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MAF's aren't simple "resistance" elements. On a GM, it's a heated platinum element with a built-in circuit that would change an output frequency/ square wave duty cycle depending on how cool the platinum element gets (that's why they are so expensive). They have very, very thin/delicate elements so that they can change readings very quickly. When they get dirty, they get insulated from the cooling properties of the air or have more mass and get "slow to change" or don't change at all. Cleaning yields varying results. Running with a leaky air intake filter or no filter element at all can kill them fairly quickly. So can an engine compartment with a lot of oil (like from leaks, or blow-by and a bad PCV system).
It is my understanding the newer megasquirts can use them as an adjunct to the MAP, or even instead of it. Only the new ones can do it because the processors are fast enough to measure the pulses.
Many people on boards don't think they improve running/economy/emissions any more than a good MAP.
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