So I've been fighting a P0308 misfire since I replaced the HGs. I've replace pretty much every component available, and I'm starting to think it may be valve train related.

While I was speaking with a fellow Disco owner last night, I recalled having problems with the rocker assembly bolt on the passenger side. All the bolt holes on the heads had garbage in them from the hot dip and I remember having a heck of a time with one almost cross threading. So I was planning on opening up the valve covers to check on my rocker assembly on the passenger side. My thought/fear is that maybe I didn't properly seat that last rocker pedestal or something is too tight/too loose.

Is there any issue removing any part of the rocker assembly while the heads are still mounted? Is there anything specific I can check clearance or fitment wise besides the torque on the rocker assembly bolts?

 

As long as you back each of the pedestal bolts out a bit at a time there is no problem doing this while the heads are on. If the heads were ever surfaced to correct warping you might need to install pedestal spacers to restore the proper push rod geometry. Also, if you rebuilt the valve train double-check the locations of the rockers to ensure the intake and exhaust are in the correct position for each cylinder. Good luck.

 

I didn't rebuild the valve train, but how can I check the rocker locations or if the pedestal spacers would be required?

 

If you didn't rebuild the valve train then the likelihood of rockers in the wrong location is probably zero, but you can check easily with the valve covers off by looking to see if the rocker surfaces line up with the valve stems. Pedestal spacers would only be required if the heads or block were skimmed at a machine shop.

 

Yes, the heads were surfaced. According to the shop notes they took off .005 but I'm not sure if they had been machined previously (I did my HG job at 170k).

 

You probably got a bad wire that got heated up and it is sparking or grounding.
Or a bad plug.

The hydraulic lifters will compensate for .005 so I would not dig into it that way.
You might have a sticky valve.

Or it could be this:

Land Rover Valve Jobs - Erik Burrows.com

 

I've replaced all the common misfire components (coils, wires, plugs, injectors, front O2 sensors). My misfire seems to remain with cylinder #8.

Is it possible to check/verify or resolve a sticky valve without removing the heads?

 

Check compression.

 

I wanted to follow up to close out my own post.

I tore everything down to the rocker assemblies. I soaked the rocker assemblies and rocker covers in a strong degreaser (PurplePower) to clean them up. They were filthy. I also replaced the oil separator while I was at it. Nothing else was replaced besides the necessary gaskets/orings. After putting it all back together my misfires are gone. It could have been a very dirty/stuck rocker assembly component or an electrical connection that was reset more securely upon reassembly. Who knows. I'm just glad to have my Rover back!

Here is a pic of my initial reassembly with half-assed cleaning of the rocker assemblies:

Here is a pic after tearing back into it and giving the rocker assemblies a proper cleaning:

 

Purple power did that? Impressive.