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Q:

coolant in oil on 1999 Chevrolet Silverado 1500

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can this be caused by an intake leak.
(2) Answers
If it is a 5.3 liter it can also be caused by a cracked head. From 99-06 in the 5.3 liter engines GM outsourced the heads to a company called Castech. These engines had a casting flaw which was known for cracks and or blown head gasket.
Yes, a leaking intake manifold gasket can cause coolant to leak in to the crankcase and mix with the oil on the 5.0L and 5.7L V8 engines.