Hi, My mother has a 1999 Lumina with a 3.1 Engine. We thought she needed the radiator flushed out because there was rust in it. Well it turns out the "rust" is actually oil. The dealer tells her upwards of 750.00 to fix. Can anyone tell me what is involved in fixing this. Thanks

How and why did this happen, what does the dealer say is the cause?

He really didn't really say except that the that is wasn't a cracked engine block or anything like that. He said something about a seal in the engine. My mother really didn't question him much. I will try and find out more info

mine 2001 lumina got same thing and I think it's a leaky intake manifold gasket

It's most likely an intake manifold gasket leak, very common on the GM 3.1 and 3.4 engines. If you have mechanical ability, it is not a job beyond a DIY, but it involves taking the entire top of the engine apart and a couple items some people don't have (in/lb calibrated torque wrench, loctite, etc...) IT would be a weekend project for your first time (I've done quite a few and have it down to about 5 hours). Organization and cleanliness are your two best friends if you attempt this yourself. If you're not too sure of your skills, it's best to leave to a mechanic. Shop around, as the price for this job varies from $450-$900. Make sure wherever you take it uses the improved design metal-framed gaskets, not the junk plastic-framed oem design (unless you want to do this again in a year or two).