I have 4.0 and gotten the same thing. Smelled like burning plastic/coolant to me. When my dad was checking it out, we discovered I have a weeping freeze plug at the front of the motor. Hoping that is the problem.
Mines been slowly disappearing too, so we weren't exactly sure where it was going. I believe its the first freeze plug on mine, it was difficult to see/get a flashlight in between the manifold tubes and block.
Forgot one other thing...I hit the A/C button the other day by accident while the heat was on (and it was like -3 F), which let to a pretty gross coolant smell until I realized what I had done and turned it off.
Sometimes the ac turns it self on when you turn on the heat(sneaky little ish)
As I roll down the trail,I hope my traction will not fail.But if I spin & begin to slide,please dear lord protect my ride
Just a thought, I had a Tahoe that had a bad rear axle seal and it caused gear oil from the axle to leak onto the brake rotor. When I drove it it caused the gear oil to burn and burning gear oil smells more like garbage than any automotive fluid I can think of.
If it comes back maybe check your rear axle seals or for gear oil leaks anywhere, really.
Add me to the coolant leak camp.
I'm thinking there's a leak that's only apparent when the system is pressurized. Small for now but could get big in a hurry. Can you do a radiator pressure test?
Sometimes a parts store will loan the equipment and it's no more complicated than clamping on a hand pump with a dial gauge to the radiator fill neck to see if it can hold air pressure for a given period of time.
2000 Laredo 4.7L Selec-Trac. 177K miles, Leather, 245 tires, tow package, crappy CD changer, full size spare, roof cross-rails & sunroof. Changes & fixes: Rear Addco sway bar & greasble bushings, upgraded headlights, upgraded tail lights, new hood struts (replaced 2X), new rear hatchglass struts, new radiator (replaced 3X), new oil pressure sending unit (replaced 4X), 2 new speed sensors, 2 new door lock actuators, new ignition key cylinder, chrome front tow hooks, Mobil 1 Synthetic 5/30, Purolator Pure One oil filter, EBC slotted & dimpled rotors, EBC Greenstuff pads, new front shocks (replaced 3X), new rear shocks (replaced 2X), new IRO steering stabilizer, new timing chain cover, new water pump, thermostat, new thermostat housing, new high pressure power steering hose, Michelin-X radials, new sunroof assembly, new single-piece lug nuts, new exhaust manifold studs & gasket on driver's side, new Durango alternator. Jeepforum.com sticker, Visit cobaltboatsforum.com when you can!