Hi,

From my experiences over lots of different engines I would say the plug is too cold of a heat range for that cylinder. But it cannot be that it’s a too cold of a heat range for this engine since the others are fine.
This sort points to that the spark plug is miss firing and thus is running very cold.
Lack of firing is about as cold as you can get. (:-)


Spark plugs do fail as I have taken Champion spark plugs right from their box to be bad. Considering the production numbers within every brand plug out there has to be about two percent of them to be bad. This number is a standard statistical average for defects of man made products.
That is except for a human itself?
That number has to be way higher due to so many uncontrolled variables covered within randomness and a pinch of Murphy’s law thrown in for good mixture. But that’s life on earth. IMHO

A Spark plugs life, used under designed conditions, are remarkably reliable to be 150 year’s old.
That why people don’t want to believe it might be one spark plug.
But… I ask … why not?

Move the plug to another cylinder or change it over to new or another known good one and then, do nothing else is the test.
Same goes for that cylinders wire if it continues.
You obviously can only do this one at a time procedure to get a valid scientific result.
You move this idea along while experiencing the laws of probabilities and gaining a sense of what is the “Most Likely” item to cause this.

With that idea I would consider that since it’s the very back cylinder it is the one that runs the hottest because it is the farthest from the radiator it thermostat outlet.
A note about the piston rings needs to be stated here.
The compression ring only does its best of sealing is on the compression stroke. The ring is put in facing up a certain way so the taper up on to uses the gas to push the ring out more. Otherwise it just riding up and down
The oil rings below it use their own tension to scrape the cylinder wall.
Since that cylinder gets hotter any oil not circulated will have a more likelihood to cook or get stickier.
With the plug telling a story you have some oil burning going on or excessive fuel deposits.
If you are using top tier gasolines that should be a lesser issue.
You don’t claim any gasoline & oil dilution so the injectors are, at this point, not a suspect.

Sometimes soaking the troublesome cylinder with Marvel mystery oil or even ATF might help
Some people use a product called “Rislone” but other oil detergents maybe available.
This product has been around for a long time so I suspect they have sold their share to long time mechanics.

As I remember this car uses a constant injection system (CIS) or mechanical pressure to open the injectors and a frequency valve to pulse and vary the flow through the fuel distributor.
On the later LH Electric injectors an electronic hot wire or plate Air Mass Meter (AMM) does the same thing. Sometimes I wonder if it was worth it as both are Lambda-Sond bound in the end.
My 1978 has been very reliable as I only fresh keep gas in the system.

So, change around things or change out that spark plug.
That one cylinder can cause a loss of 25 percent power right upon several misfirings.
A few seconds of missing equates to several revolutions per minute
The carbon deposits have dampen the heat and the path of the spark and consequently mucking things up all over that cylinder. You don’t want to burn an exhaust valve due to improper valve closures.

What may need to happen is change the oil more often or put in a better detergent in it to remove that gunk. It will not have to be a full time thing with todays oils.
Costco oil comes from a long time supplier.
The Warren oil company. I believe they are in the middle of the USA, somewhere, if it makes you feel any better.
They put oil out there under several well known name brands.

Costco’s staff of buyers are pretty shrewd in what they get for their Costco customers.
Just like SprayWay Glass Cleaner, sold @ Walmart, packages up about 2500 brands of different things.
I suspect the same glass cleaner might be under five different names and of course, prices.
You should look these things up that we buy.
As it puts branding under a whole new “limelight.”
It’s a “Theatrics” stage lighting term from the 1800s but “Retailers” of today are the same game players looking for suckers.

SO I STOP as I’m getting carried away
Have a nice day.

Phil