Hi all - I'll start at the beginning!

I bought an '02 STR in Feb this year, it's now covered 102k. Drove it home (100+ miles or so), did a few local journeys all was well, no lights on. After a few weeks, I took it up to the north coast of Scotland and on the way there (so probably covered around 500 miles), after giving it some up a slip road, it threw up the EML and went into restricted performance. Upon return, I had a local jag specialist scan it (I didn't have a scanner at the time), they said it was a code relating to lean running meaning it had an air leak. They smoke tested the inlet and said they found a leak at the EGR. To cut a convoluted story short, they ended up dropping the engine as the EGR bolts were seized and they couldn't get enough access to get them off and they replaced the 3 EGR gaskets (this took weeks).

I got the car back, did a couple of short journeys and it was fine. First longer journey I did, the light came back on and back in restricted performance. For various reasons I don't have much faith in the specialist above, so I took it somewhere else, who again smoke tested the inlet, saying they can see a leak somewhere, but they don't want to get involved beyond this as they don't want to remove the supercharger or start chasing the leak.

As it was looking more like I'm going to have sort this myself, I bought a code reader. It's showing P0174 'system too lean bank 2'. I've been doing some googling on fuel trims etc and these are the results:

At idle:
STFT Bank1: 0.8%
LTFT Bank1: 0%

STFT Bank2: 25.0%
LTFT Bank2: 19.5%

At 2500rpm:
STFT Bank1: 15.6%
LTFT Bank1: 3.9%

STFT Bank2: 23.4%
LTFT Bank2: 19.5%

It's fair to say it's only bank 2 that's affected. I don't really understand why this is, as surely an air leak from the inlet tract would affect both banks? Are there any vac lines etc that only affect or run into/from bank 2?

I've also read that typically, with an air leak, LTFT should reduce at higher rpm to maintain the AFR. This doesn't seem to be happening, so could this indicate other issues? It never deviates from 19.5% regardless of throttle position.

It's likely I'm going to have to buy a smoke tester and investigate further, but before I do, I wondered if it could be narrowed down any from the above info? Something else to take into consideration is, after reading through the paperwork, the car seems to have had a history of the EML being lit (although I cannot find reference as to why), however I have found a service invoice from 1000 miles ago in 2021 where all 4 lambda sensors were replaced. As you can probably tell, I'm no mechanic but have a basic understanding so any help appreciated

 

I wonder why they replaced all 4 O2 sensors!! Let's hope they used proper ones...

 

Careful, as this particular code isn't always caused by an air leak. Most scanners label it as such, as an air leak is indeed a common fault, but there are several other possibilities to consider.

Here's the official Jaguar document for fault codes:

http://www.jagrepair.com/images/Auto...TC_EOBD_R2.pdf


For P0174, note Jaguar's definition of "Bank 2 combustion too lean". Here is the shopping list of possible causes:

Engine misfire
Air intake leak between MAF Sensor and cylinder head
Fuel filter / system restriction
Fuel injector restriction
IP Sensor fault (low fuel pressure)
Low fuel pump output
HO2 Sensor(s) (2/1, 2/2) harness wiring condition fault
EFT Sensor fault (low fuel temperature)
MAF Sensor fault (low intake air flow)
Exhaust leak (before catalyst)
ECM receiving incorrect signal from one or more of the following components:
ECT Sensor, MAF Sensor, IAT Sensor, IP Sensor, EFT Sensor, TP Sensor

 

Thanks, quite a bit to go at there. I think I'm going to try swapping the HO2 sensors from bank to bank and see if anything changes first

 

Right, I've been having real 'fun' with this.

In a bit of a curveball, just over a week ago, the car appeared to have a bit of an electrical meltdown upon startup. I checked the battery and it was a bit low, so ordered a replacement and fitted it, which didn't cure the fault. After a lot of messing, I eventually found out the TPS plug had been waterlogged after a heavy rain shower. Getting this dry again got it starting up normally without a christmas tree display on the dash and it would now rev up normally again.

Since the above, I have not had a P0174 code, but instead had a P1647 code relating to bank2 upper O2 sensor. Looking through the history of this car, it had all four O2 sensors replaced about 18 months ago and appears to have had a persistent EML issue, although there is no reference as to why. When I checked the upper O2 sensors, they did not match. Bank 2 sensor was a Denso with the wrong part number for my model year. Bank 1 was a Blueprint sensor with the correct part number. To double check it was the sensor at fault, I swapped the upper O2 sensors over and the fault moved with it, so I was now getting P1646. Being fairly certain the sensor was the issue, I ordered a matching Blueprint O2 sensor to the one that was working.

