Rough Idle misfire 2006 XJR x350
 

Last night I had a bright idea to just pull the plugs and re-gap them....

Heres the situation: on the '06 XJR i had been having a slight issue with a rough idle and classic bad-combustion-if-you-need-a-tune-up feel to the motor. So initially I thought it must be the MAF sensor. So I checked and cleaned it. It was sending an steady number to the ECU.

Then i thought to change the spark plugs, so I did... I used NGK Iridium plugs. They came highly recommended. So i started it up and i felt the rough idle again. Honestly, I'm not sure if the guy i got to do the work for me actually checked each plug for the proper gap.

Bottom line is the car still idles roughly and I need suggestions as to what may be causing it.

By the way, there is no performance lag. Still does 0-60 in 5.3s on the street.

First thing you should do is read the long term and short term fuel trims. At idle and holding 2000rpm in neutral.

STFT is fluctuating between 5.5 and 7.0% ...now holding steady around 9.4 in Park... definitely adding fuel....


LT Fuel Trim is at 4.7 %

I fear a leak somewhere. HELP Cambo!

UPDATE... after 5 minutes of warmup... STFT has normalized at +- PLUS OR MINUS 0.0% - max 3.9 %

STILL IDLING ROUGH AND MISSING randomly.

Bank 2 LTFT coming in now at idle... I'm seeing 8.6% on Bank 2. Thank God I scrolled down to see that info on my code reader.



Any thoughts?

Which code reader
 

Which reader are you using for the trims?

actron cp9185

Plugs are likely fine (treat iridiums with kid gloves, they're delicate, do not re-gap, avoid feeler gauges). More likely to be a leak or bad coil(s).

Engine hot, parked, rev to about 2500, does that 8.6% head near 0? If so you've a leak but not a big one.

my thought was a dirty fuel injector - how many miles?

have you run a can of techron thru it to see if it helps?


Ditto on what JagV8 said about the iridiums; they come gapped for the application, I never mess with them other than some dialectic and anti seize (although you can gap them, google it).

In your case, as in most, just ignore the STFT. That value will vary widely positive and negative, that's its job. Pay attention to the LTFT. For some background you can seek out my papers on fuel trims here on the Forurm.

https://www.jaguarforums.com/forum/g...el+trim+primer

You should be OK if LTFT is less than 7-8% plus or minus. Higher than that will set faults. What you are looking for here is probably LTFT that is off more on one side than the other. Follow Cambo's suggestion of checking both banks at multiple loads and RPM's.

Since you have changed the plugs, and Iridium is the way to go on your car, you must have some kind of an air leak on one bank, or a possible ignition coil contributing to a misfire. An injector problem is a possibility, but way more rare than the other two.

Finding the reason for the roughness is the key. The fuel trims may help pin the problem to one bank or the other.

Good luck!

Thank you guys. Im currently testing at different loads and checking the LTFT. Preliminary, it seems the 8.6% is approaching 0% as I continue normal driving.

I hear this constant ssssssssss under the hood that someone said was normal for these cars (a dealer mechanic). I suspect I may have an air leak somewhere that I can't find.

I have tried this before, but should I try this on my cat as well? Spray carb cleaner around the area from where the sound is coming. And if the motor cuts off our lags that may be the leak area.

Going outside now to try. Brb.

Where is the port for me to run the fuel system cleaner? Can find a good spot.

The hiss, especially on the RH side of the engine, is very much a normal thing for the 4.2L engine. Ignore it.

Techron is an additive used in the tank, and runs through the injectors that way.

Now here is where you CAN use the STFT. Set your device to watch the STFT PIDs and while watching them, spray suspect areas around the engine with carb or brake cleaner. Don't forget to spritz around the EGR valve, too. If a leak is there, the cleaner will be sucked in and you will see the STFT spike negative. If you can RELIABLY get the negative spike in the trims at one spot, there is an unmetered air leak there.

Good luck!

UPDATE:
No results for leak tests (besides the horrible residue on the black plastic which i need to now clean).

the hissing is still there as you described.

Here is the image of the LTFT and STFT at 2500 rpm then 650 rpm




https://cimg0.ibsrv.net/gimg/www.jag...bbcbb1b077.jpg


At Idle

oops

https://cimg3.ibsrv.net/gimg/www.jag...fc2d59e51d.jpg


Idle after 20 minutes approx

From that you definitely seem to have something going on with bank 2, the LH side. Frankly, we usually see the RH side with higher fuel trims than the left. If the engine is still rough, it must be misfires on the left bank. That could be ignition misfires or lean misfires due to restricted injectors, maybe.

You could try moving the ignition coils side to side to see if anything changes, or go the tank additive route to see if things smooth out any with that. Beyond that, you have to consider the possibility of a leak somewhere around the charge air coolers where they pass to the cylinder head. That is a good sized job.

As I said before, the cause of the roughness has to be found before you can really make sense of the fuel trims.

Good luck!