I am getting heavy fuel smell in the cab of my truck. I smell it most when the heat is on. It gets even worse if you roll either window down. What could cause this?

 

I've had this happen twice since I've owned my truck. It usually happens when I get hot and heavy on the go pedal, passing somebody or something like that.

Both times for me it was the o-ring on the injector tube. Disconnect your injector line, move out of the way and brake the tube loose, think it's a 17mm or 3/4 works well. Unscrew it and slide it out. Has one o-ring and they like to leak. First one for me was #6 a year or so later #5. You can see the fuel around the head and on the injector line. Others were all dry and the suspect lines were dark, wet looking.

Check over your lines by your filter and such but your situation sounds like what I've experienced and after I replaced the o-rings the fuel smell went away.

 

Check the rubber gasket that runs across the front edge of the firewall.
When it drops down, it allows fumes from the engine compartment to be sucked into the HVAC fresh air intake.

 

It don't do a very good job Shovelhead. I glued mine in place and each time one of those injector tube o-rings starts leaking and I romp on the throttle it smells like I just poured diesel in my lap!

 

It'll help with the 'normal' fuel smells, but if your system is hemorrhaging diesel fuel, not much will cover that up..........
In any case, the firewall seal is a quick check type of thing.

 

I seem to smell more diesel smell when the temps are low lately its been 28-34 degrees. but also I get the stinks when I use Rotella (rosmella) when i do a new oil change. I see your living In MO. when I was stationed out there I know it gets cold. so maybe take that in consideration as well as checking your crankcase breather. if its stock clean it and maybe move the line farther back under the cab. or look for excessive oil or gasses. but I seem to see more gasses when cold too.