It's arrived today and I've just fitted it. The P1646 code did not clear (the new sensor was fitted to bank 1). I swapped the sensors over again and the fault has again moved to bank 2 and given a P1647 fault. This would indicate the new sensor is faulty but I find that really hard to believe (although I've ordered ANOTHER - Denso with the correct part number this time to be sure). The fact that the car has had non-matching sensors fitted and fitted recently makes me think there may be another underlying issue. I've seen that the jag repair manual suggests some pinpoint wiring tests for the HO2 sensor wiring harness but I'm at a loss as to how this could be at fault when the fault moves as the sensors are swapped. Any ideas of what to check (I am an electrical novice and not particularly confident testing because I don't know what I'm looking for but I don't have much choice).

 

Good find on the TPS connector.

With the new battery, I'd suggest hooking it up to a charger. New batteries typically are not fully charged at the time of sale. Charge it up overnight to be sure.


Wait until you've tried the newest sensor before going into FullPanicMode. Your troubleshooting seems spot on.

​​​​​​​I wish I had kept track of how many times I've had a new part bad from stock. Kudos for ordering a different brand, too. Years ago with a different car, I had the misfortune to install a bad distributor cap as part of a tune-up. I had changed a bunch of parts at the time, so it was quite the chore to figure out which new part was bad. I finally deduced the distributor cap was bad and went back to the same store and got another defective one from the same defective batch. Talk about a head scratcher! Two identical bad parts in a row. Took quite a while to figure out that one. Since then, if I suspect a new part is bad, I am very careful to get a replacement from a different source.

 

OK, it seems not for the first time, I've been a bit dense!!

I've just been messing with the three O2 sensors I have now amassed. Two of them show continuity on their heater circuit, one does not (the Blueprint one that was already fitted to the car, that the car seems to be telling me is its one good sensor). So I ignore what the car is telling me and fit the two that are not reading open circuit.

Boom...EML off on its own, closed loop fuelling resumed, fuel trims looking normal!?

I think I've been reading bank1 and bank2 wrong. As I'm looking at the engine, stood at the front of the car, which is which? I've seen the diagram of the cylinder numbering in the jag manual with an arrow at the top, is this arrow pointing as if stood at the front of the car?

I'm still not certain I'm out of the woods yet, as it's still up on stands (can't be bothered dragging the jack out the shed at the bottom of the garden), so I haven't driven it, but certainly at idle, things seem normal

 

Standing in front of car looking back (i.e. opposite of the way car directions are normally given), bank 1 is on the left.

 

Thanks, I had been reading it wrong!

I've had it out for a drive twice today. Probably about an hour each time. On the initial drive, I had a look at the scan tool and was getting P1000, so checked which drive cycles were not completed. The heated O2 sensor monitor cycle had not completed and comprehensive component monitor had not completed, all others were showing as ok. I had a look at the fuel trims and they all looked good (around +/-5% at idle). I tried carrying out the O2 sensor drive cycle as detailed in the workshop manual but probably didn't achieve as there was too much traffic and not enough space. I checked everything again when home and the outstanding drive cycles still hadn't completed and fuel trims at idle and when held at fast idle were still good.

I've just been out in it again for another go. Had a few goes at the HO2 drive cycle (drive in 3rd between 3-4k rpm then coast to a stop x 2, then idle for 11 mins). It still did not complete, so I took it for quite a hard drive. When I got home I had another look at the fuel trims. Now, at idle, bank 2 STFT was up around 15% and LTFT around 8%. I didn't like the look of that. I turned the car off, let it rest for 30s or so, then went back out for a gentle drive around the block, with live data turned on. I could see the bank 2 STFT up at 25% most of the time and LTFT hitting 19%. I got back, let it idle a bit and these were the readings:

At idle:

Bank 1:
STFT 0.8%
LTFT 5.5%

Bank 2:
STFT 25%
LTFT 18%

At 2500rpm:

Bank 1:
STFT 9.4%
LTFT 0%

Bank 2:
STFT 25%
LTFT 5.5%

I checked the codes again and sure enough I now have P0174 pending, as well as P1000. I can see that the fuel trims indicate an air leak on bank 2, with the long term trims decreasing as more air is consumed at higher revs, but there are some things I'm not understanding:

Why would it take 100 or so miles and 2hrs driving for this to show up? Surely if there is an air leak it would show immediately, yet the trims were fine at first, then slowly went out of range on one bank?

