Disco 2 V8 Intermintent Misfire

Hi all,

Chasing an intermittent misfire on cylinder #4, not sure what to try next.

I replaced all plugs, wires and both coil packs about 12 months ago, I had trouble free motoring with pretty good fuel economy until this summer.
On a hot day (well hot for New Zealand) having already driven into town I had a very lumpy idle upon starting, checking the plug leads found #4 and #8 slightly loose on the spark plugs, and #5 so loose I had to squeeze the connector together with my pliers to get a good contact.

The vehicle ran pretty well for a few months after that but recently started misfiring under load, sometimes I just notice a little loss of power but other times had quite a violent shudder that goes through the whole vehicle.
Checking the codes showed misfires logged for cylinders 3, 4, 5 and 8, but checking live data the misfires seem to only be on cylinder #4.
I put this down to the loose leads so ordered a new set of leads, checking the spark plugs some were a little fouled but nothing too major and nothing wet.
I installed the new leads yesterday, haven't done much driving but the live data still shows intermittent misfires on #4, always when driving.
I've pulled the spark plug again and it looked pretty black, I tried switching the plugs on cylinder #2 and #4 to see if it moved the misfire, but it's still showing #4 missing.


I'm getting good fuel consumption, not using water or oil, running well when not misfiring, the Oxy sensors are working as they are supposed too.
Should I be replacing the coil packs? Or trying something else.
I feel that it is an ignition problems as one would expect a fuel or air problem too effect more than the one cylinder and to happen less intermittently.

Any help would be greatly appreciated.
Cheers.

which v8 is it?
have you moved the plug to see if the misfire moves? You can also swap the coil packs to see if the misfire moves.
have you checked compression?

Are you torqueing the spark plugs to specs?
Like @kermit_rr mentioned... check for compression pressure.

My wife drove the vehicle to work this morning, said it misfired on the way too work but not on the way home.
I have new codes for cylinders 3, 5 and 8. All of those cylinders are feed from the RH coil pack, so maybe it's gone sour?

I'd rather not swap the two coil packs around as they are such a PIA to replace, but I've found the old coil packs I removed (they were fine at the time) so could try putting these back in.

I check the torque settings on the plugs and found they were all a bit loose, but they are all torqued to 15 ft/lbs now, I went for a test drive just now and all was well till right till the end of the drive and the live data on the Hawkeye started showing cylinder 4 misfiring again.
It's a bit strange that it keeps showing #4 as it doesn't log a fault for #4.

I'll get the wife to drive it to work again tomorrow, I have cleared the codes again so will see what it brings up.
I haven't done a compression test, but will do if it does it again tomorrow.

JUKE179r said:

Are you torqueing the spark plugs to specs?
Like @kermit_rr mentioned... check for compression pressure.

Click to expand...

I did a compression test today and all was well.
The RH bank was all around 165-170psi and the LH bank 170-175psi.

I did a 400km ish trip yesterday and the car ran well seemed to be running a little better than I remember, didn't notice anything wrong at all.
I checked the codes this morning and had a cylinder 3 misfire again, I didn't feel anything as I was driving so I might just call it a hiccup and keep driving as it is and see what happens.