manifold leak vs MPG

[Coastal]

I know a manifold leak probably causes some decrease in performance, but would it impact your gas milage? Both mine are leaking so bad it is emabrassing driving thru the neighborhood in the am and leaving the parking garage at work (it seals ok when it heats up, but you can still hear it with the windows down if you are listening for it)

 

[Campfire]

Yes, not much of a drop in MPG but some. Exhaust gases are a "pulse" while the engine is running. It will leak exhaust on that exhaust stroke, but as the valve closes it creates a negative pressure and sucks in outside air, i.e. fresh air with 18.2% oxygen. This then gives a false reading to the O2 sensors saying that the engine is running lean or even misfiring, and then the computer starts compensating by changing the injector pulse. Blah, blah, blah... = not good!
Get your manifolds fixed. The longer you let them go, the more problems you could have.
Sorry, I regressed back to teaching the new young bucks in the shop.