Why is there a diesel smell in the cab when towing a heavy load?

[Timothy84]

I’ve been running a 2019 Ram 3500 with the high-output Cummins for a few years now, and lately I’ve noticed a faint diesel smell in the cab only when I’m towing my 16k gooseneck up a long grade. There’s no visible smoke from the exhaust or any drips underneath.

[GregoryZT]

I had a faint diesel smell in the cab during long grades too. For me it turned out to be a tiny exhaust manifold leak near the turbo; I could hear a soft hiss and feel heat by the gasket when I had the hood up. Replacing the gasket fixed it, and there were no drips or obvious smoke after.

[Mark.P]

Could be an exhaust path leak or a small fuel line vent, or even a loose clamp somewhere. Under load with heat, the smell can sneak into the cab through the HVAC intake.

[Scott4]

If you can, have a shop do a smoke test or check the exhaust system for leaks, and inspect the manifold, downpipe, and hangers. Also listen for a faint hiss when the engine is under load with the hood up.

[Robert_C]

Does the smell show up only when you’re towing heavy grades, or does it show up in other situations as well?