I was talking with Pat at Tiger Tool recently about whether or not a dirty air filter will actually reduce your fuel mileage. Traditionally, a clogged air filter is thought to rob your engine of not only power but MPGs. But is this theory based on outdated, carburetor technology? Here's how Pat summed it up. It's a little technical for the layman but I'd love to hear what people think:
A MAP based fuel injection system measures manifold pressure to estimate the amount of fuel required by referring to a reference table, doing a few calcs and then adjusting the injector pulse width. The main restriction is the throttle plate downstream of the air filter. Add in the effects of the closed loop system and I can't see why a dirty air filter would reduce fuel economy. I do know that power would be decreased. Also if the program went to open loop or other power mode at reduced vacuum then you would see reduced mileage. I think the whole dirty air filter mileage is a hold over from the carb days where a change in differential pressure across the venture caused a dramatic change in mixture ratio.
Even more so in any system that actually measures air flow through the use of a flap, hot wire or?. The actual air flow is measured and thus upstream changes should have no effect on fuel mileage.
So can we still state blindly that dirty air filters reduce fuel mileage!
Well put, Pat! But I'm sure there is still debate out there. Let's hear it!