Spark plug diagnosis
 

Hi,Just returned from a 3,000 km road trip.Car never missed a beat. Checked the spark plugs when I got home and this is the result.The reddish coating seems powdery and as can be seen on the right hand one easily wiped off with my finger.What is your interpretation of this. I have been using what s left of my lead replacement additive just to gain a small octane boost.

Hi Arcturus,

Couple of questions first, what octane /lead booster are you using, and which brand and number type plugs? That's a weird colour probably attributed to your additive.

Assume it's a B18/20 A or B engine?

Only plugs I would ever use are Bosch (Volvo original fitment) or NGK, the latter BP5HS summer and BP6HS winter, or harder driving.

Most additives available especially from service stations are not great TBH.

J.

Sorry meant 5HS winter and 6HS summer.

J.

Woah!

I suppose if the car is running well, you shouldn't worry. But I would probably lose to Octane boost. If you are running a low compression engine, such as a B16, you probably can run on pump gas. I suggest you try that for a while and then check the plugs.

In my MGA, I mostly run 110 Octane leaded gasoline. It has a compression ratio of about 10.5 to 1 - not too aggressive but more than the stock engine at 8.8 to 1. I need the higher octane at high rpms. I have a faint red on the threads of the plug, not the tip; and if I remove the head, the combustion chamber looks a bit rusty. I attribute the deposit to fuel additives.

Slightly off topic....but this Q reminded me of the Haynes manual page with the COLOUR pics of spark plugs.
This was my favourite page as a boy...yes, i was and remain, odd.

Isn't red dusty OK, or maybe under fuelled a tad?

The dusty red is typically a result of additives in the fuel or contamination of the fuel. If the red borders on a rusty red in can indicate a contamination with iron oxide (rust).

For fuel mixture, the color of the actual insulator is the tell. Sooty black to very dark brown would be over fueling. Light brown to bordering on white would be 'in the money'. Modern EFI with O2 feedback typically run right at an AFR of 14.7 and their insulators tend to be white. Carb equipped cars lack the corrective feedback so you typically play it safe and run with a mixture that gives you a light brown insulator nose. Chronic under fueling results in a bone white insulator with signs of what looks like glazed sugar on the insulator.

In this case the residues are on the external electrode and around the metal plug body so likely the result on unplanned or intentional stuff in the fuel.

Just wondering,George.....do you ever get time to drink good wine?......😜

Frequently,but never when driving, also a nice glass often year old tawny port in the evening :laughing-smiley-011: