My 04 F250 6.0 smells like coolant in the cab when on the freeway. This smell comes and goes when accelerating at passing speeds, running a steady 70 mph and higher and driving up hills. Anyone else had this issue? Could It be heater core?? The carpet is not wet on the passenger side...

 

Maybe a small leak in your heater core, that only shows up when the coolant pressure is up, i.e. the truck is working? It could be evaporating before it drips if the leak is small enough. Heater core would be the first thing I looked at.

 

Ditto on checking the heater core. Small leak might not show low coolant for days, but will smell badly. Also the leak on the heater connectors under the hood will bring the smell inside the cabin. Go figure.
On my trucks heater core has no valve and stays hot, what add to cabin heat in hot summers. I put 2 valves on the hoses on my first truck and plan to do the same on the second. That not only will cut off the heat in the summer, but in case of core failure, will give me option of shutting it off. When my core was dripping in wet season, my glass was constantly fogged.

 

Thanks guys for the help im going to see about replacing that heater core. haven't done one before so dont know what to expect as far as difficultly level...

 

Heater core is easy on mine trucks and I believe you have the same set up. After removing the glove compartment you have to be careful on removing mixer valve as it is easy to brake, but than you have the core right there.
The only hard trick comes from the fact that the hoses "quick connections" is VERY misleading name. I spend over 1 hr trying to disconnect them before I cut the hoses.

 

its deffenetly your heater core or hoses to heater core cant be anything else

 

That is what I'm thinking cause all is good under the hood from what I can see. I even replaced the bottle cap or "degas" cap I think its called cause there was word on here that Ford made "An updated one." whitch they did however that was not the issue.

 

It could be a bad coolant resevoir cap allowing vapor to escape. If this is the case and you have your air on vent, the vapor will get pulled into the vent and then into your cab. I would replace the cap first before doing anything else.

 

Cap is pretty easy to test. When you get the engine hot, put a glove on and squeeze big hose checking for pressure. If it holds the pressure - the cap is good.
When my reservoir cracked, I drove about 200 miles with the cap removed and don't recall any smell inside the cabin. The engine compartment has no direct connection with the cabin and the holes around heater pipes are just lousy job.

 

Yea I just replaced the reservoir cap about 3 months ago and there was no change. Still have the smell in the cab.

 

I smell sometimes coolant on my truck, one day I hear a "whisper" noise coming from the resevoir cap and that were i find my smell and evaporation off my coolant. Great! Powerstroke