Lately I've noticed that when I let my truck sit and idle, there's a strong smell of exhaust. Almost smells like unburned fumes. I've tuned my engine, new plugs, wires, rotor, distributor cap, oxygen sensor, tps sensor, output speed sensor, and new exhaust. Truck does awful on gas, I think I get about 15 average about 12 city and maybe 17 highway. My exhaust tip is getting a carbon buildup, anyone out there have an idea what it could be?? I just got a new timing belt and had the the dealer and another mechanic look at it, they can't find anything wrong. Truck runs fine. Any help is appreciated.

 

Which 02 sensor did you replace? The upstream? or one of the downstream ones(CAT)? Check your other manifold too. I was told my truck only had 1 upstream and 2 downstream (3 total)... well it has 2 upstream (1 on each manifold)and 2 downstream (4 total)

That mileage doesn't seem completely outrageous for that motor. The 3.9's don't get great mileage. I get about 9 city 12 hwy with my truck, and the 5.9's actually tend to get better MPG from the factory... so consider yourself lucky I guess. lol

 

mine only had two oxygen sensors, I replaced both. I wasn't expecting MPG like a Honda, just a little better. However the smell is what bothers me, am I just paranoid?

 

17 on the hiway for a 4x4 dak is pretty good... I had a regular cab 2x4 and the best it ever got was 18's. My wife had a 2002 v6 club cab dak that never saw 17's...

 

holy ****, my dads quad cab w/ 3.9 v6 and 4x4 hits high 17s on the highway at like 75-80, so im sure it could do better if he drove it closer to the speed limit

 

the 3.9 is no gas saver. i'd say you're doing ok on gas mileage. i usually get about 15 city. one time i drove on the highway across 2 states at a steady 75 mph and got 24 mpg, but mine is 2wd.

 

my 2wd cc v6 dak (98) got 17-18 city so idk what ur talking about, but i drove it hard too

 

Just to let you know, the reason it smells more like un-burned gas is the magnaflow hi-flow cat you have listed in your signature.

 

Just to add to what I said: You probably aren't smelling unburned gas, per-say, just exhaust that hasn't fully burned up through a cat converter.