My 2.5 runs fine on the level at motorway speeds on bank 1, but throws random misfire codes on bank 2 when I go uphill and increase throttle to maintain the current speed, the CEL starts to flash but stops if I ease off the throttle. I have replaced plugs twice and coils 3 times on this bank, all O rings on upper and lower manifolds and VVT and throttle body, also checked all coil plugs for good earth back to battery negative terminal. I think I can rule out airleaks and sensors that are common to both banks, also fuel pressure is spot on, as bank 1 is fine, so trying to work out why the ECU thinks there is a rich condition just on bank 2, both oxygen sensors have been replaced but still getting negative ST fuel trim on sensor 1 when I load the engine. The fuel trims are all in spec at idle, but at cruise bank 2 ST reads - 10 to 15 and LT - 3 to 4 so the ECU is seeing a rich condition and cutting the fuel but I am running out of ideas as to what is causing it, an obvious source would be a leaking injector but the plugs all look the same pale brown colour, I have driven for miles with my Icarsoft plugged in reading off data but having spent £500 on parts so far am I am getting tired of throwing parts at it thinking this must cure it only for it to be exactly the same, so I welcome any input, I have trawled lots of threads and done the things others have suggested but now really need to get to the bottom of this, I guess it will be something really simple once I figure it out! If its an injector I don’t know which of the 3 it is so will have to replace all 3 on that bank, anyone know where to get decent injectors as my searches just pull up lots of diesel injectors or dodgy looking ones on ebay.
Please help before the missus gives me grief for spending so much time under the bonnet!
Per bank failures that manifest under load can be a clogged cat on one side.
Injectors you can switch to the other bank and see if the problem persists but knock on the cat first and if it sounds alright you should still suspect it.
Thanks for reply, I have wondered about the cat but would the engine rev easily to 4000 in low gears and do 70mph on the motorway if one cat was blocked. Also round town the fuel trims are almost identical on both banks, just when I go faster the trims on bank 2 are in the low negative figures. I took the plugs out again today and they are white so def running lean. Anyone know the Denso part number for the injectors or good equivalent and where to get a catalyser if needed without breaking the bank? I will drop into my friendly MOT centre next week and just put it o the gas analyser to check the function of the cats.
Yes, that can happen.
No, it is unlikely that all cylinders one one bank run lean because of the injectors themselves. Your money though 
The cats may work but not pass enough exhaust and then the lambda values are off, no oxygen in the exhaust, to compensate it goes leaner and leaner, etc.
It’s just a theory but it was a good one before.
Ok I will get the cat checked on a gas analyser before I go further. I just wondered if one injector was leaking or delivering too much fuel as to whether that would make the ECU cut back on the fuel on all 3 injectors on that bank giving me the white sparkplugs. This is my first Jag so still getting to grips with how they designed it to work, I have and still run Saabs for 40 yrs.
It makes sense it is a problem with the cat on that bank. When it affects all three cylinders, it must be something common to all three and that is only the cat. I had an 02 sedan with the same problem. My mechanic figured it was the cat breaking down and plugging up. The #2 bank is next to the fire wall and is susceptible to heat. It is a devil to replace.
Yes its looking like the cat is the root of my problems, however bank 2 on the X type is at the front next to the radiator so easier to get to, and this is the bank that has the plugs that are visibly running lean.
I just wondered if one injector was leaking or delivering too much fuel
Oh right, that is a good point. But then you should see it on the sparking plug.
Got a 4 gas analyser on the car today and the gas readings were spot on so no problem with the cat and I don’t believe its blocked as the car will cruise at 70+ mph no problem. Have replaced the fuel filter but no change so have now ordered a new MAF.
You’re likely on the wrong track. Your money.
If you unplug the maf does the car run much better?
I had a BMW V8 that would choke up and kill cylinders when accelerated past 80 mph or so. The cat still worked but it wasn’t flowing enough exhaust. The shop was helpless(!) and I did my best diagnosis ever. It involved kicking the exhaust after a test run and two days of googling to confirm what I suspected.
