my buddy has an 05 dmax and its loses coolant and has no exterrior leaks. ?headgasket? i want to be sure because it is a 27.5 hr job. oh has no smoke out of tail pipe and still runs really really good

 

One of the more conclusive checks he could make is with the engine fully warmed up (preferably after running on the interstate at high speed for several miles) open the hood and look at the overflow bottle for the coolant. If there are bubbles and the solution is moving around in there, he's got a real issue - cracked or warped head, head gasket, etc. Note the engine needs to be running while looking at this.

I will say that even with a "blown" head gasket the truck still may run quite well. The only conclusive way to know of an internal coolant leak is through oil analysis. I would also send in a sample before starting into heads on that engine.

Hopefully someone with more expertise could add thier knowledge. Hope this helps.