I think you may be on to something. I don't have any documentation to back up my hunch, but from what I've observed on the forum, there seems to be a hierarchy or priority for certain codes.

The infamous P0171/P0174 codes for a vacuum leak? I think those are fairly low priority and only get set when no "stronger" codes are also present. Often people report these codes but driveability is fine. That tells me the codes are fairly minor in priority and can only be detected when all else is good.

The misfire codes? (P03xx) Those seem to be a higher priority. I wouldn't say they inhibit lower priority codes per se. It's more like they are simply more obvious, typically cause rough running, etc. The general idea seems to be fix these major codes first and then see what happens.

A rough analogy from another recent thread: If you lost your arm in a gardening or chess accident, the pain your brain is processing is the equivalent of your car setting a misfire code. Sew your arm back on and all is good again until you notice you also had a hangnail. That minor sensation is the equivalent of a P0171/P0174 code. Your brain never gave the minor code when you were staring at your arm laying on the ground because it was too busy commanding you to scream.

Back to the OP: For giggles, let's say the root cause is a major vacuum leak affecting one bank. That might set misfire codes for that side, and not necessarily any other codes. More on that possibility in another post.


*Misfire Soapbox Mode [ON], as promised above:

If the computer has set a misfire code, please be aware "misfire" wasn't the best terminology that could have been used. Misfire sounds like an ignition problem, at least to me, meaning the fuel and compression were there, but for whatever reason, the required spark was missing or weak. This might lead a good-looking aspiring mechanic to troubleshoot the daylights out of a perfectly good ignition system. I do not care to discuss how I know this.

In OBD-speak, misfire simply means one or more cylinders is putting out low power compared to the others. Pretty much anything affecting the output of a cylinder can set a misfire code. It can be low compression. It can be a valve not opening fully. It could be a clogged fuel injector. And yes, it could be an ignition problem, too, but don't overlook the other possibilities after checking the ignition system. [/soapbox mode off]

 

Are you sure you want me to get involved? You'll have to forgive me if I come across as brusque, but I'm old and cranky and am convinced everybody around me would have their lives enriched if exposed to my rigid, unyielding viewpoints.

When it comes to troubleshooting, I'm cheap and lazy and proud of it. I turn wrenches for a living and do a LOT of troubleshooting, often picking up from other mechanics. The jobs that make me cringe are when those ahead of me have been shotgunning or using the parts catapult in hope of a fix.

So once again please forgive me if any of this comes across as holier than thou, but thanks to the dynamics of an online forum, I have no idea of your abilities, experience, etc. I had to shake my head in disbelief when I read you had already removed the catalytic converter just to see what would happen.

Another drawback of any forum is one can make suggestions that hopefully are helpful, and they get ignored or glossed over. Talk about frustrating... Still not 100% convinced I want to wade back into another tough one again.

So for starters, let's back up and get some more info from you. Age? Experience? Location? Your first attempt at fixing a modern car, or are you an old hand? Shoe size? Ginger or MaryAnn?

How long have you owned the car? When did the problem start? Have you been driving it for a while without any problem and suddenly this started? Or was the car reassembled from multiple wrecks as a therapy project at a chronically underfunded school for blind and emotionally disturbed teens? The forum doesn't charge by the word, so feel free to tell us lots of background details, especially the history of when the problem started.

Some basics to consider. Are you 100% sure you're working on the correct bank of cylinders? The top left illustration at this link should be the correct cylinder numbering for your car. The odd numbered cylinders are on the US passenger side. The even numbers are on the US driver's side:


http://jagrepair.com/images/Training..._numbering.pdf



Fuel trims - Lots to digest, but read through these two documents for some great troubleshooting tips:


JagRepair.com - Jaguar Repair Information Resource


You had previously said you had no vacuum leaks. I asked how you knew this. You said you did a smoke test. I'd have to reply you haven't found any leaks yet, that's all. There's no guarantee your car is leak-free. All we know is there's still a possibility and you've ruled out some sources of potential leaks, but likely not all. Something to consider is how the limitations of any test procedures can trip up even the most experienced mechanics. I do not care to discuss how I know this...

The smoke test, for example: It injects pressurized smoke into lines normally under vacuum. I don't know all the details of the supercharged engine, but most engines have several check valves in the maze of vacuum lines. If you blow smoke in the reverse direction of normal vacuum flow, the check valve will close and anything beyond that won't be tested. You can also have valves that only open under certain conditions, and block the passage during your smoke test. These scenarios have tripped up many people, self included. To get around these pitfalls, you'll have to physically trace each vacuum line and identify any check valves or valves that close off parts of the circuit. From there, you can bypass the device or work the circuit in segments to make sure everything gets tested.

Another pitfall is some leaks, such as a torn hose, only leak under vacuum but close when pressurized. A smoke test won't find a one-way leak like that.

In pursuit of recalcitrant vacuum leaks, in addition to a comprehensive smoke test, I also like to do the carb cleaner test. This is the basic one where you spray carb cleaner (or other moderately flammable aerosol) around any suspect vacuum lines with the engine running. In the days of carburetors, the idle speed would change when the flammable liquid was drawn into any vacuum leaks. With fuel injection, the idle speed stays steady, so you will have to watch short term fuel trims on your scanner instead. Same basic idea, just implemented differently.

Also keep in mind that not all vacuum leaks are under the hood. The evaporative emissions system, for example, extends all the way to the fuel tank. A smoke test is great for this system, but as mentioned above, you've got to make sure any check valves or solenoids are open. The power brake booster is another one to check. If the input shaft seal has a leak, it will will draw in cabin air from under the dashboard. Unless you specifically check under there, this one is easy to miss.

Oxygen sensors: These are very, very important. So many air/fuel calculations are based on what the sensors report they are seeing. But are they accurate? We mere mortals do not have ready access to exhaust sniffers, so we have to assume (Danger! Danger!) the oxygen sensors are accurate. When they get really bad, they'll throw a code, but not until then. On a 2003+ car, the oxygen sensors apparently use unicorn tears as the active ingredient, judging by the price. Normally I'd suggest new sensors, as they do go out of calibration with age and are considered normal wear and tear items. However, due to the cost, and seeing that your fault is mainly confined to one side, I'd suggest swapping the two upstream sensors side to side and see if the fault follows. That will cost zero dollars and maybe 20 minutes of your time.

That's all I've got for now, but if you're still willing, I'll add more as ideas come to me. Deep breaths. Take your time. In my highly overrated experience, you can make educated guesses 90% or so of the time with good results. It's that other 10% when you have to step back, accept you've got a tough one, and start at the beginning. Don't make any assumptions as you go. That last bit is tough to do, so I'll repeat it: Don't make any assumptions as you go.

Respectfully submitted.

 

+1 Karl on your stout advice!

In my opinion, for whatever that's worth, the OP is chasing a vacuum leak, which I mentioned way back in post 7 of this adventure.

 

Any updates? Or is this another thread destined to fade away without resolution?...