1997 Xj6 VDP
having a issue with STFT on both banks constantly resetting to zero percent while driving.
Bank one is usually -24% then will reset to zero and back to negatives
bank two is around +15% and will reset to zero and then back to positive.
Compression good, injectors good, coils are new and oxygen sensors are new.
cats are hollowed and I smoke tested the engine and no vacuum leaks
Tons of other new parts and sensors.

 

The numbers would suggest that O2 sensor connectors are reversed. Swap them and see what happens.

 

The Neg 24 on one bank may be caused by exhaust leaks that becomes not effecting at higher load / RPM

At low RPMs the exhaust pulses or the space between them draw in fresh O2 rich air biasing the O2 sensors to command increasing fuel to consume the O2 vs what is desired in the exhaust

The leak can come from cracked manifold casting , loose downward pointing stud nuts or the manifold donut gaskets eroded and leaking

If it was and can still be a vacuum leak the leak at lower RPMs can be compensated back to normal as the vacuum supply is increasing

You can also have a issue with the engine coolant sensor not allowing the ECU to go into closed loop mode where it begins to use the O2 sensors ( 88 C )

The O2 sensors have a heating element in each that must achieve a certain temp to read correctly , nature of the beast

There is 1 fuse for all O2 sensors and is a easy meter check through the sensor connector to verify the heaters are good

Pics coming

Easiest thing to do first is to ensure the O2 sensors are helped out

Clean the sensor connectors keeping track and aware the connectors can be swapped

Clean the sensor wire shield grounds that protect the return signal

The 2 ground studs are on the rear firewall left and right car side below the larger battery cable terminal post

Ensure the dedicated ground strap is on the ECU case or shell attached to the lower mounting bolt

 

Monitoring voltage drop while unpluging both individual oxygen sensors indicate that they are both in the correct position.

 

All grounds and wiring to ECU and straps are in good condition.
Fuel trim for bank one still doesn't breach -10% under load. Fuel Injectors and pressure regulators are all good.
Oxygen sensors seem to be maintaining correct voltage and have been replaced and Catalysts hollowed out.
However, there is an exhaust leak "sound" from the bank two flange yet running my hand over it and the exhaust manifold log, I can't feel any leak. I've pulled the cats off and replaced the flange donuts and haven't impacted anything.

 

In the all encompassing theory of Parker :

Main large engine ground strap cleaned at the body frame and lower starter mount bolt ?

10 and 13 MM

 

I'll make sure to check it out, thank you.

 

The line of thinking is the ECU and sensors run on DC power and a AC signal can be induced by arcing coils

The valve cover should be electrically isolated by design but ..........

Both a DC and AC signal can be present at the same time in a wire

Turning to a AM radio station you may hear them as a change in engine RPMs

The diode package inside the alternator and noise suppressor external to the alternator can error to give you a AC signal

 

There isn't interference (noticeably) from the Am/Fm radio.
however, apon visial inspections of all frame and engine ground straps, nothing visibly looks rusted or corroded and all are in good condition.
all voltages and readings via code reader look nominal
And connections along the ECU wire loom and ECU connectors are clean and in good standing condition.

Should I get a noid light and check voltages to the injectors?

 

The fuel injectors have power " sitting " on them from the large ECU control relay # 5 right

The ECU provides a timed sequenced ground to open them

From the print below the first power to the ECU is from the right engine bay fuse box fuse X

The second power is controlled by the ECU and brings power back into the ECU along with power for the injectors and some of your engine sensors

You can have compromised power contacts in the ECU controlled relay and can be swapped with a like # relay like a headlight

Pic coming

 

Your diagram and responses have been extremely helpful

Where is the injector relay and headlight relay? Are these the ones located in a row on the front passenger side engine bay near the Washer fluid reservoir? (US model)

 

I have this pic marked for someone else's spot of stumble under the bonnet.

You can use a Fog or AC relay as long as the same part # stamped

 

Relay swapped, no change in operation.

 

OK , but was a easy try

Thinking

I will be out doing errands until later tonight

 

I appreciate your knowledge and time.

 

I don't think it's possible to test that way.

The X300 has a bizarre design. The O2 sensor connectors are identical for Banks 1 & 2 so they can be plugged in any way around.

To get around this there is a process in the Jaguar factory diagnostics system called "O2 Sensor Orientation". This process runs the engine for a few minutes and swaps the signals internally while monitoring the O2 sensor responses. It works out which Sensor (bank 1 or 2) is connected to which pins on the ECU, and then saves that configuration, until the next time the process is run.

So it's not possible to know by checking voltages (or anything else) between ECU pins and the sensors, as internally in the ECU software they may be reversed. The only way to confirm (AFAIK) is to swap them and see if it changes things (it's free and straight forward) or run the "O2 Sensor Orientation" process from Jaguar IDS.


Apologies if you already know all of that. If so, it may be of use to someone else in the future.

 

I'm not sure I see a problem with the STFT readings, they are supposed to be moving all the time. The problem I see is the LTFT at -10.2%. Are both LTFT banks the same?

If so, here's step one.

Run the car. Drive it around and get it all hot and stuff.

Then park it, and leave the engine running. Go over to the engine, reach down, being careful not to burn yourself, and unplug the TPS. The engine will stumble, but keep running. Count to 10. Then plug it back in.

Turn the car off, and wait until the next time you need it. Start monitoring if the LTFT starts to drop. Usually takes about a week, or 200 miles?

If not, then I'll start asking how old your 02 sensors are.

If the LTFT on the other bank is not -10.2% let me know.

 

Both banks aren't at -10% fluctuation.
bank two is around 24+ and bank one -10 to -12 then both banks seemingly reset to zero randomly then slowly go back to the original fuel trims.
it's very frustrating

bank one oxygen sensor is new

 

Are you saying that the LTFT is fluctuating. This is different than the STFT. That is bizarrely unusual, since the LTFT is just a running average of the STFT intervals.

Your first picture is showing the LTFT at the B Bank at -10.2%. I haven't seen a screen shot of the A Bank.
That same screenshot is showing the B Bank STFT as -25% and, I assume, is fluctuating, which is normal.

Have you unplugged/replugged the TPS with the hot engine running?

 

I'm sorry, it's just bank ones STFT that's fluctuating in the negatives, of course. (LTFT for bank one is negative as well)
This goes for bank two but the STFT is extremely positive.

I'll have to unplug the tps when I get home from work.