Just joined the Jag family half a year ago with a 2012 XJ 5.0 NA with an amazing 19K miles. I've been having some minor (I hope) rough idle issues that I was hoping to find answers for. In the colder months the engine RPMs jump between 1500 to 1000 many times before engine warms up and RPMs drop to 600. I lived my whole life in Austin prior to moving up to NY for work so I never seen this with the 2 cars I've previously owned but I think it has to do with the colder temperatures here up north since this issue went away after the weather started getting warmer. The issue that's still occurring is a rough idle (I think?) where I feel vibrations throughout the car but the RPM are stable and don't jump at all. There's no check engine light and I feel the vibrations mainly through the seats. Its a consistent rhythmic vibration with a larger vibration randomly throw in. I went to 2 local mechanic shops and they both told me that they don't have the tools to read the Jaguar system unless it showed a check engine light. I spent $1500 already on a bunch of tests and a MAF and would like to see if I can narrow it down to a specific part or system. Changed the MAF and that had a very slight decrease in vibrations (might be placebo to be honest), They did a smoke test that showed no vacuum and exhaust leaks, checked spark plugs, checked for carbon build up, replaced air filters.

Going over budget for the repairs & maintenance I set aside for this car between brakes, tires, paint correction, and now this so ideally would like to figure things out myself if possible,


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I also shifted the car in neutral to see if vibrations go away but there was no change in vibration intensity. Was told this was a way to check if the issue is with motor mounts

 

Not sure if this is useful information but I've been getting about 8-10 mpg in local driving in NYC and as high as 30 mpg cruising 60-70mph on the highways with no traffic

 

Not sure but a guess?
Maybe engine mounts and transmission mounts like you said? Hard to believe since you only have 19K miles too.
No way to really tell until they are replaced unfortunately.
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Yea, it's a struggle figuring out what's wrong. The Engine mount looks good, tested by revving up engine and seeing if it moves around too much & idle stays the same from park to neutral. Will do some research into testing the transmission mounts

 

Are you filling your car at the same gas station all the time? I will change gas station or to a different (Top Tier) brand. I like Shell, Exxon. Mobil & Chevron.

 

I switched gas stations too, Gulf, BP, Mobil. Been using Mobil recently since its tier 1 gas but its the same issue. I don't know if the fuel filter needs changing? I also tossed in some techron fuel system cleaner.

 

I don't know XJ's fuel filter, if it's inside the the gas tank or outside.Did your mechanics check or changed the filter too? Or fuel injectors could have to much varnish on it?

 

I think the filter is on the inside so none of the 2 shops checked it. But Jaguar recommended change interval is like 10 year/100k miles I think but with this having 19k miles im not sure if that's the culprit.

 

Yours is a 2012 with 19K miles? Wow??? I have a strong feeling the injectors are clogged up due to low mileage use.

 

Yea, 19k miles, it was a vacation car at a vacation house for the previous owner averaging about 2500/year. Car has barely been driven since 2019 until my purchase. When I fill up my tank it shows 390 miles of range instead of 477 as advertised by Jaguar so perhaps injectors may be the issue. I averaged 26 mpg combined in the last 50 miles or so. Any idea on how to check injectors without going to a shop?

 

wouldn't it be easier just to run injector cleaner in the next couple of tanks of fuel?
If so, and you wonder what kind of injector cleaner to use, search existing posts for a variety of opinions.

 

I used Techron fuel system cleaner about 1000 miles ago without any changes in symptoms or performance. Not sure if using another bottle so soon would cause any damage to the car. recommended intervals is every 3000 miles per Techron.

 

First, congrats on finding such a car. I have no doubt it is awesome. March 2021 I got a similar deal on my 2011 with only 14K on it. Got 23K on it now and zero issues so far.

A few thoughts...

At what RPM does it smooth out..if ever? Is that consistent? If you hold it at the smooth point does it stay smooth?

Have you tried turning the a/c off entirely? Failing compressor clutches can lock/unlock and make for some odd vibrations, particularly at idle.

Or removing the belt and running when cold for a few minutes? Then you can check each pulley on the path for above normal drag while its off. Unlikely with such low miles but easy to check it off the list of possibilities so why not.

The Techron is safe to use more often than 3k miles. When I first got mine I put it in every tank for the first 5 fills with no concern. I did it mostly to see if it had any effect on the fuel tank level sensor which would sometimes go wonky and say empty. It did seem to help but I still have that occasionally happen right after a fill up. 3K is just the point that they would like you to buy another bottle of Techron. I would drop another bottle in there at this point.

 

Chevron Techron is a decent mild injector cleaner for normal maintenance and as Rothwell said is good for cleaning the in-tank level sensor rheostat.
But there are other injector cleaners that are more effective.
At least more Techron if not Lucas or something.
Then there's induction cleaning, but you could find more on that in existing threads.

Rothwell's good suggestions may help avoid that though, if you find it's a compressor or something like that.

