Odd yellow mold looking stuff under radiator cap

[spradilak]

Hey guys been lurking here for a while, first time poster.

I was given a 1990 C1500 that was passed down to me from my grandpa. It runs great with 560,000 miles, original motr and just had the transmission rebuilt for the 2nd time in that lifespan.

I've been putting some money into it kind of making it look Bice and replacing some things, when I replaced the radiator cap because the old one was super rusty. On the bottom side of the radiator cap it looks like yellow mold growing on it. It appears to be an oil mixing with coolant situation but I've never seen this. Anyone have any insights?

Hopefully this image will work. Thanks in advance!

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[Komet]

How do you know what it appears to be if you've never seen it before? I'd flush the system and see if it comes back.

 

[Schurkey]

Wild Guess: Ancient "Dexcool" antifreeze.

I've seen "green" chunky-looking residue in a cooling system that had green antifreeze.

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[spradilak]

How do you know what it appears to be if you've never seen it before? I'd flush the system and see if it comes back.

I mean of I had to guess judging on past experiences with coolant and oil mixing. I'll definetly try that this weekend.

 

[spradilak]

Hey guys been lurking here for a while, first time poster.

I was given a 1990 C1500 that was passed down to me from my grandpa. It runs great with 560,000 miles, original motr and just had the transmission rebuilt for the 2nd time in that lifespan.

I've been putting some money into it kind of making it look Bice and replacing some things, when I replaced the radiator cap because the old one was super rusty. On the bottom side of the radiator cap it looks like yellow mold growing on it. It appears to be an oil mixing with coolant situation but I've never seen this. Anyone have any insights?

Hopefully this image will work. Thanks in advance!

You must be registered for see images attach

That sef

Wild Guess: Ancient "Dexcool" antifreeze.

I've seen "green" chunky-looking residue in a cooling system that had green antifreeze.

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

That definetly seems possible. I feel like the maintenance was probably neglected and I'll be doing a full flush this weekend. Thanks!