I have a 2004 XJR with about 85k miles on it. I noticed surging: Acceleration from a light with more than light throttle caused power, then cut power, power, then cut power. Then around the same time I got a reduced performance message. I changed all spark plugs and coils. Surging problem slightly better but still there. Yesterday I floored the accelerator at 40 mph and it would pull then cut power then pull then cut power. It was pretty intense. Car idles perfectly. Cruises on the highway fine. But when I give it the gas it is power / cut power /power /cut power. Ideas?

 

Blocked fuel filter? Failing fuel pump(s)?

As it does it at higher speeds, probably not an air leak but check fuel trims anyway.

 

Like he said ... plus ... check for transmission fluid leaks around the transmission and the lines to and from the transmission cooler. A low fluid level can lead to a surging in the torque converter (my old Mercedes had a leaky transmission pan and every 1,500 miles or so ...)

 

Thanks, no harm in changing the fuel filter. No transmission oil leaks.

BUT, the cut off of power is so severe that I wonder if that's it. It will pull hard then cut off almost like Traction Control is kicking in, cutting power, and then allowing power back on.

For example if I stay on the gas it pulls super hard, then cuts power hard, pulls super hard, cuts power super hard, it will do this back and forth, power / cut power as long as I keep my foot down on the accelerator.

My first thought was somehow the Traction Control system is confused under hard acceleration and cutting the power.

 

If it cuts in mine flashes the TC lamp and anyway wow do you feel it. You do not appear to have anything like the same happening.

 

Yes, you really feel it. I backed off as I was afraid something was going to break, that's how hard the cut off in power is. The TC lamp did flash. So I tried disabling the TC by hitting the button but it still did the same thing. Like I said, it's like the car pulls hard then stops pulling equally hard.

 

I had a vacuum leak that caused similar issues. Mechanic ziptied it back on, works fine now.

 

I think you can't fully disable the TC. Might be a problem with an ABS sensor / failing wheel bearing which then triggers the TC/ABS.

 

That's exactly what I was thinking. I have noticed a dragging sound from the right rear wheel. I should pull it off and see what I find.

 

Scan for codes.
Not the normal symptoms for a failing TPS but it's possible IMO

or even the potentiometer on the accelerator pedal which is very rare

but then those symptoms are very rare.

There should be a code which will point the way.

 

That's the frustrating part. I checked the codes and nothing comes up at all.
Last night, with the car parked, i reved the motor and it would have a cut out/strng miss? in the upper RPM which leads me to believe it is independent of the TCS. I think my TCS was being activated by the severe surge / power cut surge /power cut.
I just replaced all spark plugs and coils thinking that was the problem as I was getting a reduced performance message. That message went away, but the problem did not

 

Sounds like blocked cats

If you have done 100,000km or more its time to replace them anyway

Cheers
34by151

 

Car has just over 80k miles. Does not feel like clogged cats. Thank you though for the suggestion!

 

When you say you had a reduced performance message do you mean "limp mode" if so there must have been a code recorded.

 

Sounds like the rev limitter.

 

No, it just said "reduced performance."
I plugged code reader in and got no messages.
After I changed the spark plugs and coils the message went away, but the problem did not go away.

 

That would make life easy....but nothing is coming up

 

You indicate that the surging as you call it is severe and happens at more than light to moderate throttle openings and that the restricted performance message came on. Have you ever noticed if that 'surge' happens when you hit 3000 rpms? If it happens repeatedly it surely leaves a code that perhaps your scanner isn't programed to see as Jaguar has hundreds of codes that are proprietary and are only readable by a scanner equipped with the SDD software. The XJ8's can approach 100mph without crossing that 3000 rpm limit if you accelerate moderately so it isn't a speed related problem but a problem to something more elusive in the fuel/air/spark timing that happens faster than the computers can react to modify and control. Since they can't tell how fast or far the RPM's are going to climb or for how long they will be maintained the computers take preventative measure and cut power to the coils.

Since you replaced the plugs and all the coils with no real improvement it would be natural to assume that the two remaining elements necessary, fuel and air, must be the problem, except that there's a lot more that might be going on with all the electronics in these cars. Anyway based on all the threads that have been posted over the years on this subject it seems that most often the culprit turned out to be a vacuum leak somewhere in the miles of plastic and rubber hoses, gaskets and seal, etc. that make up that system but without the codes that are usually thrown to guide you towards the most likely culprits you could be spending fortune finding it.

Assuming you don't have access to a scanner with SDD capability I'd suggest you find a reputable garage that can run a thorough 'smoke test' to either eliminate a vacuum leak or find where there is one and proceed from there before you start swapping out expensive parts hoping you'll get lucky. It took me a year of trying to get 'lucky' before I finally had enough and got a smoke test; found the hidden leak and got rid of the problem. That was after plugs, coils, injectors, part throttle bypass hoses, etc., along with hours of blood, sweat, and tears with my head buried under the hood.

 

Thanks for your suggestions. It happens at any RPM. Punch the throttle off the line and it does it at 1500 rpms. If I feather back, sometimes a few times, and then get back on the throttle, I can get it to pull hard. Which indicates not a fuel or cat issue.

I just cleaned the connector at the air meter that did nothing.

Don't think it is a vacum leak Those usually cause a change in idle.

What you say makes lots of sense as to why my generic scanner is not picking up the code. Running out of options.

There was one thing.....a few months ago my car would go into no gear at all. I tried lifting up the center console to see under the wood gear cover. I did not get it off. I later discovered a nut had fallen off the outside of the transmission. Problem solved......but I wonder if I might have damaged a circuit in there as I notice the traction control button is there in that wooden console cover.

 

You need to have the codes read with a scanner that will read more than the generic ones
Your best option is IDS/SDD failing that an iCarsoft i930 should do the job but I cant confirm as I dont have one

Also take a read of this thread
https://www.jaguarforums.com/forum/x...solved-137070/

End result replace the cats, problem solved
You said you have 80,000 miles thats 129,000km's
You are well overdue to replace your cats anyway, you may as well have them done as I can guarantee at that age they are choking the engine

Cheers
34by151