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MG MGA - Odd Coloured Plugs

Got bored and started tinkering with the car - should have stuck with gardening.

Warmed the car up, then looked at the plugs.

The central bodies of all 4 plugs are light rusty with a tinge of red.

What's the diagnosis?

Additional notes: I cannot weaken the front carb any more, the adjusting nut is fully tight; still some adjustment available on the rear. The carbs have done about 30k since the jets and needles were last changed. the seals were changed about a couple of thousand miles ago. The car is running great. Superb pick-up and performance throughout the range. As I said, I should have stayed in the garden!

The two photos are of the same plug; just different light conditions.


Steve Gyles

Steve -
Such a colour is some substance in the fuel; the plugs otherwise look exactly right. Fuel gets contaminated with all sorts of things, intentional or otherwise - additives by owner or blender, errors in refinery, being used as a toxic waste disposal method. If all it does is redecorate your plugs, be thankful!

The carb adjust thing means something is pushing it toward rich, like high float levels, leaky float valves, etc.

The big trouble would be if the two were related, ie, whatever is in there is eating stuff. We had a thing a couple years ago with fuel eating in-tank pumps and gauge units on a lot of cars locally. It involved many cars, but with inconsistent local distribution and effect - quite a few were towed in, others lost gauges only. It was finally said to be a local refinery error, but the explanations didn't really make sense. Cost the refinery and car companies a good bit of cash.

Some people have enough sense to drive perfectly good running cars instead of fiddling with them!

Fletcher R Millmore


98 octane but from our local nationwide supermarket. Rumour has it that some of our large supermarkets sell discounted petrol that is from 'the bottom of the barrel'. Makes you wonder. I have never seen plugs that colour before

I will probably do a carb overhaul anyway, replacing the jets, needles and seals. Floats were fine last time I looked, set at specified height.

Steve Gyles

Yep, but the waste disposal thing is rampant worldwide. Added a bunch of waste product that is expensive to get rid of makes triple money for the sellers. Depending on where in the chain it happens, they get paid to dispose of the waste, they sell stock that they don't have to buy, and they avoid fuel taxes. It goes all the way from global organized crime to local distributors, and can be highly variable by batch, so you can never tell what's what. By the time somebody notices a problem, the stock is gone and there's nothing to investigate.

Those plugs are kind if pretty actually! Iron is one thing that could cause that colour, but I'm sure there are lots of much more complex chemicals that could do it. If the glaze formed is not conductive, it's not a problem. Some glazes do conduct, and that is what lead fouling is/was - heavy lead loads would give bright yellow glaze deposits. I used to clean my impossible to get long reach 10mm plugs in the Bristol with nitric acid, because it dissolves lead compounds.

Fletcher R Millmore

Your red plug intrigued me. So I did a search. It is commonly regarded to be caused by fuel additives.It is usually found on one side of the plug. I would presume that it would be related to the head flow.

Funny thing is, I found one report where a guy found only one plug colored red, the rest were normal. So go figure. Most all references say the red deposit is not indicative of a faulty engine or a bad tune.
Chuck Schaefer


Yes, it was predominantly on one side.

Steve Gyles

Easter Bunny?
David Holmes

Last time I had plugs that color it was due to an intake manifold water leak (rusty water). But there were no other colors other than the rusty red, so this problem is probably fuel related per above.
JM Morris


Now you have me worried. I carried out a light engine rebuild 3 months ago, including a new head gasket, so leaks are always a possibility. That said the car is running great and I do not appear to have any water loss. The plugs were removed immediately after a 10 mile race around the local roads - it was a nice day.

I want to favour FRM's analysis.

Steve Gyles

This thread was discussed between 04/04/2010 and 05/04/2010

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