I bought a beautiful cherry red classic a few months back, 161,500 on the motor now. Just took it on a decently long road trip (about 1500 miles round trip) and had no issues. A few days after being back home, I started to get a rough, lurching idle. Then it started throwing the P0306 code. Changed all 6 plugs and the coil pack in the hopes that it would fix the issue, it felt like it was running a lot better but today I took it on the highway for a bit, filled up my gas, and immediately it started acting up again and throwing the same code (check engine light started flashing as well). Was advised to use some carb cleaner to check for a vacuum leak so I went over everything I could think of and didn't find anything. Not entirely sure where I should go from here, and any help would be much appreciated. I'm decently mechanical, but I don't have access to too many tools or a garage to work in. TIA

 

Welcome, Nick.

First, stop throwing parts at it.

The P30x codes can sometimes be misleading. It may point to the culprit, or it may just point to the last cylinder that misfired.

I'm not sure how the coil pack setup is wired, but I'm pretty sure you have to have a wire going to the coil pack to tell it when to fire. On the earlier XJs, and probably on yours, the coil is supplied 12v all the time, and then the PCM provides ground when it wants the coil to fire. That means you have two wires, and you need to check the 12v supply, too.

Id' check those wires for damage, for starters.

Also, what was the condition of the old plug? Can you post a pic?

 

I would start with a compression test and that is cly # 6 that is misfiring and then look at the injector and the wiring

 

Unclip the harness pigtail from #6 injector and plug it back in. Maybe spray some contact cleaner in the plug while you're at it.

 

Hey all, appreciate the advice from everyone. I went out and checked out where the wiring harness connects to the coil pack, as well as the connection on the #6 injector. I'll attach some photos of both. I don't see any fraying or damage to the contacts, IMO. Think my next step is to take it down to O'Reilly and borrow their fuel pressure kit, then I'll check the resistance on the injectors while I'm at it. When doing so, should I be checking the connection from the wiring harness, or on the injector itself?

 

Coil pack connection

#6 injector

#6 injector

#6 injector

The troublesome beast herself!

 

Fuel pressure at the fuel rail was consistent at 46 psi and did not fall off quickly after the engine was shut off.

I tested all the injectors except the 3rd (didn't feel like running upstairs for my socket set), for resistance and I got readings of 13.4-13.6t ohms for injectors 1,2, and 5, then 15.4 and 15.6 for injectors 4, and 6 respectively.

Thinking my next move is to run a compression test on cylinder 6?

 

Nope. Do them all. The variance between them matters, too. And you might as well know.