2012 xj 5.0 supercharged Rough idle and only 500 psi but as soon as I’m on the throttle it’s fine goes right up to 2100 and runs good , Once in a while it will chug from
loosing pressure but not consistent ,I’m not sure if it is the low pressure pump or the high-pressure pump.possibly filters ?? Anybody with any experience with these symptoms ??

 

The Technical Training Manual for this engine suggests that fuel pressure at idle is about 435psi and at full power is about 2176psi. It suggests to me that you may have an air leak in the or about the intake manifold. I would check the fuel trims to support my suggestion. Are there any codes?

 

Yes, lean bank one and two also low threshold catalyst system something or other , so I agree I definitely have an air leak somewhere as you mentioned ….. for some reason I thought it was supposed to be 1200 at Idle fuel trims have been 20% or more at times , for some reason when the fuel pressure rises at idle, it seems to idle better not sure if I have a couple issues. I just picked up the car a couple months ago and it sat for a year before I got it 120,000 K not sure what maintaince of any

 

I did have the intake off and replaced the coupler and deleted the noise system , I did notice the tube from the back of the valve cover that goes back into the intake at the throttlebody was leaking, so I put a rubber hose in the place of that. Can it actually be deleted and put to a catch can and plug the hole on the intake or does it need it for some other reason ,I assumed it is just recirculating crank case gases, etc.??

 

I just got my car 9 days ago and so I am still exploring, but I did have a 5.0 NA XF. That had a part load breather and a full load breather Some where almost under the supercharger there are two valves for the breathers on the supercharged engine. Personally I don't mess with the emission control stuff If you look up in the stickies at the top of this forum you may find a link to the Technical Training Manual for the engine.
Oh yes the idle speed should be about 650rpm.

 

So an intake leak or any air leak past the mass will cause the rough idle ? And obv the lean condition on both banks , it will be nice if it’s just one problem that fixes both conditions just seems to me that if it had a small intake leak at idle it would be purging up and down trying to find it spot but it’s idling really rough and trying to almost die it’s choppy like it has cams done lol

 

Ended up being the tube off the head that recirculates the gases wasn’t seated properly and problems gone ! Feel like an idiot for not checking them prior but thx for the advice !