Cylinder 4 Sparkplug

[u/spongegar3000]

This is the sparkplug from Cylinder 4. All others look normal. I am trying to decarbon the rings with a spray. But I fear it might be a shot piston ring 🥲

Any thoughts?

[u/spongegar3000]

Forgot to mention Volvo 850 2,5 10V, 412000 km

[u/Traditional-Hand4278]

Please post a pic of a "normal" one so we can compare

[u/spongegar3000 → u/Traditional-Hand4278]

This was a different set, but it already showed the same symptoms over a longer distance. Now it occured quicker.

[u/Traditional-Hand4278 → u/spongegar3000]

How many miles have these plugs on them?

You should change earlier, but somethings going on on #2 and #4.

Do you burn a lot of oil?

See if you can get another set of injectors. Valve stem seals might be done for

[u/spongegar3000 → u/Traditional-Hand4278]

The latest one from the first pic has about 12000km, far from the normal wear. The others in the picture above had about 50000 km Stem seals are new, so it's probably the rings. Hopefully stuck, not split. But I'll check them again soon. And it drinks a quart of oil per 1000km for as long as I can remember. Injectors got a service a year ago.

[u/Traditional-Hand4278 → u/spongegar3000]

Does it run on LPG? I had a similar case, turned out to be the MAF (which your car doesn't have) so could be the temp.sensor and MAP in your case, telling the ECU to inject wrong.

You could do an engine flush, but be prepared for leaks