I left a door open today for several hours which flattened the battery, so I put the charger on and while it was charging I thought I would check the spark plugs, had the XJ a year now and it's always had a very slight miss fire on idle so I thought I would check the plugs.
Removed the coil pack and checked all plugs, they were all good colour and condition so put them back in and fired it up! Ran really rough so I took the coil pack off again to make sure it was on properly, it was, re-fitted it and still rough!

Also something really odd happened, the oil pressure gauge shot up to max..could the just be a coincidence the sender failed at the same time.??

The plugs are Bosch by the way,anyone know what's happened here. Wondering if a plug was dicky and taking it out has made it worse!! dunno

 

You didn't cross the wires on the charger, did you?

 

And I'm assuming you have a 00-01?

 

Were any boot off of coils cracked causing possible spark shooting to ground causing miss

 

Make sure none of the springs inside the boot fell out or are jammed up in the boot. Also check the boots don't have carbon tracking inside. You didn't put anti seize on the threads did you? If the plugs are leaking or the threads stretched from over tightening, they'll can lose compression and cause misfire. If it were mine, I'd put Champs in and use dielectric grease. The Champs have the proper heat range and ribbed insulators. Some Bosch have smooth porcelain insulators and cause spark leakage.

 

Yeah, it's a 2000 4.0 XJ Classic 75kmiles and no rust at all

Didn't cross the wires on the charger although I did start it still connected.

With the oil pressure gauge/sender issue, it's strange because the sender is just under the coil pack but I didn't hit it or touch it even...very strange as that gauge has been spot on until now and I glance at it all the time when driving, just sits above half at 2krpm.

 

I never actually removed the coil pack as I couldn't work out how to disconnect the connector at the firewall end. I will take it off tomorrow and have a better look, also going to get some new plugs.

 

If the engine was running fine before you drained the battery then you probably did not put the coil rail back on correctly. You need to make sure all the boots are properly installed. As for the oil pressure, t is either a coincidence or you touched some of the wires going to the sender unit. Check you work.

 

It could be that your battery discharged enough to deprogram the computer. Mine did just that about 3 months ago. It took 30 minutes of driving it around two footed to keep it from stalling before it smoothed back out to normal

 

That sounds like it might be the problem, I am pretty competent with cars and don't think I have done anything daft. I will keep driving it and see if it smoothes out.

 

OK I found the problem.
When I removed the plugs, the one nearest the firewall was hard to get at with all the looms and stuff and I dropped it. This I didn't mention as I had not thought it worthy, however tonight I started the Jeep in the dark, no lights, pulled the hood light as well and I could see a feint blue flashing coming from the first plug (the one I dropped). When I got my head down it was clearly sparking to the engine block! Also the sparks were in sync with the missing.
So I seem to have damaged the plug when I dropped it.
Could the voltage going to the block have fried the oil pressure gauge?

 

possibly. If you have a ground issue the spiking voltage could have caused the sensor to be damaged...