Missing coolant.......I think

[2000tahoeZ71]

Alright so I did the lower intake job about 6 weeks ago. Since that time, I have had to fill the reservoir up twice. Well, check it again today and it was low again. Changed the oil and didn't have any signs of coolant in it. Looked like oil. Poured the old oil into the empty jug ang measured at 4.5 quarts so don't think it's going to the oil. On startup I don't see any white smoke until maybe a minute in, and it smells like exhaust. Goes away when it is warm. I don't have any visible external leaks, no leaks from heater core. I have not kept up with how much coolant I have added. I have ran the heater control wide open, I really don't thing it still has air in the system. Any ideas, things I should check next?

 

[2000tahoeZ71]

Have been reading where cracked heads can cause this problem with eating coolant. Any way to check without pulling heads? The engine has never been over heated, runs cool (~185). What should I be looking for if I pull plugs?