purple junk on side of coolant reservoir
 

I noticed some pinkish purplish stuff on the insides of the coolant reservoir. The coolant itself is green but there's this stuff on the sides like mold. I tried getting it off with my finger but it doesnt come off. Anyone have an idea what this could be?
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Perhaps some crazy colored stop leak?

Probably time for a thorough coolant flush :D

I did the plenum repair not too long ago and flushed the radiator and put all new green coolant in. I just noticed this recently. I hope its not transmission fluid, but why wouldn't the fluid be darker reddish, and not just the sides of tank? Does this image work?
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What year is your truck? And is that the original (as far as you know) radiator?

The inside of mine is black but it's fairly new too, can't remember for sure on the stock, but I wanna say black too.

Yours is white, maybe the inside coating is peeling and turning weird colors? :icon_shrug:

the truck is a 2001 1500 sport. The radiator, as far as I know since I have had it, is the original one. I did have it pulled it and repaired for leaking a while ago, but it wasn't even the radiator, it was one of the connections on the passenger side. there is little spots like what mold does. I tried rubbing it but it doesn't come off.

Weird. I have no idea.... Someone else will chime in eventually and may have more info :)

Have you ever noticed the inside of the plastic tank being black? Odd that yours is white.....

The tank has always been white. Yours is black though? how do you check the level?

I just pop the cap and if it's full I don't fuss with it. Yup, mine is black.

:( I just now seemed to realize you said "coolant reservoir". Which explains the clear sides... :icon_doh:

i would guess that somewhere along the way, a previous owner used some red dex-cool coolant for GM. it doesn't mix with with green, and forms a nasty sludge. you probably have some remaining residue in the block and radiator.

you might want to flush the radiator and block and heater core to get out as much as possible. there are 2 block drain plugs hiding behind the engine mounts. spray them liberally with penetrating oil like PB blaster or other for several days, and when you attack them, don't round them off. the drain plugs are hard to reach and are at a bad angle.