So my d2 just hit 160k and been having issues with losing coolant.... I did a pressure test and found the TB heater leaking... so I bypassed it, still losing coolant, Its not overheating, temps stay under 190 when its actually full.

The water pump, radiator, and every coolant line has been replaced, 180* T-stat.

Been noticing a coolant smell from the engine bay, had little bit of exhaust smoke but it didn't smell like coolant, so could it be the head gaskets? I can't really see anything leaking...

 

cracked coolant reservoir or bad reservoir cap are possible culprits​. My last coolant reservoir had a crack that would only leak once the truck was good and warm then would leak overnight. That said, I expect you'd be finding some coolant somewhere in the engine bay if that were the case.

 

x2 with cap and draining from the small drip hose above the exhust manifold. OR, head gasket leak behind engine, vapor goes and can smell coolant. Check under the truck between the firewall or coolant visible on the cross bar underneath.

 

I actually did that the other day, and did find it was leaking in the back but I'm not sure if its the valley pan gaskets or the from the head gaskets, but I ended up ordering a head gasket kit and probably just do then since its been almost 100k since they've been replaced.