Last year my 95 xj6 x300 while driving to work and sitting at a red light motor started shaking and lost oil pressure. Popped the bonnet and realized i had a big hole in the valve cover. Had it towed home to get it fixed.

Searched forums to track down possible reason and best i could get was timing chain tensioner needed replaced. Ordered replacement and my mechanic cracked open value cover, replaced upper timing chain tensioner and found old tensioner to still be working fine.
After replacing spark plugs since we found oil in two of them from worn down valve cover gasket. Followed the steps for putting in time, replaced valve cover gasket, and repaired hole in it as well. Car still ran like it had before with the rough idle. Took out for drive had to drive with both feet and at a stop if the rpm drops below 1000 engine starts shaking and sputters and dies.
Thinking i had a vacuum leak inspection time to locate. MAF in working order, found cracks in exhaust manifold and hole in muffler. Ordered replacement downpipe from exhaust manifold and had the cracks welded back. Replaced egr gaskets as well as cleaned trottle body and checked the vacuum hoses by ether test and unable to locate any leaks.
Now after all the repairs still not getting it figured out because still running like it had initially. Connected code reader and 4 codes are available



Mechanic said he failed to mark O2 sensors where they were on old downpipe for when replacement one was installed.

Could this be the entire issue with my rough idle and running rich? If so, it still doesn't explain what made the hole in valcue cover that started the problem of rough idle initially.

Someone have any ideas ive missed? Im scratching my head.

 

The hole in the valve cover certainly needs to be explained. Do you have any photos of the damage?
Is your battery fully charged? I would make sure it is, clear the codes, and scan again

 

You can tell if the O2 sensors are connected the wrong way around by checking the STFT for both banks (you need a code reader with Live Data capability).

The STFT will be maxed +ve on one bank and maxed -ve on the other if the O2 sensors are attached incorrectly. (Usually about +24 and -24).


Also check that coils for cylinders 5 and 6 have not been reversed. This is a common mistake when replacing spark plugs/valve covers. Coil 6 sits in the opposite orientation to all of the others and it should have a piece of yellow tape on the wires.

 

According to the guy who worked on it it was due to my upper timing chain tensioner letting the timing chain listen enough to come up and smack into the valve cover. But when I replaced said tensioner he said the tensioner was in perfect working order. Says it must have malfunctioned but it was still working when I replaced it.

 

Looks like 3 separate issues happening.

I've never heard of a loose timing chain putting a hole in the valve cover, so it must have been very serious! I suppose a tooth jump could have occurred too. That will cause rough running.

Code P0400 for the EGR probably means it is very dirty and possibly clogged or otherwise failed. That could cause rough running. An EGR recall (R493) was ordered and you should have a visible label above the radiator if it was done sometime in the past. The update made the EGR valve cycle after engine shut off to help keep it cleaner and operating freely.

Codes P1191, 1192, 1194 are all related to the downstream O2 sensors. They were either damaged from the repair or connected improperly, as Brendan said. Certainly a cause of rough running. Your scan tool doesn't describe the P codes very well, not even mentioning the O2 sensors. Code 1192 is for O2 heater short circuit. Codes 1191 and 1194 are for O2 heater resistance fault. I think this is the most likely cause of your problems now.

 

I hooked up the scanner and my life data had more than one STFT but both were reading 24.2