Throttle Body buildup & need for cleaning

[Montego Murph]

Hey all,

The other day I was working on a friend's car. The throttle body was so caked up with black buildup (carbon?) that it would stick. Sometimes open, sometimes closed. Either way, bad deal. While poking around and checking things out, we realized that the PCV valve was also stuck & clogged, so we cleaned that out with carb cleaner and the little ball now rattles around in there. (This may be relevant info to answer my question, but I don't know.)

While looking at the system, I'm a little stumped at exactly HOW this buildup accumulates on the throttle body and butterfly valve. It's just air -- filtered air -- passing through it. Fuel enters at the other side of the butterfly valve, and exhaust is on the other side of the engine. What is the source for the heavy black buildup, and would the clogged PCV filter play a role in this, and why?

I just don't understand how this would happen as I can see no lines from the crankcase or fuel system going to the air box before the butterfly valve.

Sorry for the rookie question. This is clearly something I should know after 25+ years of wrenching, but I just don't understand.

 

[UrbanRunner651]

Can be amplified by bad gas/bad filter but typically just a result of neglect in PM.
The buildup is mostly from a backflow of oil & exhaust gasses in the case, which is normal. If you noticed a bad PCV valve then that is likely what is causing the excessive deposit.

 

[Montego Murph]

I thought I'd do a little follow-up. Since the weather was nice, I decided to remove the throttle body and bring it down to the basement for a good thorough cleaning.

One thing I did notice (pointed out by the red arrows) is it looks like the gasket isn't sealing as well as it should. I should pick one of those up and replace it... I'm wondering if that seepage around the top photo accounts for the liquidly black stream passing through. That's the coolant return orofice, correct?