 

Congrats on your car! Seems to be a similar situation as mine where its a vacation/winter car. Mine isn't as up to date on servicing as yours though with oil changes every year or 1.5 depending on when the previous owner came back. The RPMs smooth out to 600 ( not sure if this is correct idling RPM or not) after about 10-15 full seconds and hold steady at 600. RPMs also seem to be stable while accelerating and maintaining speed in correlation with the pressure I put on the gas pedal. Stable 2k RPM while I'm cruising at a consistent 70-75 mph. I did also notice a delay when flooring the car from a red light onto a highway on ramp while in Dynamic mode but whenever I hard accelerate while car is in motion there doesn't seem to be any hesitation in Dynamic mode although in normal mode it does seem sluggish.

I haven't tried turning off A/C completely but will try that sometime today or tomorrow.

Can you provide some insight on removing the belt? I'm usually wary of touching components like that since I have no idea what I'm doing LOL.

I do notice that my fuel gauge read out is a bit wonky sometimes too, it would show that I have slightly more gas in the tank than previously stated

Should I use any other fuel system/injectors cleaners? I heard on other forums that I should use something specifically made for direct injection engines and I heard BG 44K has good reviews with the members here.

 

Surprisingly enough I have pretty much the exact same issue with my 2011 XJL NA 5.0 currently at 17k miles, bought it at 14.5k miles. My RPMs also used to jump between 1100-1500 before settling at 600. I think the vibrations feel more like engine vibrations coming into the cabin through the seats and floor rather than a misfire/rough idle. In my situation I had both MAF changed since they showed different stats and that provided a slight decrease in vibrations (think 10-15% reduction in vibration intensity) The mechanic shop I went to basically did the same thing they did for you (checked spark plugs, MAF, smoke test, checked for carbon build up with a camera) they said with a 10+ year our luxury European car there could be a bunch of reasons as why the car vibrates as it does and without a full tear down they can't guarantee identifying the cause so rather than spend more money into the car they suggested I get insights from a Jaguar dealership or come back if symptoms get worse or check engine light comes on. I have a bunch of old school mechanics in my area who do really well with traditional cars but aren't so keen when it comes to electronics.


A full tank for me shows about 386 mile in range so its either normal or there's something wrong from our cars sitting too long. Interesting to see a few low mileage (sub 20K) XJs pop up on the forums as well as car sale sites

 

You get the belt off by releasing the spring tension on #3 in the drawing. Then it just unravels off the pulleys. Getting it back on is not hard just tedious.
If you haven't done it before probably best to get some help - a shop could do it in 15 minutes or less.

Seems like you should keep treating the fuel a few more times regardless. My fuel gauge was so bad it would not even go to Full after a fill up. It would eventually creep up there due to the varnishing. Much better now and continuing to improve I hope. Techron or BG is fine.

 

Also - to access the belt, you need to first remove the top engine cover. You lift-up on the front end of it until it releases, then pull forward to remove it.
Then you have to remove the air intake ductwork.
For the Naturally Aspirated (NA) engine, there is one worm gear clamp on each side, where the ductwork meets the air filter box. Loosen the clamp at each side w/ screwdriver or nut driver & pull back on ductwork until it comes off of the airbox (MAF sensor with wires stays with the airbox side).
There is one small bolt (10mm) that secures the ductwork to a metal bracket at the top-left side of the ductwork. Remove that bolt & when removing ductwork (after the next steps), pay attention to guiding the ductwork out from under that metal bracket.
Then there is one more worm gear clamp where the ductwork meets the throttle body. Loosen that clamp too, and wiggle the ductwork CW/CCW to pull free from the throttle body.
Then there is a vacuum hose that connects to the right side (as you're looking into the engine compartment) 'inboard' of the ductwork - you have to look at the lower-backside of the duct to find it.
This hose is released by squeezing the two sides of the quick disconnect connection on the hose & pulling it from the ductwork connection.
Then there may be a hose routing clip under the left side of the ductwork that the cooling hose is kept in place by. Before pulling the ductwork out of the way, make sure the hoses are clear of the keepers that are on the bottom-side of the ductwork.
After those are released, the ductwork should easily be removed (remember to guide it out from under the metal bracket by pulling it forward enough to clear it) and set-aside.
Be careful to NOT LET ANYTHING get into ANY of the air ducts that are now exposed. You could cover them temporarily with sandwich bags or something if you like.

With that out of the way, you can easily see the belt.
Since you're there, you may want to go ahead & replace it unless it's been replaced within the last few years. Belts are pretty cheap. Paying someone to replace them, not so much.
And maybe the same with the air filters, at least check them out since you've got the ducts off of the air boxes.

Put force on the tensioner in the correct direction to relieve the belt tension, if it takes a lot of force, it's probably the wrong direction. IIRC it takes a 1/2" (or was it a 3/8"?) drive ratchet (I use a breaker bar) into the square hole on the tensioner that's there just for this purpose.

BE CAREFUL with your fingers (of course), not that anything will bite you in there, but just general safety.

Re-installing the ductwork & engine acoustic/vanity cover are just the reverse of removal.

I don't think I forgot anything.

 

Vibrations still on going with AC turned off, had a mechanically inclined friend take off the belt and inspected the pulleys, both appear fine.