Afternoon everyone

Me again!

Have a weird one for you. When the car is hot, and I lightly accelerate in any gear (from a roll) between 1500-2000 rpm the car bucks (jerking back and forth) but after 2k rpm it stops. No fault codes.
Car is a 5.0 XKR
Thank you in advanced!

 

Also, to add, it only happens with about 1/4-1/3 throttle anymore and it's fine

 

With no further information, it MAY be a vacuum leak somewhere. That, or dirty MAF sensors. Bad Fuel? Hellz, could be a lot of things.
Start easiest and cheapest first; clean the MAF sensors with the appropriate cleaner and method, then use the best fuel you can find.
Several ways to check for vacuum leaks, but maybe try MAF first.

 

I seem to recall 5.0L XKR's have two fuel pumps.

It could be the range it doesn't work it isn't getting enough pressure for the demand.

The range it works it is running on both pumps, or below demand.

Could be as simple as a clogged fuel filter

I'd follow Cee Jay's advice first since it is a maintenance task that doesn't require parts.

 

All good suggestions but I would think any of these issues would throw a code, except maybe the bad fuel. You never know though and easy enough to check/clean...

 

I thought it was fuel as well but I use Shell V Power, yeah I was thinking if it was the MAF there would be EML?
I'll clean them though and see what it does! But it only happens when it's warm

 

Yes, two fuel pumps, one low pressure and one high pressure for the injectors. Chances are that fuel starvation would occur at HIGH demand rather than low demand. The fuel filter in the 5.0 models is inside the fuel tank, so not really a Maintenance Item nor an easy repair.

 

On many cars that would be misfires.

 

It's definitely not a misfire, it's more the whole engine just slugging. The throttle is a bit snappy as well

 

Think of it as a curve where there is an expected output and expected transition where the outcome requires the out put of both.

At WOT there is tolerance in the fuel pump spec the car would drive fine at WOT with both pumps even with a restricting fuel filter because the design tolerance is higher than the expected peak load.

However, at the range of the curve below where the second pump is activated the system would be weaker than expected and would be waiting to activate the second pump until the throttle/rpm load would demand it.

I mention it because without the tools of proper diagnosis all we can do is throw ideas out there.

I believe Jaguar has a six year maintenance interval on the fuel pump for my 4.2L. I haven't read the schedule for a 5.0L that I can recall.

Six years ago would be 2013.

 

So you're saying it could be the first or second fuel pump?

 

No, and follow Ceejay's advice first on cleaning in the intake sensors.

If you have a way to watch fuel pressure I'm saying it might be the fuel filter.

 

Could it be a faulty drive-by-wire potentiometer?

Seems like fuel filters / fuel pumps would have problems when flooring it rather than 1/4-1/3 throttle.

 

Here is an unmarked illustration I quickly threw together since my words seem to fail me.

The yellow line shows fuel requirement, WOT is demand on the right side of the image.

The dark blue is fuel pump one, the dark purple is fuel pump one and two combined.

The light blue, and light purple, show the same amounts respectively provided by fuel pump one, and both fuel pumps when encountering extra resistance from a clogged filter.

If in a given yellow line the car expects a particular fuel pump performance, the area where extra resistance from a fuel filter in need of replacement would cause issues is marked in red.

This is where fuel pump one is activated, but not providing the flow needed to match what the car expects fuel pump one to provide.

Once expected fuel need reaches the point where fuel pump two is activated by the car's computer, the problem goes away because now the system is capable of providing fuel up to the light purple line.

At WOT, there is extra tolerance of fuel pump one and two combined so even with the extra filter resistance the problem is not seen.

Instead the problem occurs in the area before enough fuel is required to activate both pumps together.

 

Gotcha. Great explanation!