What causes black crud buildup inside the oil cap?

[OilStainedNapkin]

Since the filler neck is up so high, there is likely very little to no oil splash on the cap. The dark residue is likely oil vapors. Those will leave nasty deposits, just look at the DI engine intake valves.

I think you nailed it. There is absolutely no oil splash in this zone.

 

[ZeeOSix]

Since the filler neck is up so high, there is likely very little to no oil splash on the cap. The dark residue is likely oil vapors.

And it doesn't get nearly as hot as the rest of the engine, so it acts like a vapor "condenser".

 

[TomYoung]

Not worried as I realize this car is almost 20 years old and nearing 200k rather it’s more curiosity.

gotcha: Then do a liqui moly engine flush, fill with Valvoline Restore and Protect, add a can of Liqui Moly Ceratec, or MOS2 and keep on trucking. (Also, brakeclean and paper towel to the oil cap)

 

[OilStainedNapkin]

gotcha: Then do a liqui moly engine flush, fill with Valvoline Restore and Protect, add a can of Liqui Moly Ceratec, or MOS2 and keep on trucking. (Also, brakeclean and paper towel to the oil cap)

Tbh I think the furthest I’d go is maybe use valvoline r&p or Mobil 1 0w40 and call it a day. I’m sure the seals are tired as well. Would hate to do all that and then end up with a RMS leak or something else. I’ve been using basic synthetic the last 50k miles and will likely switch back to syn blend after the next oil change.

 

[TomYoung]

Tbh I think the furthest I’d go is maybe use valvoline r&p or Mobil 1 0w40 and call it a day. I’m sure the seals are tired as well. Would hate to do all that and then end up with a RMS leak or something else. I’ve been using basic synthetic the last 50k miles and will likely switch back to syn blend after the next oil change.

I can see skipping the flush... but the additives are awesome.

 

[lbhsbz]

Deleted...posted to wrong thread