Fuel Trim and O2 Issues....I am really Stuck....

Jaguar XK8 1997... 4L V8

Here's the story so far....

Note my ECU OBD Tool is a cheap Chinese Bluetooth dongle and iPhone software. I have checked this on my Porsche and the readings are as I would expect and everything is working normally, so I am confident that I can read data with this tool.

Previously, I had a dodgy battery, this has now been changed.
Also developed a fault in the ECU, it was throwing out all sorts of random error codes relating to either bank or both at the same time - spent today making sure that was the case, pulling it out and replacing the faulty components.

ECU and battery now seem to be working well again and I am back to where I was a few days ago....

So, now what I have is that the car's fuel economy is down the pan, the exhaust stinks - you can't stand behind it.

Hooking up the ECU and reading the data reveals the following.

Engine Temp reporting as expected
Air Temp reporting as expected
Revs reporting well
Ignition advance retard as expected
MAF sensor putting out sensible style data
Throttle position data seems right

STFT - pegged at about 14.5 both banks
LTFT - pegged at about 19.5 both banks
NO ERROR CODES, NOTHING, NO P1000 etc.

O2 Sensors - here's where things are interesting.

Fuel system is reporting all well and in Closed loop. Hence why, I think, there is no error code....

I have checked that the right O2 sensor is in the right plug etc....so that's all been verified thanks to images and wiring diagrams from you guys already.


O2 ODB reading is 0 - doesn't change for either bank. Am I right in thinking this should be average 450mV, and swing from 0.2 to 0.7mV? That's my understanding of the operation.

Heater coils on the O2 sensors seem to be ok....

When I hook up a DVM to the sensor and check the output the O2 sensor is reading about 150mV - but the ODB is 0... This is live comparison as I have back probed the sensor for the volt meter.

When I rev the engine to about 2500 rpm, it sees a wide open throttle condition, goes back to open loop and then the fuel trims respond as expected. The O2 sensor readings also move, but only a little. Back to idle and the fuel trims climb again. O2 sensor reporting as 0.

I also cannot see any ODB sensor listed for the down stream sensors - is this normal? Am I not able to monitor those?

As I understand, if the O2 sensor is reading low, it should back off those fuel trims and richen the mixture to make them switch and then when they go high, lean them off again etc etc - closed loop control - which is what it is saying it is in... But I am not seeing it as such.

I realise I am looking for a Vac leak, on idle - but to be honest, that is one hell of a leak to max out the trims, right? and I have made an inspection of the normal pipes, some small leaks to address but nothing like this.

Also the O2 sensor data is not corresponding with the trims - right?

This is also common to both banks.


Any one any ideas please - this is driving me nuts.....

thanks,

Matt.

Do a water test to quickly see if it an obvious leak. Nothing pops up then a smoke test. Or whatever test you want to use to find a leak... propane, carb cleaner, brake cleaner. I had a problem once with the upstream O2 heater circuit. It actually caused it to go into "open" loop. Is your fuel pressure in spec.? upstream O2 sensors do have something to do with the trims too. When you rev your engine do the upstream O2's react properly or kinda hold within a small range? A dirty fuel filter and bad fuel pressure will increase trims as well.

Sorry man, just read more and what you're describing is bad upstream O2's sensor. But this probably isn't your only problem.

I basically have the same problem with trim values on my truck. When my upstream O2's are working right, it lowered some of my positive fuel trim significantly on my STFT's. But I still have positive LTFT's.

I haven't moved on to checking for leaks, since this seems to be a bigger problem at the moment.
And my real issue is why are the fuel trims being driven opposite to the O2 sensors?

Also, I can't see the down stream sensors on the PID listings....but I can see the connectors for them on the car and they are plugged in....Seems strange to me that all 4 sensors would be bad and both sides would be reading wrong by the same amount....

Fuel filter was replaced a little while back. Not sure about fuel pressure as I can't test it, but the ECU is controlling the pump properly, I can hear that. It runs up pressure quickly and then shuts off the pump...(before you start the car of course).

When I rev the engine the fuel trims start to behave - once I come out of closed loop...but I can't see much change in the o2s, it's a bit like everything is reversed.....

Maybe check the fuses for the O2 heaters? Cause it sounds as if they are not working. I don't think it should come out of close loop so easily. My truck also runs like crap in closed loop and better when it's in open loop. In open loop, on my truck, My STFT are holding steady at zero and my LTFT are pegged at 25%. It really sounds like the upstream O2's are no good to start with cause there should be more movement. It is odd that they all don't work. Check the fuses

You’re in global OBD which is good because it seems like your 02 being at zero may be pointing to an open. Use a test light on battery positive and use it to drive the 02 signal high At the sensor or even by unplugging the sensor and touching the test light to the connector going back to the engine computer. Your scanner will tell you if the engine computer is receiving the signal.

If I'm wrong on this sorry... but if the O2 sensor was Open would that be the same thing as disconnecting it and see if the scan readings change?

Potanist wrote: If I'm wrong on this sorry... but if the O2 sensor was Open would that be the same thing as disconnecting it and see if the scan readings change?

He stated the 02 was reading “0”. This indicates it may be (most likely is) an open an open on the signal wire. It is the same as it being disconnected. However, using a test light to drive the signal wire is the key here. It will verify the integrity of the connector on up.

An 02 should really oscillate anywhere from 100mv to 900mv. Anything under 100mv can set OL FAULT.

Got'cha man... I only blurted that out because I just ran into that myself on my problem truck. It was more of a check on what I did...