GQ TB42 runs well but has one corroded sparkplug

[LandRoverGhost]

Hi All

TB42 carby on dual fuel.
The spark plugs are less than 5000 k / 6 months old.
But the number 3 cylinder plug is sort of corroding away - or covered in some kind of rust like coating - impossible to scrape of.
All other plugs are beautiful smooth - almost new.
- Starts at first go
- Runs super smooth after first 15 minutes
- Pulled off the #3 Ignition lead to see if it possibly does not fire - all OK , fires normal
- Engine hardly uses any coolant if at all - uses zero oil.
- Compression is about 120 psi on that cylinder
- Car is becoming a tad lazy lately

I'm puzzled.

[mudski]

Cracked head or gasket letting a little coolant into that pot?

[LandRoverGhost]

Thanks!
That was my first thought as well. But the almost no coolant use ... ?
Would even a teeny weeny bit of coolant in the one pot cause that even though it runs smooth ?

[PeeBee]

I would pull the plug and simply replace it and monitor. I seem to remember build up can occur from the 'corona' of the spark itself if the plug gap or plug itself are faulty, gap usually being too wide. Maybe jump onto the web and do a search on plug defects and condition analysis - might be a lead, and save hours of heartache also.

[LandRoverGhost]

Thanks - Sounds like a good start. Will do that today .

[mudski]

Thanks!
That was my first thought as well. But the almost no coolant use ... ?
Would even a teeny weeny bit of coolant in the one pot cause that even though it runs smooth ?

It could just be leaking into that pot when the engine cools, then the warmth in the pot "steams' away the coolant giving the spark plug a corroded coating. Corrosion needs moisture to happen.

[PeeBee]

Have a look here
https://www.onallcylinders.com/2012/12/20/reading-101-how-to-read-your-spark-plugs/

[mudski]

Have a look here
https://www.onallcylinders.com/2012/12/20/reading-101-how-to-read-your-spark-plugs/

Thats handy Phil. I would think if there was corrsion on all plugs, whats on that guide would be it. But only on one plug, gotta be something else.

Would a dodgy lead possibly cause this? Or help with corrosion..

[PeeBee]

Thats handy Phil. I would think if there was corrsion on all plugs, whats on that guide would be it. But only on one plug, gotta be something else.

Would a dodgy lead possibly cause this? Or help with corrosion..
If the spark is uneven such as a lead breaking down you would probably not see anything. But if the plug is faulty or gapped too close you could see a metal deposition, not only from one terminal to the other but also reaction with the air itself as the arc is quite high voltage and arcs of electricity can change the format of metals at high voltage. This is getting on the fringe of my knowledge as its 40 yrs since I was dealing with high voltages and corona treaters but we had similar issues with arc variance and the principle would transfer to a simple spark plug as well, but then, it could be moisture but I think it would dry out in the combustion chamber unless there is some slow leak and the car is left for ages.