2000 7.3L Map Sensor Operation???
 

Hello all. Looking for a Factory Ford Tech or other knowledgable person to explain the operation of the Map sensor on the 2000 7.3L engine. Some background is in order; I've had the truck for a few years and had noted some electrical issues related to crappy connections. I had previously cleaned the in-cab connections as well as those located at the rear but had not done the engine compartment. This cleared up a number of problems. The fun started after doing same under hood. My fuel mileage dropped from a reliable 18 city to 12Mpg or so. I have examined the input to the ECM using both a Snapon MT2500 scantool & the Ford NGS tool & see some strange behavior. I notice that the ECM when first powered has a default value of 57Hz for the variable freq MAP input as well as the Variable DC Voltage (0-5v). The 57Hz value is present until the engine is shutdown & restarted, at which I see a value of 0Hz (I suspect it's the computer locking the input out as out of range). I have been all through the factory service documentation & have searched other places. I have seen 2 different answers to the question as to what signal the MAP sensor passes to the ECM. One lists a DC voltage between 0 & 5v while the other lists a variable frequency signal of between ~50Hz & 150Hz or so.Now for the reason for the interest; my fuel milage sudenly dropped from ~18mpg in town to ~12mpg. That's when I ran the scan tests above. What I am trying to remember is what I saw for the MAP variable freq input when I looked a year ago - I keep thinking I saw readings in the ~80-160Hz region. So, I am trying to determine just what I should have for the input; variable freq or DCV signal (or both?). I keep thinking this may be it as my mileage is very good when I first fire it up (after reseting the ECM) while my performance is every bit as good as it is with the signal suppressed by the ECM (0Hz) but my mileage goes to pot. I have tried both a new MAP sensor & a new ECM with the same results as above. The wiring also checks out, hence my question to the Ford Tech Community above. Thanks in advance for any input.