Hi all, my 4.0 S Type (2001 model year) is not throwing up a "Check Engine Light" yet, but a very slight miss or "fluff" at idle is now being observed.
Some background: I recently had the timing chains, tensioners (new metal ones) and guides all replaced, the timing has been checked and is bang on. New factory spec spark plugs, gapped correctly, were installed as well. So I don't suspect ignition. I believe it to be fuel system related. I've run some Lucas Fuel System cleaner into the gas tank and ran it 30-40 miles. The car runs like gangbusters, except for the occasional fluff at idle.
So I suspect that an O2 sensor(s) may be just beginning to head south, but not yet enough to trip a code. My question is, after a fair bit of looking around and seeing information on wrong sensors, which are the correct upper and lower sensors to get for a 2001 4.0 S Type? I'd appreciate any correct part numbers so I get the right ones. I'll look at replacing all 4 just to be on the safe side, as they are all the same age.
Or, should I just run it more and see if the Lucas stuff "cleans" the injectors more and hopefully corrects the idle issue. Many thanks in advance.

 

Chances are you have an air leak.

What do the fuel trims show?

Then, if they show no issues at all, what do the O2 readings show?

It's not cheap to replace potentially good O2 sensors...

BTW the "Fuel System cleaner" is unlikely to help but you could get lucky.

 

Ahh, thank you, that air leak idea sounds like a good place to start. I agree, its rather silly to spend $$ on things that may not be the problem.
Is an air leak on a Jag 4.0 the sort of thing I can spray carburetor cleaner around to see if engine RPM changes? Or is it better to explore some of those "hidden menu" items outlined in the Tips Section at the top of the forum? I'm kind of new to the Jag world, so all help is appreciated and apologies in advance for "dumb newbie" questions....

 

With your symptoms if it is an air leak it will be a small one. So I doubt you can find it with the old spray carb cleaner trick unless your monitoring the fuel trims real time.
I had a rather large air leak in my old 2005 STR and the engine ran really well other than the check engine light but when I monitored the fuel trims they were large and swinging all over the place. The cars electronics are very fast and compensate quickly for any changes.

I would plan on getting a decent scanner for Jaguars as they are not too expensive anymore.
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No hidden menu will help at all.

Get used to OBD II - it's been on all (petrol) cars for over 20 years and can really help with issues.

 

Turns out I have an OBD scanner (that I'd forgotten about) that I bought a while ago for some VW's I have. Hopefully it'll work for the Jag. I'll report back in a few days with what I find. The miss is getting a bit more pronounced now, both at idle in Drive and a bit in Neutral as well. Let's see what we find

 

Follow-up on the stumble at idle... well after having fuel trims checked at a shop that is familiar with these Jags ( I was told the trims were ok), fuel filter replaced (was pretty grungy), fuel injectors flushed twice, throttle body cleaned, the slight stumble at hot idle remained. So I began to think that it maybe wasn't fuel, but spark. There were no warning lights or codes being thrown yet, but looking through the car's service records (now have 204 000 km's on it) I saw one or two coils were replaced along the way a number of years ago, and the rest were originals. So I located a new set of 8 Denso coils and coil tubes, and spent the afternoon installing them yesterday to go with the new iridium plugs I had installed in December when the timing chains/tensioners and valve cover seals/plug seals were replaced.
Result -- stumble is now gone and pickup is noticeably improved. When I floor it the V8 now revs freely up to 6500 before it shifts, it hadn't quite made it there in a while as it had shifted a bit earlier at WOT. (I don't normally drive it like that all the time, but it was fun to see...)
Anyways, it now purrs like, well, forgive me but... a cat.
When I reconnected the battery, I did the "press the throttle slowly twice to the floor" trick to reset the shift (?) parameters that I read about in the Sticky at the top of the forum, then took the car out for 30-40 km drive. The transmission still seems to hold on to second gear when I come to a slowdown or stop for a little bit before it goes back into first. I'm assuming the transmission is still re-learning itself and this will settle down as I drive the car more (?). Aside from this minor issue the car now drives great.
So anyways it turned out to be a coil issue... like I'd read so many times on other posts here. Add this one to the list. And thanks to all for their suggestions, I'm always listening and learning!!