Meaning of yellow green residue inside thermostat housing?

[loc182]

I started removing everything cooling system related from my car today and found that the inside of the thermostat housing looks like this:



(Pics are with and without the flash.)

The yellow green stuff just wipes out fairly easy, but I was wondering if it has any meaning? I haven't seen it in hoses or the radiator and it almost seemed like mold.

The car is a turbo automatic and it suffered two blown headgaskets before I bought it. (Last was around 2007.)

This thread mentioned green at the ends of hoses can mean internal breakdown, but the thread doesn't really have any answers. It seems more like a checklist:
http://www.supramania.com/forums/showthread.php?71402-Heating-and-A-C-notes

Thanks in advance!

 

[suprajztwenty]

it looks like rust. my last motor had severe block decay. i was thinking it was due to the previous owner constantly driving on a bhg and the coolant became acidic...not flushing your system for years on end will cause this too but mine was pretty wicked, it was rust jacking the freeze plugs.

yours def isnt near as bad. id run a flushing agent through it once or twice and keep an eye on the fluid color.

 

[loc182]

Darn, rust is not good... I just ran a flush about a month ago that seemed to do a lot, but it looks like I am in for more.

Thanks for the info!

Oh, by the way, why is it that yellow green color? I had always though rust was red or white.

 

[IndigoMKII]

Take sure that you use toyota red coolant. It'll prevent the head from being eaten away as well as the block.

 

[loc182]

Yeah, nothing but Toyota Red for me. But no idea what the previous owner used. He grew weary of Toyota after paying for two head gaskets and started going to scary places like Les Schawb... I know the coolant did not look good when I bought the car.

What I drained out before I started working had only been in for a month but it was nice and red still

 

[suprajztwenty]

if youve already done a flush then id just drive it awhile and watch the coolant color every couple weeks to assure you get no surprises