The amount of exhaust is directly related to the power generated. Cruising needs next to no power, accelerating does and it will need a few seconds at full load so first or second gear won’t do it but when you accelerate hard in top the exhaust fills and the cylinders misfire/ can’t breathe.
By the way the MAF works both banks right
I agree with Dave. It is the cat that is causing the problem. The MAF would affect both banks. There is no way for it to just affect one bank. The only thing that is common and exclusive to the one bank is the cat. Anything else would either affect both banks (everything upstream of the inlet valves) or not all the cylinders on one bank (injectors, coils, plugs, ect).
And besides it’s load related. I can always be wrong… but I’m 98% sure it’s the exhaust.
Hello Steve - read thru your list of items checked and wonder if this might help - not familiar with the wire harness configuration lengths for the O2 sensor plugs, but if possible, can you swap the harness connectors around, so the plug for bank 1 is on bank 2 and vice-versa - just thinking you may have a faulty wire or plug connector in the O2 harness path.
Isn’t obd2 able to diagnose itself in that regard? And while the idea is good, how can it only happen under load?
I figured that since the O2 sensors were changed (along with all the other items he mentioned) that there might be the possibility of a wire harness, or connector problem - not sure if the OBD2 would detect such a condition - as to only happening under load, I was of the thought that, with the engine moving on the mounts during torque application, and then returning to previous position when he backs off the throttle, that the harness may have a wire condition that is causing this problem.
Thanks for the replies, Unfortunately the the O2 sensor wires are not long enough to allow swapping over. Yes the MAF feeds all 6 cylinders but its a cheap swap compared to the cat. The problem with living in Norway is that there are no spares suppliers other than a dealer where our prices are 2-3 higher than the rest if the planet! So I have to order parts from the UK, and am trying to find a front cat for a decent price. Still haven’t figured out how a partially blocked cat would give negative fuel trims, I can see it causing excess back pressure but that would bog the engine down. If I can get one for a good price, but I need to be sure its needed before I risk £6-700.
I really appreciate all the input from this forum , I have been reading the official Jag workshop manual and it lists this exact fault, lean mixture on bank 2, and I have done most of the things suggested, it does list the sensors that are common to both banks as possible causes, but I guessI am being steered toward replacing the cat ££$$!
It feels like the engine is running out of breath and the unevenness isn’t easy to notice due to the inertia of the engine.
The back pressure could kind of eat up all the remaining oxygen so the sensor thinks it’s way rich. The second sensor only checks if the cat works by comparing values to the first - OBD2 requires that. By leaning out the engine will start to misfire. You could try to hook up a vacuum gauge, that may show something?
If parts are more expensive (I’ve been to Norway…) all the more reason not to throw parts at it. Disconnect the maf, see if it works better on standard values.
I replaced the MAF 2 days ago and its definitely better than it was, I can drive for miles without the CEL lighting up and its gone from random misfire on 2,4,6 to just 2,4 and today just 2.
The CEL doesn’t flash going up hill like before, its does it when I kickdown from 5th to 4th and the revs are around 3000, it goes out when it shifts to 5th or I back off. It cruises at 80mph no trouble and the fuel trims for both banks are nearly identical. Short term trims go positive and negative about the same values and the long term bank 1 is about +8 and on bank 2 about - 2 to 3 and all read very close to zero at idle. Is it possible that it is recalibrating itself in the 60 or so miles I have done since the new MAF. The huge negative trims I had on bank 2 have reduced to nearly normal and the readings seem to update quicker than bank 1 which has only had a new upper oxygen sensor about 2 yrs ago and the lower one is possibly original. The only things left are a leaking injector or the catalyser both of which I am having trouble sourcing if needed.
Yes that learning in thing is possible, at least on other cars, and the prime suspect is still the cat because what you describe is still exactly what happens.