Oil in the overflow tank? on 1998 GMC 1500 Pickup

I bought a 98 pickup and it has been sitting for about 6 mos. I checked all the fluids and it had about a 1/2" of oil floating in the overflow cooland tank. The motor runs great and the radiator water doesn't look bad. Where could this have come from? I cleaned out the tank and hope that it won't do it again. Any other ideas? Thanks-K

by in Twin Falls, ID on January 09, 2011
Get Your Car Fixed at a RepairPal Certified Shop.
We certify shops based on expertise, quality work, and verified customer reviews
1 answer 1 comment
ANSWER by , January 09, 2011
GM vehicles have had problems with Dexcool coolant eating through their intake manifold gaskets and then leaking oil in to their coolant. If it persists, I would think about replacing the intake manifold gasket and possibly the whole manifold, if it is pitted, ASAP.
COMMENT by , January 09, 2011
This pickup had green coolant in it. Did the 5.7 have this problem too? I thought it was a V6 problem. Thanks-K