I have run the car about 550 miles and noticed this slimy stuff on the radiator cap and inside the radiator. Car runs fine, stays cool at 190 degrees but this has me concerned. I am running the green antifreeze at 50/50 mix. No antifreeze in the oil and the oil level hasn't moved so I don't think it is oil in there. Is this normal from a new engine build or is there something else going on I am missing? Could this be from the chemicals and sealants used in a new build?

At first I thought it might be a head gasket but all fluids are at the proper level. Any thoughts would be appreciated.

Wade

Looks like oil. Brown and not mixing with water.

Maybe you had some oil in the engine cooling jacket by accident.
I would flush the cooling system till she’s 100% clean then reload
with new anti freeze. I put in 100% good brand (Prestone) antifreeze
not the mega expensive. No water in the cooling system
makes it pretty impossible to corrode.

Fire her up again and see what happens.

What does the oil look like?

I agree with Gus68, looks like oil. Was the engine broken in by the builder ? If not it is possible a bit of oil got in the engine water jacket. Flush it, then flush it again. Use hot water, it will help lift the oil into the mix. Clean the cap ( or install new clean one) fill her up, then go for a drive. When cool open her up & see how it looks. Also make sure the oil is new looking any tanning of the oil will tell you there is a problem. Any water will settle down to the lowest point & not show up on the dip stick. If in doubt drain the oil into a clean pan and inspect for water bubbles or tan oil. Good luck Be safe

Is your car an automatic? If so, don't forget the transmission cooling circuit in the radiator. Maybe you have a leak somewhere there so you get transmission fluid into the coolant. Maybe not the first thing to suspect, but Worth checking, in my opinion.

thanks for the ideas guys. I rebuilt this engine so I may have ended up with some oil in the water galley somehow. The oil is clear as when I poured it in from the bottle. I did use prestone green anti-freeze and will probably stick with that.
Ronnie, yes it is an automatic, I never thought about that aspect of it possibly being a hole between the trans cooler and anti-freeze.

I will give it a flush this weekend and start over to see if maybe it was from the engine build or if I really have a problem somewhere else.
Thanks for the input guys,
Wade

It looks like rust to me, see a lot of cars with it

As stated earlier, if concerned, flush it and start over. You should use a quality antifreeze. I mix it with distilled water verses tap water. If your water is soft probably not a big deal but if it is hard you run the risk of having stuff plate out. I would not recommend 100% coolant. It has less heat transfer and can also plate out.

It could also be bacteria. It will grow in antifreeze