results swap banks then I would know.

 

Not really, since if you swap the sensors, the control will be adjusting the "wrong" bank. It will then probably peg. One trick is to alternately remove oil stick (make lean), then introduce unlit propane from a torch (US use, not a flashlight!) into the intake air horn and watch the sensors.

 

Unplug and swap then replug in? Surely that wont cause disruption?

 

Naw, it won't cause any problem, but I don't think it will lead to any information, either, since normally it will peg sensors high or low, randomly as the wrong bank responds to the wrong sensor. IE. if you could move your thermostat to your neighbors house, and move his to yours, still hooked up to the original unit, set each to the exact temperature currently in each house, what would you expect the two AC s(heaters right now, I guess) to do an hour later? Undefined!

 

Sorry mate, getting a bit confused here, if bank1 O2 sensor is faulty but bank 2 sensor is ok.... unplugging and moving bank1 sensor to bank 2 would make bank 2 faulty would it not?

 

This has happened before to other guys on here. The tranny gets swapped or something similar and the O2 sensors get swapped by accident. What happens is the fuel trims will go nuts because the banks are fueled separately by the ECU. If bank 1 says it's lean (O2 sensor) the ECU sends more fuel (injector pulse time) to bank 2 so bank 2 gets rich, but it's sensor tells ECU to lean it up (sensors swapped), so ECU leans bank 1. Anyway, I'm saying the same things as Sparks. Might be worth a try, swap the sensors and do a hard reset and see what happens. Is it possible that when they did the HG they removed the O2 sensors ? By swapping the sensors, I meant the sensors and their harnesses.

 

I think you are right L80ous.
To go back to the thermostats if you had a faulty thermostat and switched it with your neighbours and the fault switched to his house you would know the stat was faulty.

 

The wiring harnesses have been checked and are sat at their correct banks, the jag man did this check when it was in last week being inspected.

Will pull the plugs this weekend and see if the lean bank swaps sides moving along with the sensor (Will leave harnesses as they are).

 

If you mean move the sensor and where it is plugged in, then YES, it is a valid test. If you just move where it plugs in (or just where the sensor is located), it tells you nothing! I read your original post as if your plan was to only swap the wires. Sorry!

 

:-) No problem buddy, thanks for your help, will report back on Saturday, hopefully its just the O2 sensor now, we'll soon find out.

 

New MAF and O2's and let's see the numbers. I've learnt to stock both as spares, with these car's it can happen anytime. I've a stash...hope it cures it once and for all.

 

Getting sick of this now, popped out to get some fuel, on the way there fuel trims working ok on one bank as before, on the way back now both way out again!!!

Havent even had chance to swap the O2 sensors over yet so must be something else.

Lifting the dipstick out makes no difference to the trim nor does holding down the fuel cap tight.

 

OK, so are they reading high or low LTFT?

 

High both banks

 

Should a 98 xjr have 2 O2 sensors or 4? Cause I only have 2 (upstream/precats).

Wondering of 2 have been disconnected when having the exhaust work done.

What else could cause one bank to be correct on minute and high FT's the next?

 

Air leak, bad MAF, bad Lambdas, serious mistime (failure to burn O2) and al least half a dozen others.
And... since I assume "lincolnshire" is in the UK, Yes, the early MY98 x-308s apparently had two lambdas on your side of the pond. At least that is what I am lead to believe.

 

yep, our old AJ 26 supercharged engines only have the 2 pre cat sensors. so in theory ( and for off road use only ) we don't need to have cats fitted for these engines to run properly. don't know what the emissions would be like without them? but on a race track I don't think it matters.

 

Going to take her to the main jaguar dealers I think as the specialists didn't seem to have much luck finding my problem.

Just out of interest, will I be running a few more HP's while its lean? It seems to be running well at the moment when the foots hard down, turned a Boxster S 2007-Reg (295BHP 0-60 in 4.9) into dust tonight off the mark and down the motorway....