When I check the coolant level in the radiator I can see a little bit of oil floating on top of the coolant. After further inspection the under side of the radiator cap has a thick black grease film. The inside of the filler neck has this same film. There is no signs of water in the oil and the engine runs strong. The engine appears to have been well taken care of. I'm thinking maybe it previously had a blown head gasket and the radiator was not properly flushed. Can this grease be flushed out of the radiator ? Should I just take the radiator out, fill it full of a de-greaser, and give a good flush. Or maybe just buy a new radiator.

 

Most likely not oil. When oil mixes with coolant you end up with a tan milkshake color. Sounds more like dirt or some carbon.

FYI oil in the coolant is very unlikely even with a blown head or intake gasket. Unlike some other designs there is no pressurized oil in the intake or the head. Oil in the coolant is very common on cars with over head cams. In those design there is oil pressure going through the head gasket since the oil pressure is greater then the coolant pressure it will push oil into the rad.

On a plus note and good flushing is all you need.

 

flush your radiator, heater core and engine separtely. dont want to be running crud from any to the other. there is some help with bleading air from the lt1 in the how to at the top of the section.

 

Thanks, it's certainly not a chocolate milk shake look in the radiator. I have been down that road before on previous cars. Under the oil filler cap is very clean no signs of water. The coolant actually looks clean except for the little bit oil I can see floating on the top. It's the filler neck and under the radiator cap that has a very dark black grease film on it. Makes me suspicious if it's also coating my thermostat. When I get some time I'll do a good flush and bleed the air out of the system.

 

You sure it not just a little left over dex cool? The original coolant that GM put in these was orange and it turns brown with age. You would be had presses to finding a 4th gen that does not have some dex cool spots in its coolant. I had my motor and radiator out and back flushed them and I still see a few spots now and then.

Coolant sould be done every three years so flushing is always good either way.

 

Don't forget to clean all of the old coolant and smutz out of the overflow tank. It is located under the battery tray and once that old dexcool stuff congeals in there, it is a pain to get out.

When I flushed my cooling system, I took the overflow tank into the kitchen sink and filled it with scalding hot water and took a flexible bottle brush to the inside. All sorts of crud came out.