I'm 73 and sort of a techno peasant. Worked on cars all my life but these codes and many other things about my wife's Jag are getting to me. Could use help for sure. So I have low fuel pressure at rail that drops off when engines off. Codes were lean banks and misfires so I started with new fuel filter... what fun!?!? Car took off like no problem codes went away. Week later...fuel pressure below 40 and fading again when off. So I figure fuel pump and regulator too? I can't find anything on a 04 regulator location. Also no video on fuel pump replacement. Here are the new codes that showed up!?!
P1260...P1582....P1672...B0300..B1316
Battery went dead yesterday but took a good charge last night. I know this is a lot but here I am so any help would be much appreciated guys..... Thank You

 

Be aware that low battery voltage can do all sorts of odd things on these cars, incuding running behaviour. The various modules (and there are many) like to have full battery voltage at all times. When they do not get it, all sorts of random problems come up: warning light for vaious systems, touch screen issues, window and seat module problems, and when certain drivetrain-related modules are starved for voltage, they throw codes.
The fact that your battery went flat suggests that it may have been discharged for some time. I always kept my 04 XJ8 on a CTEK battery minder to avoid these problems, and indeed it worked!

 

I'm thinking that's why I have a fob not locking my car doors too. Now that I have a 14.4 volt read I still don't have a working fob. Wonder if that Factory reset code P1582 has anything to do with it? Thanks for your advice

 

I think you my not have the right meaning for P1582.

14.4V sounds like the alternator but you need to know about the battery.

Many cars now have no fuel return line but instead can pump an exact full pressure - if the battery is good.

Similarly, the coils only really work properly with a good battery.

Next worry after the above would be an air leak - so check fuel trims.

 

Thanks Jag V-8
Yes I don't understand the meaning of code P1582. Correct me if I'm wrong but a low volt battery has nothing to do with my fuel pressure dropping off below 15 pounds when the engine is off.... right?
I have a charged battery showing 12.5 volts now and had it tested yesterday where I purchased it....four year old battery though. I'm getting moved over to the XJ8 03-09 forum soon thanks to the help of Graham...your advice is much appreciated.

 

Have a look in http://www.jagrepair.com/DTCforms/X350_P_DTC_OBD_II.pdf

 

You have two bus comm failures, CAN and SCP, which is possible however more likely battery or grounds/earth related. No comm will also affect key function. Could also cause low fuel pressure and random miss.
As a start, if you are 100% sure the battery is good - including voltage during cranking, check the ground/earth connections behind the headlights.

 

Well this is good information...even though the battery takes a full charge of 12.5 volts it is fading fast on amps when turning the motor over. It's being charged and my amp meter shows no draw on the battery when off....so I guess four years on a five year warranty battery is all I get. Not going to AutoZone for another battery even if their a block away. This is my second battery of theirs failing in two different vehicals in last two months.
THanks for your information "Old Karzs"
I really appreciate it.
​​​​​On another note....I need information on replacing my fuel pump in her 04 XJS stock auto. Can someone lead me to a write up if not a video...It looks like to me the pump is in the back left side of trunk?!? Do I need to make a new post for fuel pump help. I don't want to scatter posts again is all. Thanks for advice.

 

So I finally just replaced battery in trunk but I still have low fuel pressure with misfire codes. I have a fuel pump replacement. I purchased a download manual for my 04 XJ8...It shows the fuel pump under the rear seat floor? Really spotty information with removal drawings. Anyone ever replaced a fuel pump on this model Jag please. Do I really need the round tool they show to remove the pump? Where would I find one?

 

So I finally just replaced battery in trunk but I still have low fuel pressure with misfire codes. I have a fuel pump replacement. I purchased a download manual for my 04 XJ8...It shows the fuel pump under the rear seat floor? Really spotty information with removal drawings. Anyone ever replaced a fuel pump on this model Jag please. Do I really need the round tool they show to remove the pump? Where would I find one?