This is a bit after the event since you've bought new coil packs but...
Brian_H has mentioned using some sort of spark quality test equipment to identify failing coil packs on some of his cars.
Think I'm right in saying a P38 coil pack only has 3 LT input wires, would it be possible to swap the LT input wires and HT plug leads and see if the potential fault condition (sooty plugs) moves to the other set of plugs... I.e. swap which cylinders the good coil and the suspected dodgy coil (within a single coil pack) supply spark to. Or just swap the 2 coil pack positions lol... Going on memory I'm thinking they have 2 coil packs and each has 2 channels, each channel sparking 2 plugs.

Gilbertd wrote:

What plugs are you using and how old are the leads? I reckon on NGK BPR6ES and change every 10,000 miles and generic 8mm HT leads (Island specials) and change every couple of years. On the old school Crypton tuner that had a proper CRT scope, that would show a higher voltage on cylinders that weren't running as efficiently as others. Might be worth a try if your scope can handle the voltage.

I'm using ERR3799NGK's from Island. Can't check the exact NGK part number without pulling one, but that part number sounds familiar. Also brand new generic HT leads from Island.

I'd need a heck of an attenuator to connect to my scope.

Lpgc wrote:

This is a bit after the event since you've bought new coil packs but...
Brian_H has mentioned using some sort of spark quality test equipment to identify failing coil packs on some of his cars.
Think I'm right in saying a P38 coil pack only has 3 LT input wires, would it be possible to swap the LT input wires and HT plug leads and see if the potential fault condition (sooty plugs) moves to the other set of plugs... I.e. swap which cylinders the good coil and the suspected dodgy coil (within a single coil pack) supply spark to. Or just swap the 2 coil pack positions lol... Going on memory I'm thinking they have 2 coil packs and each has 2 channels, each channel sparking 2 plugs.

That sounds like the Thor coil packs - GEMS has four coils, each feeding two plugs. I'd try that if the problem was more obvious, but a tiny bit of soot on a plug probably isn't going to be definitive, plus I don't know how long it'll take to soot up - the old plugs were at least a year old.

I might try hooking the oscilloscope up to the inputs to the coils, that might give some clues.

These are what I used - https://www.amazon.co.uk/Laser-2780-Ignition-Spark-Tester/dp/B003AN1VPW/

They are a useful tool to have (better than my mates way of diagnosing a spark or lack of spark by holding the lead near the engine which frequently ends up zapping him and is so hit and miss to be nearly useless). They proved my suspicions that the coil pack on the Disco had started to fail (and more importantly which one had failed at the time, though the other one followed shortly afterwards) and can show up very slight faults that you can't necessarily see at idle which show up under load.

If you have 4, you can stick them on one side at a time and compare - I've used them about 3 or 4 times now (got to use them tomorrow actually as well), you can see a weak spark by a dull light or flickering thats different to its neighboring cylinder. You just put them onto the end of the lead and then onto the spark plug.

I don't know what the GEMS coil packs are like though, they fit the Thor ones easily, but some other vehicles with long spark plug wells may make them impossible to use.

But, you could just spend the money spend on them on a new coil pack and get an answer that way!.

Incidentally the last MOT the Disco had, ended up with replacing the whole exhaust due to failing emissions both CO and HC too high (the best of the local testers I use as the others I've tried I didn't want to go back to a second time, won't test on gas and I really can't be bothered to argue the point with him any further now) when tested on petrol, the cats on that had suffered misfires both from before I brought it (as a spares or repairs with misfiring) and the failed coil packs, to the point where the exhaust rattled when idling - a sound like a stone caught inside it is typical of a cat thats broken up internally, so I wasn't really surprised that it failed emissions after scraping through the previous year (when the same garage had a different tester on, who did test it on gas once I'd told him what to select off the test machine as he didn't seem to know if it was CNG or LPG he was dealing with).

It sailed through with a new Cat, took the opportunity to replace the rest of it as the middle box was more exhaust putty and rust than steel where it had been patched up before and hacked about where the flange on the end had rotted away. Only other thing I did to it was give it an oil change and a new air filter. If you wanted to check the cat, really the only option you've got is to stick a camera up it and look that way, or take it off and tip it up and see what falls out though that only tells you its broken up. They can get fouled up with other crap and stop working that way as well of course (oil being the most likely cause since you can't buy leaded petrol anymore)

Tnx Brian, I picked up a set of those Laser plug testers the other day, not used in anger yet. I was wondering if it would be more effective to use them in their coil pairs, rather than left bank/ right bank? Semantics i suppose, I may just buy another 4 and cover all the bases

It helped in my case to have one side visible - simply as you can compare good to bad that way, and you can only look at one side of the engine at a time anyway. The advantage with having a second set is you can put them on before you get the engine hot but its probabbly not really needed. If the coil arrangements on the GEMS are like the Thor (as said, don't know on that as I've never seen a GEMS one but I'd guess it is), then its paired across banks anyway. Coil packs usually fail in such a way that both plugs on that coil pack (bear in mind the Thor ones are really 2 coil packs in a single package to give 4 cylinders and another seperate unit to do the other 4 cylinders, I'd gather the Gems ones are a single coil pack to do each pair of cylinders) so as long as you've got one bank covered, you should be able to see your fault. It can help if you can do it somewhere dark if possible to see the light from them more clearly.

I've got a set of those, hadn't really thought about using them for this though as I didn't expect to be able to see a difference. Will give that a shot, ta.

It generally takes a few minutes to get your eyes used to what your looking at, but once you get the hang of what pattern you should be seeing, you will soon notice a difference between the good and bad. Just used them on the focus this afternoon to find out if it was a spark related issue I was getting at cold starts, and it looks like the coil pack has died (again). Noticable that sometimes it would spark ok, others it would miss on two cylinders, checking the obd logged codes against cylinders 3 and 4 it was noticable that those two were missing some of the time with the car sat at idle. Now to see what it does in the morning with a different coil pack on!

Tried them on another car and could see a difference in intensity. On mine though I didn't see any noticeable difference.

Got the new coil pack today, and it made no difference whatsoever. :(

So back to square one.

Do they flicker any differently on your car compared to the neighboring cylinders? Like an occasional failing to light when it should fire? If its suspected the cat has been damaged by misfires, do you know if its one side of the car that seems to have suffered more? (as that might give a clue where you should be looking for the source of the misfires)

Hi Brian,

Will need to test that when I get a chance. I'm pretty convinced that the fueling off idle is pretty close, but at low revs it's still rich. So my current thinking is dirty/leaking injectors on one or more cylinders. Need to build a test rig for them to check that theory.

Got my HT lead probes for the scope today, a quick test on my car suggests they're all firing about right, for the most part.