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Matt
Matt, Mechanical Engineer
Category: Jaguar
Satisfied Customers: 22470
Experience:  BEng hons Mech engineering, in auto industry 22 years
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2004 jaguar x type V6 petrol. Loss of power at 2900 revs.

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2004 jaguar x type V6 petrol. Loss of power at 2900 revs. Speed reduces despite pressing accelerator. Heavy vibration until revs reduce to 2500.

Hello and welcome to Just Answer

I'm assuming that you don't have a check engine light or other fault light on? as you haven't mentioned it.

if you do then get this code pulled off with a scan tool as it will greatly help the diagnosis

This sounds like a lack of fuel and could be a lack of fuel pressure from the electrical pump at the tank.

1st thing to try is to remove the fuel filler cap and then flick the ignition on whilst listening for the brief ‘buzz’ of the pump

If the connections to the pump are corroded or damaged then the pump could stop at any time or could not be running at full speed. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse,

Ideally measure the pressure at the injector rail (should usually be around 3Bar / 45psi +) and if this is low check the pump as described and also consider changing the fuel filter as if this partially blocked this too will reduce pressure.

You can also get this issue with contaminated fuel , if there’s water or dirt in the tank then either the petrol floats on top of the water or the filter sock on the pump is choked up – so if the other things check out then I’d consider pulling out the fuel pump and cleaning the filter sock

And one other thing to try is to loosen off the fuel filler cap, as there’s a 1 way valve built into the cap to let air into the tank and so prevent a vacuum from being created as the fuel leaves the tank to the engine

If this helps then replace the cap with a new item

My goal is to give you the best advice that I can and I hope that I've helped today. Please remember to rate my service by selecting from the 5 stars at the top of the screen before you leave. If you need more help, use the reply box to let me know.

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Thanks Matt

Customer: replied 1 month ago.
The juddering of the engine begins at 2900 revs every time and continues until revs fall to around 2500. Then everything seems normal again. I should have said that the dashboard engine light flashes during the events and for a short while afterwards.

Hello

OK

if you can get a scan tool onto the car to grab the fault when its flashing then that would really help

from your description I'd say its still worth checking the fuel pressure and also check that you're getting full throttle by pulling the intake off and having your assistant slowly push down on the accelerator pedal, this year should still be on a mechanical / cable throttle but its still possible to have issues

Customer: replied 1 month ago.
Scan tool says "No Codes Found"! Today I noticed it was VERY slow to change up a gear (automatic gearbox), but no doubt that's another problem. :(

Hello

if there's a general lack of power then this will make an auto box slow to change gear too as the transmission controller judges everything on the amount of air / load the engine is pulling

and if there's a lack of power then you'll get slow / lazy gearshifts too

try measuring fuel pressure next and let me know

Hi
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Thanks Matt

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