Engine running Roughly.

[enigmas]

My Mk3 Rover P5 Coupe hybrid (P76 engine) started to run roughly during a short trip to visit some friends. The overall drive would have been no more than 20 kms. It's an LPG fueled car and has been for 28 years. So no complaints. It's also a cared for vehicle, but now that it's on club plates it's not used as a daily driver. I usually service it once a year.

Interestingly when I arrived home I gave the engine a quick look over and couldn't really find anything amiss. When placed into Drive and under a load, the engine felt as though it was misfiring. Its usual steady idle felt erratic.
I checked the usual sources that would cause this behaviour, shorting plugs, leads, vacuum leaks, faulty coil, etc. All appeared fine. I then removed the distributor cap to check the distributor terminals. (The ignition system is fully electronic with a locked distributor.) Inspecting it carefully I noted there was a white powdery residue on the internal cap terminals. This white powdery coating seems par for the course for most traditional ignition systems that use a distributor cap. I cleaned the terminals with a mini wire wheel using a dremel tool. Sprayed WD40 into the cap and wiped it clean. Restored the cap to the distributor and fired the engine. It started and ran perfectly.

All I can think of as a cause of the misfire was contamination within the cap causing electrical shorting between the terminals. Wiping the inside of the cap and cleaning the terminals fixed the issue. Although I wiped it clean there was no obvious (to the naked eye) dirt or residue within the cap.

A question for anyone with specific knowledge with regard to high intensity electrical energy/arcing. Does a spinning rotor inside a distributor cause/create conductive debris through terminal erosion or similar process?

[Mike’S-a-loon]

My dad always told me the carbon from the central contact brush, that contacts the center of the rotor arm, could cause conductive dust inside the dizzy as it wears. For what it's worth. Presumably if this was your issue there would have been some black dust.

The white build-up on the inside dizzy cap electrodes seems pretty common and scraping it off standard procedure.

I know you're way ahead of most of us, Vince, you must be having a bad day mate!

[enigmas]

Hi Mike, no, not at all.
Some things just make me wonder! I know there are a few boffins on the forum that may have an in depth grasp of what electrons may get up to inside the confines of a distributor cap.

At various times in my motoring life I've had all sorts of electrical gremlins upset the smooth running of the engines in my cars and motorbikes. Most of these I've learned to comprehend.

Obviously there's erosion within a distributor cap over time for various reasons and cleaning the inner terminals at regular service periods makes logical sense to me. The inside of the cap looked fine to my eye, yet (cleaning the terminal posts...something I did later), squirting the cap interior with WD40, then wiping it dry made all the difference.

So what goes on? There must be something within the cap after a period of use that promotes or allows cross-firing!

NB. I'm aware that cyls 5 & 7 can cross-fire due to the firing order of the V8 if the leads have poor insulation.

[djm16]

It is possible that the white residue and cleaning it off had nothing to do with fixing the problem, but disturbing various leads in the process of doing so has led to a temporary improvement in a poor contact somewhere.

In answer to the other part of your question though, the combination of coil and capacitor (yes I know yours is electronic) forms a resonant circuit. The current through the points and through the HT rotor arm / leads and spark plug alternates rapidly. When negatively charged electrons jump across an air gap, heating the metal surfaces to above melting point locally, positively charged metal ions go in the opposite direction. If there was no resonance, then the electrons would always move in one direction and so would the metal ions. The result: points would rapidly find all their platinum on one side and become ineffective, ditto for the rotor arm and plugs. The trick is to choose values for the capacitor and coil that lead to approximately equal amounts of metal moving each way so that there is no net erosion of either side.

While positively charged metal ions are flying around, some of them inevitably combine with atmospheric oxygen to form the white powder that is the metal oxide, at a sheer guess, I would say is zinc oxide (brass is a copper and zinc alloy). Zinc being more reactive than copper will oxidise preferentially.

[enigmas]

Thanks David...that was informative and what I was looking for to a point. 👍

Interestingly though I only initially wiped the inside of the cap and didn't clean the white residue off the terminals until my second road test. Merely wiping the inside of the cap was sufficient to restore smooth running. I still believe there's something obviously invisible to the eye that either coats the interior of the cap or inhabits the internal air space of the cap and effects electrical distribution from the rotor to the appropriate terminals after a lengthy period of service.

Yes...it all does sound a bit spooky and a bit like a conspiracy theory! What are those electrical gremlins doing sight unseen within the distributor cap? Hmm! 🤔 😉

PS. There were no loose or eroded terminals and the HT leads are also high efficiency spiral core.