I'm leaning more towards combustion gas in cooling system at this point. I popped open the reservoir tank after letting the car sit for about 2 hours in the garage and I immediately smelled exhaust gasses from the opening.

I'll do a leakdown test this weekend to confirm this, but I already went ahead and put in orders for new parts.

104 010 20 20 - Headgasket kit - 1x
104 050 01 58 - Valve stem seal kit - 1x
104 203 00 08 - Metal hose to thermostat - 1x
103 203 00 82 - Rubber connector hose for thermostat - 1x
104 990 00 12 - Cylinder head bolts - 14x
102 016 03 76 - Cylinder head washers - 14x

Additional notes/information:
- I've been seeing random puddles of coolant on the floor everywhere I park. I haven't determined exactly where it is coming from since I've seen the puddle form on both sides of the engine.
- This motor was originally from another car (1991 300CE @ 200k miles) and I swapped it into my 190E that was running just fine. I did a basic compression test prior to installing the motor and the numbers looked good so I did not tackle a headgasket job.
- With above in mind, my radiator, expansion tank/cap were all relatively new (5 years old) and working properly with the original M103 motor. No clogged radiator or leaves stuck between condenser and radiator. I also did flush out the block with a garden hose attached to the water pump inlet with the drain plug loosened until clear water came out.
- Driven around 70 miles since motor install (this last Saturday). Issues started developing roughly after 40 miles. (Monday morning trying to commute to work).


@MCallahan: Issue is not transmission. I have a freshly rebuilt (0 miles as of last weekend) 717.411 transmission attached to the motor. It feels and drives normal so I am ruling that one out, but I do understand your viewpoint with extra engine load.