why is my gas milage down?

If your "exhaust" smells? Than this would be a Cat gone bad. They usually go bad from mis-fires or running too rich.

 

If the smell is a rotten eggs smell, this ^^^ If it is a raw fuel smell, look for fuel leaks. Raw fuel evaporates quickly so a leak may not be obvious.

 

i cleaned my MAF, new air filter, Deckplate mod, and new exhaust and im suddenly getting 20mpg HW. i used to barely ever see 18..

 

Those are all good things to consider, with the exception of the Deckplate Mod, as that doesn't apply to all Yota's.

Fink

 

If you have an exhaust leak pre-O2 sensor, will the sensor richen the fuel mix as the leak draws unwanted air in? Reason I ask is, I'm also suffering from a sudden mileage decrease. I replaced my cracked manifold a while back, but now after my last Seafoam treatment, I can see the new one is cracked. Any 2.7 liter owners have input on headers while you're at it?

 

If it smells like gas and makes your eyes hurt, it might actually be a vacuum leak causing the engine to run too lean. I know it sounds funny, but I had that happen before. I thought I was chasing an over rich condition. All it took was a new intake gasket (on a carb though).