If none of those work, here is a problem I had with my 99 suburban. I had a
combination of a slow leak in the heater hose fitting that goes into the
intake manifold on the passenger side front of the intake. Look and see if
there is a ball of corrosion around it, it is kind of hiding under the air
intake. Real easy to see with intake off. Second was my intake gasket
leaking. I didn't have any signs on the oil dipstick but looked like peanut
butter on the inside of the oil filler cap. Once I removed the valve cover
it was coated with the same peanut butter looking crud. Either one is a
pretty inexpensive fix.
I had this happen on my 99 Sierra 5.3 and it was the plate on the back
of the water pump. Took the water pump off and removed the 5 bolts
holding the back cover on and sealed with some gasket maker. Problem
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