So my V12 has been feeling not quite right for some time. There was a roughness about the engine. Both at idle and throughout the rev range

Checked everything I could think of. I also had a persistent FF44 error code. Replaced the O2 sensor in the A bank, but it reappeared shortly.

Last week, I pulled the old plugs as a last resort. One spark plug was gapped at 0.015, the other at 0.040

Regapped them and took it out for a test. The difference was astounding. The engine is so much smoother. And the idle is very stable.

I have never seen an engine so sensitive to plug gap

 

It's because of the combustion chamber design and the relatively high compression of the HE engine. That .025" gap is specified for a reason!

 

A simple way to check the tune of your V12 is buy one of those $25 infrared thermometer’s point it at each cylinder’s exhaust manifold. If they are all the same. Then all 12 are firing properly and everything is as it should be .
If one or more is cooler then you need to check the plugs and or the injectors. I have a cheap pen like device that set on a spark plugs wire tells me if it’s getting a good spark.
I also have a cheap stereoscope to set on each injector to listen to it firing.
It takes me about 10 minutes to check everything.

 

Your right about the IR. I have one on my sailboat, where I use it to monitor coolant flow during engine checks.

I do use a stethescope for the injectors. That is how I found (with my noid) my injector harness was bad.

I also have one of the spark pens. I also found a defective lead in a new set of plug wires.

These procedures should be part of checking my tuneups.

 

I've often speculated that there is no such thing as a "Marelli Failure", ie it isn't a failure of the Marelli system or parts. The real culprit is incorrectly gapped plugs which create the conditions of failure. Akin to how a light switch is burned up by poor connections and high resistance, and we never refer to that as a "switch failure".

Cheers

 

those simple aides sure make diagnosing a V12 easier.