It's an '03 XJR, last of the X308s. I am 99.9% sure the system has a typical return - it has a typical pump and regulator. The regulator *is* vacuum controlled - the vacuum line is recent, silicone, and in good shape - unlikely to collapse or kink. Last year I did check fuel pressure when chasing that other issue, but it's definitely worth testing again.... high fuel pressure could be responsible here. Shame, because that little tiny vacuum hose is a PITA!

I agree the cats are unlikely - one clogged cat, maybe, but two? Seems like a stretch for a car that is *quite* fast. I have a few very quick cars, and I don't have reason to doubt the Jaguar's ability to process air. Same with the injectors - two banks at the same time doesn't typically point to leaky injector. And O2 sensors... both misreporting? Nope. I'm looking for something upstream that both banks have in common.

My experience with failing MAFs is that they typically lose range - they lose sensitivity to both low air flow *and* high air flow and report in a narrow range. Result is rich running at idle and lean running at WOT. This has played out with a number of Bosch- and EEC-equipped cars, and I'm gonna assume Denso hotwire systems are subject to similar problems.

So I have the MAF coming. I could reasonably move onto a MAP sensor, although in all the cars I've owned I've almost never had a MAP sensor fail. I really. Really. Hope it's not the throttle body. Thing is, the codes + (apparently high) calculated load REALLY think it's gonna be one of these.

 

Two blocked cats and it would fall on its face at or near WOT I reckon.

MAF is a fair suspect.

It's puzzling that getting a DTC was hard, though once OBD monitors are unset a lot of DTCs can't be flagged.

It's an AJ27 from the sounds of it.

Don't suppose you know the info on the PCM's label? Not suspecting it, just being nosy.

 

Strongly agree - I was surprised to see a DTC before the monitors set, and even post-DTC the monitors aren't set. It definitely raised an eyebrow about the ECM. I may have a photo of it somewhere - I was in the compartment a long time to clean the fan.

 

So, the conclusion....

Got the new MAF and started driving around, watching the data stream. Initially mixture was SUPER negative, but after 2-3 trips I started seeing far more sane values. More sane, in fact, than I have ever seen with the car. In five years it's always been around -14% total (STFT+LTFT), so always quite rich. With the new MAF, I was seeing more like -2% total, with offsetting positive and negative values, like one wants. Also, the calculated load value dropped a couple points which seemed desirable.

However, after another 200 miles the EGR monitor would not set. Boo.

I was still pretty hung up on the high calculated load, and since I know the MAF is good and the MAP tests suggested it was good, I looked at the TPS. The only thing I had to work with was an absolute TPS value of 4.7% with the engine off... on EVERY other car I have owned this is well within range, but it was something....

I carefully adjusted the TPS from 4.7% to 1.3%, reset the ECM, and headed out. Calculated load had dropped from 26% at idle in D to 20%, so that seemed like an improvement. Also, interestingly, MAP dropped from 6psi at idle to 5.3psi... no explanation for that. Idle increased from 600rpm in D to 650.

The EGR monitor set within 10 miles. Most of the others within 20. Evap at 30, at cat at 40. So, all monitors set within 40 miles... save comprehensive, which is exempt from testing on these cars so whatever.

Knowing what I know now, I probably didnt "need" the MAF, but at the same time I'm certain it was on the edge so replacement wasn't able waste.... mixture is much closer to stoich now, so hopefully that prolongs cat life.

 

Heyyy! Good result!

Kinda weird, but the pain of finding out (unless you love a tough puzzle) is likely worth avoiding.