Trying to pick apart my missfire issue. I've owned the car for ten years, it has 150k miles, and I have gone through some of the typical xj issues with rather minimal maintenance, and has been quite a reliable beast. I was tracking down a slight shudder on idle , which turned a spark plug change, into a valve cover gasket replacement (Oil in and around the coil boots, seems common).

This issue in particular will happen 30-45 seconds after a cold start, right after the idle settles down from 1300 to about 800-900 rpm. The entire car will change it's smooth idle into a shudder for a good 20 seconds, and then it automatically kick back into a smooth idle for ten seconds before continuing this same pattern. 20 seconds of misfire/entire car shaking, followed by 10 seconds of calm idle and it's repetitive like this. It seems to become a bit more severe the longer the car is on for, and another symptom is the car drops 500 rpms randomly while going uphill almost in a choking fashion, and I have to push the gas at least a quarter to 1/2 of the way down further which then jolts the car up 700-800 rpms instantly. If I depress the gas down much less than that when this occurs, let's say only a 1/8th increase , the rpms will not increase at all and it feels like a choke/sputter until I get to about the 1/4 way mark on the pedal in which it then corrects itself. This happens approximately every 4-5 KM.

Unfortunately this problem has reared in importance due to emissions test requirements while importing the car to canada, and the car is running rich, which I suspect may certainly be the largest cause to my miserable emissions tests failures. I could have a bad cat as well, but this missfire certainly isn't helping , so I want to diagnose it first before replacing the cat.


What I've done and replaced since the valve cover gasket and plugs in order-

-Fuel Filter

-Injector cleaners

-MAF sensor (cleaned not replaced)

-Air Filter (new)

-Plugs again (4 months passed of daily driving on new plugs, and the new ones showed snow white carbon built up on the plugs, bought a new set anyway)

-EGR Valve (It had a code supposedly and was plugged completely to the point of no suction, noticed a huge power increase after cleaning the valve, but still missing and having uphill choke issue still)

-O2 Sensors (Factory old ones showed white carbon also-strangely the problem got much worse after switching the sensors. All of the power I gained now seems squandered and the car now misfires harder during acceleration, and I took a huge performance hit along with a 10 MPG drop in fuel efficiency. My trusty local Jaguar extraordinaire says this may have been due to the old O2s being a problem for a while and the car was running them in a fault state which provided a confused and cautious rich mixture permanently which is now corrected, but still shows the problem to be elsewhere as it's still missing)

-Coils (had 2 spare known good coils, replaced one by one, problem remained regardless of which coil was replaced)


My next step from here is to take apart the fuel rail and manually clean the injectors, I've also been told to disconnect one of the two thermostat sensors when the car is hot to check for any difference in idle issues.

I don't have the exact engine codes it was throwing when we ran the obd, but my mechanic said it mentioned a cylinder misfire code, egr, but otherwise was clear.

Any advice or direction would be greatly appreciated.

 

The fuel trims would help understand what the lambda sensors are "thinking". When you say running rich, do you mean be exhaust analysis or OBD measurement? What is the actual OBD code number?
Out on a limb, I might replace the CKPS. Normally I ridicule throwing parts at a problem, but the CKPS is so hard to actually diagnose, and is relatively inexpensive, and is showing up in a lot of reports these days, i would be inclined to give that a try.

 

If you suspect one or more clogged cats, run the car to operating temp, then use an infrared thermometer to record temp readings of the cats.

Be sure to take readings at the front end of the cat and at its rear portion. Many clogged cats will have a higher temp forward.

This same trick can be used to measure temps at each exhaust header pipe and intake runners (cylinders firing equally?), and so on. I do this with my motorcycles too, for example, before and after a carb sync or throttle body sync.

Let us know!