My 2001 Jaguar S Type 3.0 started driving really rough a couple of days ago. Its stutters while driving and idling. I thought it was just bad gas or something. I put some fuel injector cleaner in the gas tank but it did nothing. My MPG has suddenly gotten worse as well. Can anyone help me out.

 

Probably a vacuum hose gone bad. The awkward one needs the intake manifold off and is buried lower down. Lots of threads about this. Good advice in them about what else to do while it's apart.

 

Is your check engine light on and flashing? Have you checked for error codes? On a normal car this could be a bad coil pack. On our S-Types this could be that, could be worn-out spark plugs, seeping upper intake manifold gasket, or the infamously cracking PCV valve hose... If you have the time and skills or the money and somebody else's skills, I'd get them all changed out at one time while the manifold's off.

 

Same problem and same solution mentioned one post below yours fixed it. Not a hard job but an involved one. And has given me a few more grey hairs (broken bolt in the head for one) there is one more "document" that I found on cadillac vs Lincoln that was the best piece, it was done on excel and I printed it and followed along. I also found it was worse the colder it was. Oh and I was in denial for a long time trying everything but the elbow!

 

It's almost always the elbow.

LTFT values will confirm but at the age of your car it's due replacement.

 

I've had it happen twice. First time it was a vacuum leak from the bottom of the airbox because the tube was not seated correctly. Second time it was a coil.

 

You have a completely different engine and different year. The faults like this don't tend to be alike.