I have always encouraged my wife to run high octane fuel in our 2001 4.0 S type. However when i drive the car i noticed it runs poorly , when accelerating the car wants to buck or maybe misfiring , when i floor it, it seems to kinda run better but i know its burning lots of extra fuel and in most cases the bucking or misfires continues. of course i blame the wife for putting the cheap fuel to save money, We recently paid a local shop $200 for a fuel rail or fuel injector deep cleaning that i was told would really help.. After that i did notice some improvement and again told the wife , from now run the high octain premium fuel.

Now when we driver the car it still wants to not run smoothly, kinda bucks, seems to misfire or fall on its face when trying to accelerate lightly . What do you all think? Are there maybe a few fuel injectors still dirty or maybe bad or warn out? , or they need to all be replaced ? Could i be wrong and maybe its more on the ignition side like some defective coil packs.

Unfortunatly i dont live near a jagaur dealer to take for a diagnosis. And hate to start throwing new parts into if I,m not confident it will fix the problem. I need your all help.. Thanks ,if we need to talk you can call me at 812-430-5744 ,8a to 8p Scott

 

I would get a code reader and see what that throws up, it could be a coil failing

 

+1 on that, BUT... If the restricted performance light or check engine light has not come on, there won't be any stored codes... You may see some pending codes that will help, but won't be true codes without the light.

See, considering the age of the car a rough run and stumble condition that you describe can be a ton of things... Anything from the previously mentioned failing coil to dirty or bad MAF sensor, dirty or failing O2 sensor(s), failing catalytic converter, and the list goes on and on....

While technically running regular gas should not hurt the car, it will cause it to not run "at it's peak"... But wouldn't cause random miss-fires or stumbling like you describe. I do highly recommend running premium, since that's what the car is programmed to run on..But I wouldn't come down too hard on your wife since age is probably more the root cause of your issue than 87 octane gas. Again, running 87 doesn't exactly help the situation either...

 

+2 on the above - sounds like a coil issue to me. Is your check engine light on? Be aware - a intermitient misfiring coil will not throw the CEL but you will realize the engine is not running smoothly.

 

I suppose the other not so likely thing could be timing, as I think the car will have the old tensioners and they're a weak spot. Go for codes first. If none, I'd then look at fuel trims at idle & 2500rpm (in P or N). They'll reveal whether you have an air leak - which you likely don't on a 4.0 but diagnosis goes a whole different way if you do.

That said, odds on it's coil(s).

 

It may very well be failing coils, but I would still clean the Mass Air Flow sensor, especially if it has never been cleaned before. Our vehicles always run a bit better and smoother after I clean our MAF sensors. On our two Jaguars, I do it at every oil & filter change. Remove two screws and out comes the sensor - very easy to do. Get a spray can of MAF sensor cleaner at any auto parts store and have at it....

 

Good advice- and there's NO connection between octane rating and 'quality' of fuel. All fuels contain sufficient cleaning additives to get the job done and there's no inherent technical reason why a high performance engine needs more cleaning additive than a low performance engine anyway.

Look elsewhere as already suggested.

 

And I am guilty as charged... I tend to not speak in octane...but in "regular" "mid-grade" and "premium"... To clarify.. In my language..Regular=87 octane, Mid-grade=89 octane, and Premium= 92 octane... Again, this is US octane which is different than the European measurement. Also, when I say premium, it is octane and not quality I speak of.....

 

Fuel in Europe is also frequently identified as 'super' or 'premium' or some other superlative that infers a connection to quality. Clever those marketing people...........

 

It's true

 

thank you, that is exactly what i will do first. Good place to start

 

thanks for all the great advise, i will clean the mass flow sensor and definatly get the codes read. the check engine light has been on a few years, i replaced 2 of the 4 o2 sensors years ago when i had the codes read. i took the car back to a local car repair to have the engine light reset. soon after that the light came back on. im hoping the codes have locked in some defects. if not, im thinking the some of the coil packs may be bad. However i need to find a place that is familiar with jaguars. or take to a dealer to have the codes read.

 

Are the coil packs that were installed OEM or aftermarket? Many aftermarket coils are sketchy at best! Stick with OEM - I know the costs are higher but you will be highly satisfied.

 

It's not wise to ignore the light. First, real damage may be done (such as destruction of cats). Second, if further codes occur you may really struggle to diagnose the confusing mess.

 

+1 on JagV8 - you have to get to the bottom of the check engine light issue - not good to have that light on all that time - I'm sure there are codes stored - let us know what they are.