x300 performance problem

[QuiteQuietCerb]

hi i recently purchased a 4.0 xj6 with lpg kit. the car runs great on both and accelerates ok to 90 but after that the revs dont rise as fast and acceleration is slow and doesnt want to go past ...any suggestions ? could it be lack of use or plugs?

[Jaguar steve]

Any loss of power due to spark plugs will be accompanied by vibration under load due to missfiring. Plugs either fire the fuel mixture or they don't and with high efficency coil on plug systems plugs have to be in a terrible state not to work.

Loss of performance is difficult to define and diagnose so think about what are you comparing it with? Sure, 4.0 litres is a big capacity engine but the X300 is a big heavy car and may not have the top end performance you might expect from such a large engine even when in good condition.

You could try substuting the air flow meter for another known good one but I'd suggest a diagnostic check is probrably worth doing. If you do have a failing AFM causing a weak fuel mixture and lack of performance and drive the car harder to overcome it you'll burn valves out. If you do actually have a problem I'd suspect a fuelling fault rather than ignition from what you describe.

[QuiteQuietCerb]

thanks for that, not expecting huge performance,but was not able to keep up with my mates diesel vectra, i will investigate your suggestion though

[SamPet]

I have the same car and can sail past 90.

MAF is a good shout

What about a cracked manifold - somewhat common on this model I believe

[Jaguar steve]

SamPet said:

I have the same car and can sail past 90.

MAF is a good shout

What about a cracked manifold - somewhat common on this model I believe

It is, but you'd hear it and it wouldn't affect performance

[nc67]

i also had x300 on lpg until recently, it would fly into treble figures without a problem, and it was only a 3.2, it did suffer once with low peformance, it turned out to be air flow meter, but as j s, says prob best to get it on a diagnostic

[QuiteQuietCerb]

Thansk for this guys i ll get that checked, only other Jag i had to compare against was my xjr6 and that was a different animal, anyway does the maff not show any other symptoms (i.e idling issues etc)