When P0174 appeared previously, the LTFT was staying static at 19%, but this O2 sensor then failed very shortly after and I had P1647 for that instead, indicating that it was already failing, hence odd LTFT readings. The car seems to have a bit of a history of eating O2 sensors - is there anything that could be making the sensor read wrong and causing it to then fail (although the LTFT on this new sensor is now moving around so appears to be acting as would be expected)?

In order to complete the O2 monitor cycle, is it looking for the trims to stay in range for a certain time under specific conditions and this is why it would not complete in this case?

I don't really understand why a new O2 sensor makes it fine for a short time, then the trims creep up again

 

As it's just one bank you can rule out things common to both.

At higher revs the LTFT may still be learning, judging by STFT.

So, you may or may not have an air leak. Don't be sure either way as yet.

The DTC PDF shows quite a list of causes but most would also flag P0171 (or have readings near to flagging).

I guess you could have a damaged cat or an exhaust leak or ... (er, not sure).

 

The reason you don’t see the P0174 show up immediately is because it takes a specific set of conditions to be set before it will put the code out and it needs to do that on 2 Consecutive Drive cycles so if you don’t meet all those entrance criteria you won’t get the code because it won’t run the test or it won’t meet all the entrance criteria on 2 Consecutive Drive cycles

 

Thanks, I understand why it didn’t throw the code immediately. What I’m struggling with is why it takes 100 or so miles of driving each time after a hard reset for these conditions to appear (I.e high trims bank 2) then once appeared they stay that way? Whatever is causing the problem should either be there all the time or intermittent depending on conditions, but what is happening doesn’t seem to follow that.

 

For your car obviously I don't know, but maybe something like parts of a damaged cat don't always move around straight away, or an exhaust leak doesn't always open up, or not enough driving under the right conditions (Load, RPM, etc) occurred to train the trims.

You can clear the trims by disconnecting the battery - but it ALSO clears all the monitors (often needed for smog and until they set many codes can't flag).

That last part might be relevant as to why the P0174 can be slow to appear - many codes can't be flagged until enough monitors have set (essentially because the PCM isn't sure what is wrong i.e. which of several or many codes to flag).

(And as has been posted, many codes need to occur in quite similar circumstances at least twice (see the DTC PDF) to be flagged. Part of the OBD II rules and for good reason. You may see them as pending beforehand.)

 

Well I wish I had an answer for you but I don’t. But if it makes you feel better I’m in the same exact boat.

in fact I can go and make this even a little more bizarre. After my check engine light has cleared I noticed the car will run fine on several trips with no check engine light but when the check engine light does come on I swear the car was actually idling differently just before that occurred. I have a gate at the end of my driveway that I need to open and close to get in and out of my property. Are used to have an automatic gate opener but after it broke for the fifth or six time I got sick and tired of paying the $300 to get it repaired so I just open and close it manually. But this gives me an opportunity to actually get out of the car and listen to the engine after I have just started it and I swear that when the check engine light is off it idles a lot more smoothly and the exhaust smells a lot cleaner than on the trip that sets the check engine light. on the trips that do you set the check engine light for the 0174 code my car did not idle as smoothly as it normally does and the exhaust doesn’t smell as cleaned up. It’s almost as if it’s got a vacuum leak but but how could a vacuum leak be intermittent like that where do you have one out of every seven trips? So I haven’t figured it out but I know I’m close. It could be as simple as the expansion and contraction from heating and cooling cycles has actually created a small vacuum leak that caused the check engine light. Or maybe I have a bad wire to one of my sensors that’s intermittent and throwing off the fuel map. Or maybe I have a sensor that flaky or a ground that’s flaky? All I know is it’s intermittent and I can physically hear and smell the difference between it running perfect or when it is about to throw a code.

But don’t feel alone because I’ve got the same exact symptoms in my car and I have not yet solved the problem

 

As suspected, as soon as I started it up today I got an EML, restricted performance and P0174. Fuel trims on bank 2 are more or less permanently 25% short term and 19.5% long term at idle. The long term trim will drop to around 6% when held at 2500rpm.

