Sorry for all the acronyms. AFM = Air Flow Meter. The AFM is the large device located on top of the air filter element container ( right behind the driver's side headlight in the engine compartment on 3.0 equipped 4Runners). There is an electronic plug attached to it. The way it works is: there is a spring-loaded door inside that opens wider as a larger volume of air is drawn into the engine. The motion of the door is what transmits a signal (via the AFM electronics ) to the ECM (engine control module or the 'computer'). It helps determine how much fuel should be delivered to the engine. ( On the other hand, the O2 (oxygen) sensor in the exhaust pipe is determining the amount of unburned fuel by measuring exhaust by-products. The ECM is constantly balancing these readings, along with many others, to give optimum performance, mileage and driveability. Just make sure the hinge on the AFM door is lubed and working smootly and freely ( no binding). If it's not smooth, it will cause driveability and mileage problems.
The 'computers' on these older (pre-1996) model vehicles are what is known as OBD-I. They are not as sophisticated as the newer OBD-II models. That's why in order to get the diagnosis out of an OBD-I computer, you read the CEL (Check Engine Light) flashes by jumpering the T1 and E1, terminals in the 'Diagnosis' box under the hood by the fuse box. Also for that reason, there is not a 'chip' or way to just plug in a re-programmer and 're-set' the computer. There is a company ( cant remember the name right off ) that can recondition your ECM for more power, but youhave to send the whole thing in to them. The way I understand it,, once it's set,, it's SET. And I think it's pretty expensive -- a few hundred dollars to get this done.
Most have found that it's just better and more economical to make sure everything is working properly,, maybe do headers and new exhaust ,, and let it go at that. The 3.0 was not the greatest performing engine that Toyota ever built. Having said that, I like my 3.Slow and it has served me well. Mine has 183,000 miles on her and still gets over 20 miles per gallon ( highway mileage). Just have to remember, these older 4Runners are HEAVY vehicles. It costs mileage and performance to carry weight.
Mike in AR