I have a 2000 Jaguar S-Type 4.0 and have had ongoing rough idling issues. So far, I have replace four coils to no avail. No codes are coming up but my idle is still rough. Performance on the road is fine. I finally noticed, however, as I set it in park and pressed the gas at a steady rate, the rough idle disappeared and reappeared as soon as I took my foot off. After doing this several times, I am now thinking I have a fuel issue, maybe the injectors or something related. I have gone to 3 different mechanics and no one has found the problem. I keep paying for new coils, new plugs, oxygen sensors, etc. but to no avail. Does anyone know what might be causing this problem and what I can do? I have tried a fuel injector cleaner before but figured I may indeed have had coil problems and now that those coils were replaced, I am going to try a different cleaner once again. Besides that, any ideas?

 

I would do a basic tune up! spark plugs - fuel filter - air filter. You would have got codes on a miss fire if the coils were bad. Maybe even a injector cleaner , Good luck

 

More likely an air leak. Would be much cheaper to spend money on diagnosing what's wrong than throwing parts at the car in the hope of getting lucky.

You're short of hard data. Ways to get some include hunting for air leaks - covered here many times if you Search - whether by squirting stuff or OBD. Or a competent tech (too often a "mechanic" knows mechanical stuff to do with engines but not how to find other issues, TBH they're only part-competent).

You can check O2 sensors using OBD, too, but I'd use it to look at fuel trims (good words to use with Search).

 

Yup, that's what I was thinking also. Check your vacuum hoses for leaks.

 

Had it scanned but no codes are showing. Shouldn't a vacuum leak trigger a code? I have only put about 10k miles on a set of double platinum plugs but will go ahead and change the fuel filter. If, in fact, it is a leak, wouldn't it still be idling rough when I give it gas? Like I said, I can put it in park and hold down the gas pedal and it runs smooth but as soon as I take my foot off, the rough idle is back. I also just tried some fuel injector cleaner in my gas tank but as of yet, nothing has changed. Could a faulty valve cover gasket create this problem? Can someone address the possible reason why the idle is only rough when I don't give it gas?

 

You need to do some reading. Search is you friend, just need to use it intelligently and with some effort. Much cheaper than parts. Or second-guess instead as you just did.......

Easier to guess wrongly, though, than read others' experiences and fixes.