2012 F250 6.2L, 3.73 - towing my 9.5K trailer through the Siskiyou mountains on I-5 in CA and OR today and noticed a burning smell on the long uphill grades. At first I thought it was oil or transmission fluid and pulled at the next rest area to check. Fluids were all full, clean and no leaks and no fumes from the engine compartment nor leaks or dripping underneath. Both the engine and transmission temps were completely normal. I also checked the tires and brakes - all normal.

I have had this truck and trailer for over a year and towed over many mountain passes - thinking back now I have noticed it before but not as strong. I always use the manual mode on the transmission in the mountains but one thing I have changed recently is that when approaching a long grade I downshift to 3rd at 65mph/3500rpm and really open the throttle to hold the gear as long as possible - when it slows to 50-55 I then shift to 2nd and the rpm climbs to 4500 or so. I then back off the throttle and let it climb the rest of the way in 2nd gear.

I am thinking that at full throttle and only 3000 to 3500 rpm there is some unburned fuel in the exhaust and that is what I am smelling. Pulling hard burns quite a bit of gas in a short period of time.

Any thoughts?

 

Have you serviced the transmission lately? Any signs of oil under the hood especially around the PCV tubing after a long pull? Any signs of oil in the air filter housing? really doubt your smelling unburned fuel with the exhaust exiting all the way in the back of the truck, also if you had unburned fuel it would more than likely set the CELL. How many miles on your truck?

 

77K miles on the odometer (2nd owner). Transmission fluid changed at 65K. Fluid is nice and clean and no burnt smell or color. No obvious oil under the hood but will look closer - I will also check the air filter. Agree about fumes from the exhaust pipe not likely to be smelled up in the cab when moving at highway speeds.. No CEL notifications.

Also had my wife sniff under the hood with her superior sense of smell and she didn't smell anything. She easily noticed it in the cab when towing up a long hill.

 

most likely.. exhaust being trapped between low air pressure between truck and trailer.. along with the direction of the wind..

no cap on truck.. and nearly flat noise on trailer..

I have had it going down I 77 in West Virginia hills.
I have a similar setup..
my opinion.

 

Its possible it a vehicle in front of you...even way in front of you.
Every once and awhile ill get a whiff of exhaust and start looking up ahead to see where its coming from.

 

My exhaust is dumped under the truck, and even pulling my 37 footer, I never smell any exhaust fumes.

 

what about.... the added 'work'... and any light oil coatings on engine and/or transmission cases/blocks... and is burning off ??????

aka, heat soaking... stuff on outside of heat producing parts.

 

I have a 2011 CC SB 4X4 and tow a 34' toy hauler BP 11,800 pounds loaded. My exhaust exits in factory location. Neither my wife or myself have ever smelled any exhaust fumes. I might not notice fumes as much, but my wife would. She has the nose of a blood hound (in function only, not looks).
Maybe the OP has a fifth wheel, shorter truck, leak in the exhaust camper shell, I don't know?
Mark

 

Use a max AC setting or re circulation when you notice it, not outside air or defog.

Your headers are glowing cherry red on climbs, might just be road dirt, grime, oil burning off. Not to mention all the plastic bits of Prius building up on the cats which are a perfectly good table for dirt to build up on.

 

I am towing a 34 ft travel trailer - somewhat rounded front cap (see picture in my signature) and no top on the truck bed. I first thought the fumes were from a truck ahead of us but the Siskiyou mountains have quite a few long climbs and we smelled the fumes every time. Outside temps in the upper 30's - windows were closed with heater on but fan on low setting.

It could have been some spilled oil from the last oil change (I take it to the shop) but I didn't see anything obvious or smell anything in the engine compartment. No oil in the air filter/housing. For safety reasons I didn't pull over on the shoulder during the climb but waited until the next exit or rest area so the engine would not have been as hot then.

Anyway, we are home now - the rest of our trip was I-5 N to the Seattle area which is just rolling hills. Fuel consumption was normal and the check engine light never came on. Will look around closely with the shop light later today and see what I find.

 

Could be the problem - I will check for this

 

Inspected the truck today with a bright drop light; other than the Prius debris and cat carcass buildup everything was in first class shape - no sign of leaks at all. Will keep my nose peeled the next time I tow through the mountains and see if I detect the fumes again. Thanks for all your input.