Hi guys.

I bought an XJR a few months back and was getting P0171 codes. At the time these cars were totally new to me, but after researching and cross-checking information, plus many hours spent with a Jaguar specialist garage, I have come to the conclusion that this is caused by the aftermarket exhaust and 200 cell cats fitted by the previous owner providing insufficient backpressure for the o2 sensors to read correctly. (no vacuum leaks, injectors tested OK, MAF fine, o2 sensors swapped out etc).

I am now reluctantly looking into getting spacers for the o2 sensors, has anyone else been down this road?
all input greatly appreciated. Thanks very much.

 

I would not put a spacer on your wideband O2 sensor (the pre cat sensors) -- this isn't an emissions O2 sensor one is trying to trick, rather it's the upstream wideband O2 sensor that tells the ECU if you're running rick or lean. I would invest in an appropriate scan tool and continue to work the problem. Monitor short term and long term fuel trends on both banks. The P0171 is telling you one bank (and not both) is getting too much air (leak) or not enough fuel (injector issue, fuel pressure issue). Also check the manual for a rundown checklist.

Lean conditions can be frustrating but they are ultimately resolvable.

 

Thanks for the reply.

You must've missed the bits in my post where I said there was no vacuum leak and the injectors are fine.

In summary, the engine isn't running lean, the o2 sensors are sending incorrect data. If anything it's now running rich due to the fuel trims being almost +20%.

 

+20 means it’s desperately trying to compensate for a lean condition by adding fuel

i have no cats and no lean code

 

Guys I know what the code means.

It LOOKS like a leak, but it's definitely 100% not. We've already done a smoke test.

I honestly think the only thing left to blame is the aftermarket exhaust. Wideband o2 sensors are sensitive to backpressure.

Has anyone actually been in this position before? Pulling my hair out here. It has the Jag specialist stumped, he spent a full day on the car and couldn't get to the bottom of it.

 

I'm with Xalty, you have one bank that thinks it's seeing a significant lean condition. Off the top of my head, here are things to check off. And while you are confident in no intake and injector issues, there are other ways to cause a lean condition. A partial list without looking up the manual:
1. intake (pre supercharger) leak / bad MAF <-- unlikely b/c you only have this on one bank.
2. post supercharger air leak -- did you change injectors recently? If not installed with oil, the seals can rip or bind and create a leak at the injector.
3. low fuel pressure due to fuel pump or FPR issue -- with a scanner, check fuel pressure at the rail. Also, if this were to cause a lean condition with only one bank, it would be with the bank at the *end* of the fuel rail, not the one fed first (at least if in cases of only partial failure of the pump / FPR)
4. exhaust manifold crack or gasket leak -- this would allow air into the exhaust and make the sensor think that bank was lean. To check this, pull the cat pipe, plug the intake, and run a smoke test into the exhaust manifold.
5. bad upstream O2 sensor