What causes antifreeze to get under the plenum on a 98 buick 3800 eng ? thank's Luther

If it has the 3800 engine, usually the issue with this engine is at some time, the upper intake manifold (plenum) developed an internal coolant leak, from the plastic housing just under the throttle body, from the EGR tube that flows in to upper plenum. This causes the coolant to seep into the lower intake manifold ports and into the engine cylinder, on top of the piston, and when the engine turns over, sometimes this coolant causes this concern. If at any time you started your engine, and it stopped turning (cranking) over for a split second and then cranked over to start, this could cause the connecting rod to bend slightly causing the internal engine knock. Was your coolant/antifreeze ever low?? Is your engine oil have a milky color?

what year? make? model? what engine? if this is a 3.8L V6 ( 3800 II ) then the PCV valve is under the map sensor on the top front of the intake plenum.

Blown head gaskets cause this symptom.
Also, oil leakage on to the hot parts of your engine can cause this problem.

The smoke can be either antifreeze or oil that accumulated in a plenum or low spot that gets hot when you start your engine.

If each car has an identical 8th digit in VIN, then the motors are exactly the same.

3.8 is under the belly pan nearest the front of the engine...why don't you just have it changed by nice mechanic and if your nice, maybe he'll let you watch!
Good Luck

the color of the antifreeze will have zero effect on the problem you are having.  I suspect the replacement engine has a leaking manifold gasket or a blown head gasket.  Why would you buy an engine with that many miles on it?

should be on or near breather box

Replace upper intake and sparkplugs and wish it well!

Hi you have replaced allmost everything ,(try fuel pump relay)

check throttle pos sen this will couse stalling at idle and low rpm assuming you have the 3800 gen 2 motor