How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Matt Your Own Question
Matt, Mechanical Engineer
Category: Jaguar
Satisfied Customers: 23195
Experience:  BEng hons Mech engineering, in auto industry 22 years
Type Your Jaguar Question Here...
Matt is online now

When running cold seems ok, but as soon as engine starts

This answer was rated:

When running cold seems ok, but as soon as engine starts getting warm, i have a problem accelerating through the gears. It seems to be fuel/air/heat related. I've had all the lambda sensors checked and replaced, new spark plugs and even a new MAF sensor but still having problems accelerating through the gears. I'm going to check fuel filter but as from that point i'm struggling to pin point the fault


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 its also worth trying loosening the fuel filler cap slightly as they have a 1 way valve built in to let air into the tank and so prevent a vacuum. if the valve is stuck the resulting vacuum can stall the fuel pump

do you still need help?
Bear in mind that the site takes a deposit from you at the beginning and this is held by the site until you rate my answer at which point the cash is split between the site and the expert. I am only paid for my work on this question if you rate my answer, using the star system at the top of the screen. Please do not forget! Thank you

Customer: replied 7 months ago.
Fuel filter was changed and big improvement is noticed but still a bit laggy after 4000rpm and above. Does the car brain need time to adjust to new settings/mixture as it’s been running rough prior to new filter and lambda sensor?


the fueling should adapt quite quickly within a 100 miles or so

I'd measure the fuel pressure if you can as this will show if the pump is OK or not

also worth changing the oil for fresh 5W30 too as the VVT units are very sensitive to the oil cleanliness and weight and also check the connections to the variable inlet actuators on the top of the inlet manifold

Matt and other Jaguar Specialists are ready to help you