What happens when spark plugs "decompose" in combustion chamber?

[audi4fun]

Again I'm looking for some kind of support from you, guys.

Some weeks ago I determined that one spark plug is not working correctly when engine is cold and I decide to have a look on them. When got them out they all were black. When started to search answers I realized that my NGK PFR7B spark plugs have bigger gap than needed. I had around 0,75mm when new have 0,8, but particularly for 20VT gap must be reduced to 0,6 for standard engine (as per catalogue).

Then I continued to read and found that if there is tuned engine gap must be less that original (<0,6mm). NGK says - it must be reduced and do it on your own risk.
Somewhere else in the internet I found that gap must be reduced for P0,01mm (0,004") for each 50hp (it was sad like NGK advice).

So, as I have somewhat of 325hp on flywheel and wanted to go for more soon, decide to stay with 0,5mm as a test.

When started engine, it was running perfectly smooth. Much better then previously. Then I went some km to highway and did some acceleration 1-2-3. And engine just stopped working normally. Engine started to work like on 4 cylinders. At first I thought it is a problem related to coil or commutator. But when get back to home realized following picture.

First 3 plugs were nicely clean and light brown (I put them back black). But other 2 knocked me down. Especially that on the right side.





Questions:
1. Did I right when reduced gap?
2. What consequences this can make to cylinders, head, valves and turbine?

I'm going to buy BOSCH F5DP0R plugs in a few days. But before that I would like to know what I have to check in advance.


Thank you!

[driftin jo]

Looks to me like you could be running lean on those cylinders, caused by injectors
clogged up and or just the fact that they are the wrong plugs and not suited to this engine.

[Rusty]

Oh dear, that's really not ideal! Looks like the metal has become brittle or you have stress fractured it when changing the gap.

If you know someone with an endoscope then you can get that in the piston and have a look but really, the only way to know if there's damage or not is to pull the head off and have the turbo dismantled and inspected If you're going for more power then you really should do this, otherwise you're in line for a big bill later on.

[audi4fun]

Looks to me like you could be running lean on those cylinders, caused by injectors
clogged up and or just the fact that they are the wrong plugs and not suited to this engine.

Can't be lean on Porsche orange injectors. I was thinking it is too rich (porsche orange in place of RS2 injectors and FPR 4,0 in place of 3,8). Fuel pump also changed.
NGK PFR7B = BOSCH F5DP0R as per catalogue.

Plugs were black when I put them back and I went maybe 20 km in total while got home. For me they look almost ideal now. I mean these three which survived.

Rusty, thanks.

[quattro84]

Hmm it even destroyed your nose insulator on the last one...
Are the electrodes melted or do they show a fractured surface?

Looks like pre ignition combined with some knock due to running lean and so overheating the plug. Is this the right heat range?
Reducing the gap will not help in this situation neither. Keep them at the OEM specced gap or higher when running uprated coils.

[audi4fun]

Reading comments I'm thinking all the time how can I get lean...The only thing I did together with gap change was that I changed fuel pump also. The only thing I can imagine is that the new fuel pump does not provide needed amount of fuel.

From another side- before I changed gap, spark plugs was looking like I'm running rich even with standard fuel pump.

Problem was/is with 4th and 5th cylinders. Unfortunately I do not remember which dead plug for which cylinder.

First 3 plugs - electrodes are fine.
The one with insulation left - electrode is fine.
The one with insulation missing - top of electrode is missing also. I checked remains of electrode inside plug- it seems that insulation has cracked in three parts.

These missing parts melt or just cracks into smaller particles?

[Billman]

If you gapped used plugs then like Rusty said!

[quattro84]

Hmm you have to run insane amounts of timing or have massive pre ignition for longer periods to make the entire plug melt. But it's only the electrode that's affected so that does not make sense. The worst one even has the center electrode melted.
Another thing which can lead to electrode failing is running leaded fuel. Is that possible? (not very likely, but i don't know Latvian fuel quality)

The broken electrode can be because of the reduced gap.
But because it's the last 2 cilinders with the issues the thermal heat soak theory is plausible.

I am afraid your turbine and valve/seat might have damage due to the insolator parts getting in contact, these surely don't melt.
I would suggest to check the combustion chamber with an endoscope, do a compression test next and based on the outcome to think what to do next.
At last i would suggest never to decrease the plug of 0,75mm on an engine like this.

About the lean running, i don't know how the Motronic reacts. It can't compensate for low fuel pressure for sure until dutycycle on the injectors max out, and i can't imagine it detecting this (if this happening)

I have seen plugs fail on engines before, but not 2 at a time combined with a shattered central electrode...

[audi4fun]

Thanks, guys.
I gapped used plugs. Don't know how old they were. I got them with a car. Running 750km per year over last three years.
Fuel quality should be ok. Bought from best fuel retailer.
Ok, will start with compression test and let you know about results.
Regarding spark gap - as I told, I took this information form catalogue and NGK website. I assume that something went wrong during gapping.

[driftin jo]

It is possible to go lean if your injectors are clogged.

A friend of mine changed his fuel pump and filter and 3 days later he had an injector
fail, it went very lean and destroyed a plug, (luckily as it could have melted a piston) it seems that rust was dislodged when changing the filter and this travelled to the
fuel rail and clogged up an injector screen. All the injectors were removed and sent for
cleaning and all showed signs of rust in the screens, the thing to note was they all flowed
different amounts due to this .

I can't say if this is what has happened to your engine but it could be.

[audi4fun]

Thanks for advice. One more thing to check.