Does the electrode of a spark plug break when the engine runs too hot?

Question

I have an Audi A5 1.8 TFSI Sportback from 2011, bought it recently (4 months ago), which ran fine until two weeks ago. The engine went through diagnosis and the operator showed a broken spark plug to me.

The electrode broke off inside the engine (see image below).

The question is: does the electrode (circled in the image) break off? Does it break off when the engine runs too hot?

To answer the question, you should know that there's another defect with the Audi: four weeks ago I noticed that I'm frequently running out of engine coolant. I checked it at a mechanic and they found, that the water pump was leaking. I was unable to afford getting a new one. They said I could wait a couple of weeks with that, specially because it's currently very cold (+5°C outside) and I would have to fill coolant liquid myself until then - which I did.

Could that have been the cause? Because I was low on coolant, the engine went too hot and the spark plug electrode broke off?

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Secondly I have another (legal) question which might suits here (or not, please correct me): Do (by germanys legal warranty) I have to pay for this or the dealer?

Here's some facts:

• Date of purchase: 16.09.2019
• Date of first registration: 16.08.2011
• Date of incident: 17.01.2020
• Car was on 88.000 km when I bought it, now is on 98.000 km
• Chequebook says, that spark plugs have never been exchanged
• Chequebook says, that I have to exchange spark plugs for my model every 90.000 km or 6 years

I dropped the car off at the car dealer and went for legal warranty. He now claims, that the spark plug wasn't the issue - instead the engine went to hot.

Is he right? My temperature gauge showed no more than the usual 90°C as always. I was pretty low on coolant fluid (like 30-40%).

The other thing is... Can I claim, that he sold a faulty car because the spark plugs were past lifetime? All of this might now have happened when I fixed my waterpump - but I'm not sure if it's actually my fault or the dealers fault.

Answer 1

Because I was low on coolant, the engine went too hot and the spark plug electrode broke off?

Yep that's possible - running low on coolant (particularly with what amounts to a malfunctioning water pump) could create "hot spots" in the engine, potentially leading to the spark plug failure. If you're frequently running around on less than half the coolant the engine requires you're going to run into problems. Expensive ones. Frankly if it was leaking that badly I think your mechanic did you a bit of a disservice suggesting it was okay to be driving around for a few weeks.

To be honest my gut feeling is that the plug is more likely to have broken due to going way past life on both age and mileage. But any overheating is only going to accelerate the wear on the plugs. You might have got away with only pushing the limits on one of those three factors, but pushing all three is (unfortunately) a recipe for disaster.

Can I claim, that he sold a faulty car because the spark plugs were past lifetime?

IANAL but I think you're unlikely to get anywhere with that line of thinking, but from my understanding of German law on this the dealer is responsible for defects determined (usually by a third party appraisal) to have been there at the time of sale, so if (say) the water pump was leaking when you bought it that would be covered. But I think you're going to have a hard time proving it was leaking 4 months ago when you only noticed coolant loss 4 weeks ago.

A serviceable item being past due isn't really a defect and there's a reasonable argument that you're the person who chose to do another 10,000km on the plugs before the failure. The plug clearly wasn't broken at the time of purchase, and they are a wear and tear item.

Answer 2

If the fluid is low, then the temperature gauge will not, in many situations, report the actual temperature. This is because the temperature sensor is usually located in a water channel in the head and if it is sitting in air then the rate of heat transfer from the head to the air and, subsequently, to the sensor is not the same as when the water is in contact with the channel and sensor.

This means that while the gauge showed a "normal" reading, the engine could have been overheating and that was due to the low coolant level.

This means that basically you should have replaced the water pump to prevent the overheating and subsequent problems.

In terms of the mileage you show,it could be suggested that you should have had the plugs changed as soon as you got it at 88,000 - driving to 98,000 was not inline with the service schedule.

Answer 3

The electrodes are Inconel so immune to any temperature that can be made in a engine: The same applies to the platinum center electrode. A defect in the plug or some mechanical damage would be required to remove the electrode. Long ago I found 3 bad platinum electrodes in 6 Bosch plugs after modest mileage ( like 20,000 miles), I concluded some defects. Otherwise platinum plugs easily last over 100,000 miles = a bunch of kilometers. Your photo shows an old Inconel center electrode but I strongly suspect the car has platinum plugs.