can someone explain why certain 4.0’s have a MAP sensor?

[stamina]

certain 4.0’s in the junkyard, specifically 1990 and 1991, have a MAP sensor (and the MAF sensor) and a plug for it on the wiring harness. in every picture i see of a 4.0 with the MAP the vacuum line to the sensor is not even hooked up and has a small white piece on it. furthermore, i took the computer out of a 91 with the MAP and sensor plug, stuck it in my truck which has a harness from a 92 explorer with no MAP sensor plug, and it runs absolutely perfectly, no CEL or anything. is it necessary at all?? i find it very peculiar and was wondering if anyone had any input on the subject. both trucks + harnesses are Federal emissions 4.0 4x4 manuals with no EGR.

 

[Uncle Gump]

It's used to read barometric pressure.

 

[RonD]

+1 ^^^

Ford was hedging bets on MAF sensors ability to compensate for elevation

 

[stamina]

+1 ^^^

Ford was hedging bets on MAF sensors ability to compensate for elevation

so it’s intended to not be hooked up to vacuum?? and to read actual atmospheric pressure?

 

[Uncle Gump]

It is...

 

[stamina]

It is...

thank you very much sir

 

[RonD]

+1 ^^^

Gasoline's air:fuel mix is 14.7:1 ratio

This is a WEIGHT RATIO, not volume
14.7 pounds of air to 1 pound of gasoline
14.7 grams of air to 1 gram of gasoline

Every wonder why fuel injectors are sold by lb/hr, pounds per hour, that's why, fuel flow into the engine is based on grams per minute or pounds per hour
A 4.0l engine can pull in 4 liters of air every 2 RPM at wide open throttle, lower RPMs are just math, computers are GOOD at math, lol
But.................have to know how much each liter of air weighs so you know how much gasoline to add

MAF is "MASS" air flow, sensor, mass is another word for weight
MAF weights the incoming air, well 10%-15% of it anyway, lol

As you go up in elevation there is less "air" oxygen, so its lighter
If you HEAT air, it gets lighter, i.e. "hot air rises", it's how Hot Air Balloons work/fly

Vehicles that only had a MAP sensor read outside air pressure with key on, to get a baseline of outside air weight, once engine is started the computer can then calculate fuel mix based on that baseline using engine vacuum levels
Barometric pressure numbers were programmed into the computer
If you drove from 500ft elevation up to 2,500ft elevation you would notice engine starting to struggle above 1,500ft or so, you would pull over, shut off the key then turn it back on, get the NEW baseline for 1,500ft and engine would run better

MAF sensor didn't need that, computer could adjust for lighter or heavier air "on the fly"

You still have less power at any higher elevations, ask people who live in Denver, lol, because air weighs less so have to add less gasoline, so you get less power
This is why turbo anything is popular in Denver or other high elevations