So the pas few weeks I have noticed a HUGE drop in mileage. From about 16-17mpg down to 11mpg. I am trying to find out what is causing the drop in mileage. I have just put new plugs, wires, cap and rotor in, new ox sensor aswell. I did however have my intake mani off, and after reading a few threads, I'm thinking I may have broken off the barb for my TVV.

I'm wondering if the TVV not working would cause the huge drop in mileage? I have also found it may be my fuel damper that is leaking. I am getting a slight STALE fuel smell from time to time, so I am thinking fuel vapor and EVAP system before the damper that would be raw fuel.

Any thoughts?

 

maybe a gas leak into the exhaust??? id have the fuel system pressurized to look for leaks

 

Just checked my TVV and it is all good. I have not noticed any fuel dripping or pooling under the truck. I have put a 3" lift in my truck and 33x12.5 on, but after the lift the mileage had maybe dropped by 1mpg, would going up 2" in tire diameter really affect the mileage that much? 200miles per tank is no fun

 

hahaha horrendous what a mint word, also going to a 33x12.50 tire would make your gas mileage suffer that much, stock gears?

 

running 4.56's with a 5spd, they were on my buddies 22r with 4:10's and never had this kind of mileage

 

I get a solid 14mpg in town and 18.5 highway with my 3.0 and 32x11.50 ATs. That's without correcting for tire size difference, which is negligible IMO. 11 is a bit low, but close.

Andy

 

quick Q, you noticed ty gas mileage after you got the tires on?

 

The tires I had before said 32x11.5 but the actual measurements of the tire was 31" tall and about 11" wide, the BFG muds are a true 33x12.5. Even with overall diameter difference and speedo differences, I am at about 12mpg, which I think is terrible for the mods I have done. I was expecting maybe a 1-2mpg drop, that is why I am thinking that this fuel smell is my issue. It only began after I removed my intake plenum. Over fueling maybe? I have looked for a fuel leak, but found none. The truck still drives great.

 

Possibly a leaky injector seal? I had a Honda Prelude with that issue (smell but I couldn't see a leak) and after a $2 O-ring it was all better.

 

I know the tires are going to kill mileage, it's the smell that is new. Only started smelling it after I took the mani off to replace the by-pass hose at the back. So i don't think a injector seal would fail right as i took my mani off. I was thinking I didn't hook a line back up or I crossed a vac line, but I have checked and re-checked everything. I am 100% positive that the lines to the side of the mani are hooked up correctly now. Any other ideas?

 

I dunno, plugged cat? Vacuum Leak? Bad wheel bearing? manifold leak? There's like a million things that it could be.

 

have you been warming it up in the cold mornings?

 

hasn't been a cold morning yet. the fuel smell is all the time, warm or hot. I smell it stronger when I punch the gas.

 

Larger diameter tires will also cause the speedo/odo to read low i.e. you might be going farther than you think. That wouldn't cause a 40-50% drop though.

 

Throwing any codes? A bad o2 sensor could drop your mpg in a hurry.

 

Have you checked your fuel lines and filter(Frame rail) in case something has hit them and damaged them and caused weeping that is barely leaking / a rusty line in need of replace - not enough to drip ?

 

Those would be things I would check if it just happened randomly, but this smell started as soon as I put my mani back on and fired up the truck.

 

you musta hit sumthing while reinstalling then.. take a good look for fuel leaks ill bet its leaking sumwhere and burning up. it must be. the only explanation for the smell.. Perhaps get a fuel pressure tester and see if it holds pressure?

 

There is a line that leaves the Charcoal Cannister on the bottom of the can near the Firewall , its a vent line (at least on my '91) , supposed to be tucked in frame rail , think maybe that has been pulled out of its hole in frame or pulled loose from bottom of cannister ? , that would be right in the path of the passenger manifold , just a thought .
Other than that maybe it would have to be a small leak/weep somewhere close to the area you worked I would imagine since it started right then unless it is a coincedence .

 

Check your fuel pressure regulator and fuel pulsation damper.