hard carbon on valve heads

[bentvalves]

when a head lands on your bench and the valve heads have a HARD, thick layer of carbon burnt onto them, does this immediately point towards oil getting past the stem seals, or does it immediately point towards oil getting past the piston rings?

[modok]

It MAY be burning oil, or it might not.

Engines that alternate running rich and lean can have some massive deposits.
Runs rich and builds up carbon, then runs hot and lean and bakes nearly melts that carbon...........repeat, repeat. Like with spark plugs "high speed glazing" as they say or said..
Funky fuels, propane, also can make rock hard deposits.

[travis]

Lean/hot with badly worn guides can do this too

[user-23911]

When are carbon deposits not carbon deposits........when it's lead from leaded fuel or when it's baked oil deposits from oil.
Carbon is black and dry and doesn't melt.
Propane burns clean and doesn't leave carbon deposits.


The first step is to know what it is?

Photos?


I'll take a guess using my crystal ball that it's from the rings not sealing properly.
Or if it's a direct injection engine it's extremely common, worn rings and EGR blocking the intakes.

[mag2555]

What does the back of the valves look like and the port walls of the Intake and Exh runners?

[bentvalves]

I didn't get any pics and it already went out the door.

I was just curious if I should be pushing for a stem seal install when I see these symptoms.

the deposit is so hard on the exhaust valves, that a knotted wire wheel on a pedestal grinder has trouble breaking through it.