Kind Regards
Danny
Diagnose before replacing parts. Inspect the intake gasket first, since the 3.1L pushrods pass through the intake edge and can let oil into the coolant. Do a compression or block test for exhaust gases in the radiator and pressure-test the cooling system. Also check the radiator transmission cooler for an internal crack, transmission fluid level, and the engine oil for milky contamination.
A 1996 Chevrolet Lumina 3.1L owner found oil mixed into the coolant, with no external coolant leak or steam from the exhaust. Experts suggested the intake gasket as a common cause, a possible head gasket, or transmission fluid entering through a cracked radiator cooler. Recommended checks include inspecting the intake, compression or exhaust-gas testing, a cooling-system pressure test, and checking transmission fluid and engine oil.