I have a V6 3.0 litre XJ X350 2005 Sovereign.. It stutters and doesn't pull under load. It ticks over fine bu6t struggles to get to 70MPH ! Therese no smoke out the back and it is not overheating, Petrol consumption is down by a couple of MPG.
I've had full service carried out, new stick coils but it still does it. The guy who serviced it reckons he changed the Fuel Filter but I haven't seen the old one so cannot be sure.
Its been on two lots of diagnostics but with no error codes apart from the Stick Coils.
Does anyone have any suggestions? I have had people mention blocked Cats but at £700 that's a large punt to take!
I am in the Leicester / Northampton area of England and am stuck working on the side of the road with no MOT at the moment, if anyone can suggest a way of getting this sorted or someone who can diagnose without just throwing bits at it, Id love to hear from you!

 

Have you got any fault messages at all, and any warning lights ?

What had you last done related to the car before the problem started ?

Normally with symptoms like this there will be a fault light and message of some sort. So puzzling.

 

You don't state the mileage but if it's over 100K might be worth checking

fuel pressure at the injector rail.

 

Hi, its only got 83,000 on the clock Full Service History for what its worth !

 

OK, latest diagnosis is blocked Cats..... watch this space lol !

 

If they're blocked it ups the back pressure and (partially) prevents the exhaust leaving the cylinders, thus reducing vacuum (and you can figure the rest) - particularly at when flow rates would be high.

Sometimes cats break up and then you can hear them (may have to be under the car).

When blocked you can try:
1. removing an O2 sensor and looking in (e.g. borescope / USB camera)
2. IR gun to see the temperature drop across the cat

To some extent you can look at MAF (air flow) values under load to see they're not where they should be but you need to know what they ought to be / can just think it's a bad MAF.

 

Yay, happy days the beast lives!
All of those who suggested blocked Cats, feel free to pat yourselves on the back and look smug down the pub tonight
Now I need to pop yet another kidney on ebay to pay for it

 

Sort of "good" to hear (not the kidney part).

Once fixed and the trims have had a chance to relearn (or you've done a hard reset and a few drives), check the LTFTs are OK, in case they're not and are why it failed. (The car only flags very bad trims...)