1992 6.0 XJR-S running rich on B bank

Just came back from the MOT with an emissions failure - the B bank is running way out of spec on the fast idle test.

A bank
CO %: 0.08
HC ppm: 30
Lambda: 1.020

B bank
CO %: 9.95
HC ppm: 285
Lambda: 0.731

Any ideas where to start, and how to know if it is fixed without emissions test gear? Note, the ECU in the XJR-S is a Zytek unit, not Marelli or Lucas.

How does it seem to be running? Noticeable misfire? Does a folded $100 bill held over the exhaust tip show any difference between banks? Any black soot in the exhaust tips?

Can you get inside that Zytek box? A quick check for blown capacitors or dry solder joints might be helpful.

Seems to run and idle OK, I don’t notice any misfire. But I have been away for a couple of months so it is the first time it has run for a while, so it could be a new thing I didn’t notice on the way to the test station. I can’t get to the bank to exchange a $100 bill, I’ll try and make do with a £50 note instead. If anything, there is a little more soot on the A bank exhaust tip. I’ve not had the Zytek box apart before, but I can check the usual suspects on the PCB. O2 sensor maybe? - although that should have thrown an engine fault code.

It is overdue for a spark plug change and I also have some new plug leads as well to install. Pulling the plugs for examination would be a good next step in any case.

You’re not alone. Mine also failed the mot on emmisions today for running too rich. It drives ok, no misfires.

It’s a 5.3 Marelli though. I’m not sure if one bank is the issue or both, but they seem to be running similarly.

This is already with brand new plugs. Going to try resetting throttle balancing (i know it’s currently wrong), TPS adjustment, coolant tempreture sensor, just basic stuff like that and give it a good thrashing, it hasn’t had a good run in many months.

The place that does my MOT gives me two print outs from their emissions test machine, one for each side.

I’m not sure whether to be happy or just confused. I fiddled with all the plug leads I could get easy access to, poked at the injectors, and waggled any sensor leads that had LHS/RHS pairs. I then tried a very basic CO monitor that I use to use for setting the carbs on an old VW I had. I wasn’t expecting to see the same absolute values that a pro machine would, but I was surprised that I couldn’t see any relative difference in the two banks. Which seemed odd as there was an order of magnitude difference at the MOT test. Without much else to work with, I took it into a local shop with Jag experience and asked them to figure it out.

They can’t find a fault, both banks now running within spec. I had previously taken a plug out to see what type it was, I guess it may not have got the plug lead on correctly, or maybe it was one of the sensors, or maybe … dunno. Happy it is fixed, frustrated that I don’t know why.

Any time before you get an emissions check, be sure the engine is fully warmed up, and drive it hard (Italian tuneup) for a few minutes. That will ensure the cats (if so equipped) are fully activated, and the O2 sensors are hot and operating correctly. I’ve seen this fix emissions fails more than once.