I have a 2001 XJR the supercharged 4.0 V8 model, I've owned it for about a year with almost no problems other than some misfires from water and old spark plugs and things ect, recently I started having problems with fuses blowing and my car not even running, it would drive off and on for 20 mins but once heated up it would die like it ran out of gas. We changed the Wiring Harness, and it started running perfectly again, but since I got the car back from the shop I took it too I've had nothing but Lean codes on my check engine light and occasional "Restricted Performance" and Failsafe engine mode warnings. For the year ive had the car I knew that the airbox had a certain bracket that was broken, but that didn't cause any codes or problems up until I got the wiring harness replaced from a shop recently.

I've looked up and down the airbox/boot ect and can't find any leaks or anything there is a slight Tssss sound but we don't feel any air coming from anywhere. Also the restricted performance light turns off after 40mph+

Is it possible that a sensor had just been broken and that's why I only just now started to have the check engine light and restricted performance turn on since the new wiring harness got it fixed or something??

I haven't had any problems with how the car actually runs either. Drives great and still feels powerful.

Please get back to me if you have any ideas💪

Here's Whatever pictures and things I have.

 

Sounds like you have an air leak somewhere but it will be a vacuum leak letting unmetered air into your intake system, so the hiss will be impossible to feel being that it is a vacuum and not pressure.

The common suggestion that will be offered to you by most members will be to do a "smoke test" to see if you can detect where the vacuum breach is.
Crudely, you can just light a cigarette or cigar and then let that smoke drift in and around the various pipes and fittings associated with the air intake manifold, associated small plastic vacuum lines, EGR and PCV hose etc. to see if the smoke is drawn in anywhere.

The small gauge plastic hoses do get brittle with heat and age and will fracture or snap if flexed, so be careful putting pressure or moving any of those hard lines.

I assume that if you were getting any error codes that you would have posted them in your original description of the issue, but if you do have codes, please post them in a reply so the members here can begin to offer some more defined guidance.

Good luck

 

P0171 p0174

 

Low fuel pressure or air leak between MAF sensor and intake manifold affecting both bank 1 and bank 2.
The fact you think you can hear a hissing sound, my money is on an air leak.

Start with the smoke test and see if that will steer you in the right direction.

 

Although the basics are probably the same, please be aware that you have posted in the wrong forum and that there might be more help in the correct one for the X308.

This is the forum for the X350 from 2003 onwards.

Best regards,

Thomas

 

Oops my bad