Tried to do a bit more ruling in/out. I can't detect any misfires and I've never gotten a misfire code but thought it would be an idea to rule out the coil packs, so I swapped all coil packs from bank 2 to bank 1 and vice versa to see if a bad coil pack would move the fault to bank 1. Other than discovering that 1 coil on each bank has been replaced (googled the part numbers and they are correct), there was no change. There is some oil in the plug wells so it probably needs the cam cover gaskets changing, but I don't think that's affecting anything. I pulled a spark plug on each bank. They look relatively new, are NGK IFR5N10 which I think are correct. Only thing to note was bank 2's plug was very fuelly.

I got some brake cleaner and sprayed it all over the seal between bank 2's charge cooler and the manifold that splits the air flow to see if it would reveal an air leak. No change to the trims. I sprayed a load of brake cleaner towards the base of the charge cooler to try to access the gasket between it and the head. No change. I sprayed brake cleaner at the crank case ventilation tube on bank 2. No change (I also later pulled this out and checked the O-ring which still looked good and wasn't flattened).

I took the MAF out. It looked fine but i gave it a very gentle spray with electrical contact cleaner and re-fitted it. It made no difference to the trims, but I initially thought I had killed it, as on live data, the air flow rate and intake air temp were both reading 0 - I had just forgotten to plug it back in! Once plugged in, these values returned to what they were (incidentally, I though if the MAF was unplugged, it would change to open loop fuelling, as it would not know the volume of air to expect, but it didn't). No change to the trims, although I now have a pending code for MAF and Intake Air Temp!

I jacked it up and had a good look at the bank 2 exhaust manifold to cat joint in case there was a leak. There definitely is no leak there; there is no evidence around the joint and I was able to feel around it with my hand with the engine running and it's not blowing. I couldn't see the manifold itself as the heat shield completely covers it. The bolts are very rusty and I don't think it'll come off.

I'm going to try and cobble together my own smoke tester to rule in or out the air leak. The only place I can see a leak now being likely is the gasket between the charge cooler and the head, as you can't really get access to it with brake cleaner with everything in place. If the smoke test does not reveal an air leak, then I think injectors are the only thing left?

Couple of questions, is it possible to remove the charge cooler without removing the supercharger (is there a guide anywhere?) and is it possible to access the injectors without removing the supercharger?



I'll post up the full live data stream below with the car at idle, in case anything jumps out that I'm not seeing that could be another clue:

 

Where are you with the whole P1646/P1647 issue? Seems you had isolated a fault to one side, and the problem changed side to side when you swapped sensors. There was some confusion about which side was actually at fault, though. Was this ever fully resolved?

 

Yes, I had bank 1 and bank 2 mixed up. Of the three O2 sensors I had, I tested the heater circuit on all of them, one was open circuit. This was the dead one, as soon as that was replaced the P1646/7 code went away and the car returned to closed loop fuelling.

 

Unless I am watching the fuel trims in real time I have never been able to hear any differences with the engine idling while spraying anything around the engine?
When I had a similar but different problem I had spent hours with Propane and carb cleaner going all over the engine and I could not tell that anything changed.

But when I hooked up my cheap ELM327 and got the real time fuel trims up on the screen it was very easy to see things change. I can only assume that these electronically controlled engines can react very fast to changing conditions and unlike an old carb. setup it takes a massive problem to make the engine run rough enough so you can tell any difference.

Just a guess but do try to read them in real time if possible.
.
.
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One caveat. Make sure the stuff you're using is flammable. Lots of newer eco-friendly formulations are also non-flammable, or nearly so. Without realizing it, you're actually spraying an inert substance that will have no effect on combustion, and thus no change in fuel trims.

I've always been partial to using a propane torch setup. I slip a length of flex tubing over the (unlit) torch and then I can deliver the propane exactly where I want it.

Even better, a smoke machine is nothing short of awesome. If checking vacuum lines, be mindful of any check valves or solenoids that may stop the flow of smoke with the engine off. You may have to work around them to do a full test of everything.

 

Thanks, I’ve cobbled together my own smoke tester so I’m planning to have a go with it tomorrow.

Does anyone know if the charge coolers can come out without the supercharger or are the fixings underneath the supercharger? I think that’s where it’s going to be leaking.