Fingers crossed!

Odd that they were, though.

 

Update:

I've been too busy with my elderly mother's out-of-town estate sale the past few days to spend any additional time working on my wife's 2006 XK8. But the new NGK iridium spark plugs I ordered from rockauto.com showed up in yesterday evening's FedEx delivery. I'll make the time to install them later today and then see if they indeed clear the P0171 and P0174 codes for good. Stay tuned....


P.S. - My new $62 amazon.com Denso MAF sensor has now done nearly 500 test miles in my 2005 S-Type since Wednesday without a burp, so I think we can now declare that these units are indeed exact fits for our Jaguars....

 

Update:

Unfortunately, the new NGK iridium spark plugs did not cure my P0171 and P0174 codes issue. Both codes returned during a 20-mile test-drive that included three drive cycles. So did the "Restricted Performance" dash message and the illumination of the Check Engine icon....

I need professional diagnostics to get to the bottom of this problem and plan on booking the car into Flying Circus at their earliest opportunity. They stay very busy so it may be awhile before they can get to me....

The car is due for its annual state inspection by February 28th so this will be the first time I will miss the inspection deadline for any vehicle we have owned. There are financial penalties for doing so but I have learned that there may be a waiver available if one can prove they have spent $200 or more attempting to fix the issue. With the new MAF sensor and the new spark plugs, I've spent a total of $138. I'm sure the Flying Circus diagnostic fee will be more than the remaining $62 to reach the magic $200 threshold so we'll see what happens....

Thanks to all who have offered suggestions to date....

 

Jon, in diagnosing your codes, you need to have a couple of things. One, SDD or an ELM327 so you can see fuel trims, especially short term. Then get yourself a can of carb and brake cleaner, and lightly spray a little at each of the intake runners where it meets the head. If you squirt and then see trims make a jump, you have an intake leak.

Here is a video that might help you understand this;

 

Update:

A smoke test by Flying Circus revealed leaks at both VVT O-ring seals with the drivers side leak more prominent than the passengers side leak. I did not initially suspect these seals (and neither did Flying Circus) because I had replaced them both in March 2015 when I performed the car's 90,000-mile service and while they are infamously known to begin leaking small amounts of oil, they are not generally known to begin leaking air. And after only two years of service on the car....

Both faulty seals were replaced with OEM ones on Wednesday evening and since then, the shop has done multiple drive cycles with no codes being thrown. The various monitors have not reset yet but that is typical. We plan to pick up the car from the shop by early afternoon today. My wife should be able to do enough drive cycles this weekend to reset the monitors so I can finally take the car in for its annual inspection one morning next week (and then go to the DMV with all of my repair documentation in order to get any late penalty fees waived)....

I ordered the short-lived seals from motorcarsltd.com for about $11 each and will not be purchasing those again. While I have always believed that these VVT seals are significantly overpriced regardless of where you purchase them, my recent experience may prove that in this particular case you get what you pay for....

So we hope the new VVT seals have indeed fixed my P0171 / P0174 issue. While I regret replacing my factory NGK iridium spark plugs so soon as a potential cure, they did have more than 108,000 miles on them and I will never have to replace them again....

We'll give the car a week or so to prove itself once more and then I'll report back on any results....

 

Hello Jon89. I too had the same P0171 / PO174 codes and I couldn't find the leak. I finally took the car to an indie garage, as you did, and they used propane to find a leak in the EGR valve. Air leaks are hard! Hope this fixes your problem.

 

Update:

My wife's XK8 passed our state's annual inspection this morning with no issues. All monitors finally cleared and reset during a 40-mile drive yesterday and the only code now showing on my scanner is the always-good-news P1111....

So be sure to check the condition of your VVT O-ring seals on a regular basis. If they are looking and feeling brittle even though they are not yet allowing oil seepage, replace them immediately with good-quality ones....

 

Quick update for me: I bought one of the Denso 197-6030 air flow meter from eBay, reputedly new, and actually a Toyota part. It works perfectly. The engine acts way better, much more throttle response, and a "noise" going uphill at low rpm is now gone (ping?).

Of course, I am not completely sure of the cause as I also replaced the VVT seals. The old ones leaked a little oil, but once removed, were obviously hard and soaked in oil, so probably original. I had ordered a pair from eBay (Jaguar OEM part# AJ82856), so I popped them in. Finished the job with contact cleaner on the plugs.

Altogether, it feels like time (and a "pocket change" money) well spent. Thanks for the heads-up.

 

Glad to hear that the new Denso 197-6030 mass air flow sensor is doing a fine job for you. My amazon.com spare is now waiting patiently on my shelf for the time that I, Rick, or Wayne need it. This version also looks to be a perfect fit for the MAF sensor in our 2013 Subaru Outback 3.6R. Sooner or later it will be put to good use....