Scotty I have a 99 f150 with 4.6 engine I bought it and put in a fuel pump ran fine 2 days later the truck stall out and now only runs with the mass air flow sensor unplugged I've looked and found no vacuum leaks and replaced mass air flow sensor and crank sensor and temp sensor I've asked many mechanics and no one can find the problem plz help
Did you use an OEM MAF sensor? Did you already have the vehicle scanned for any codes?
One thing that's easy to miss is a brake booster internal vacuum leak so maybe disconnect the booster and plug the vacuum line to see if that helps.
Testing the MAF sensor wiring on your '99 F150 is easy enough to do with a $10 multimeter.
Here's a link to a step by step tutorial on how to do that
https://easyautodiagnostics.com/ford/4.6L-5.4L/maf-sensor-tests-1
1) MAF Signal. 2) Ground from ECM. 3) Chassis Ground. 4) 12 volts.
That's a fine way to test the wiring integrity and the MAF sensor but here's an easier and better way if you also have a cheap scanner.
Backpin that #1 wire and hook the multimeter up to it (as described on page 3 of the tutorial under TEST 4: Testing The MAF Signal.
At the same time hook up your scanner and look at the MAF Sensor Voltage (MAF (V) ) that the computer is seeing. You don't need that expensive scanner. A cheap one will work just as well. Like this:
In this screenshot (with the engine running) you see that the yellow multimeter that's backpinned to that #1 wire on the MAF sensor connector is reading 1.3 volts.
The scanner is showing that the computer is also seeing 1.3 volts.
That shows that the wiring integrity of the signal wire between the MAF sensor and the computer is OK.
In your case, the engine won't even run with the MAF sensor connected.
So with Key ON, Engine Off the voltage reading on the multimeter AND the scan tool should be about 0.02 volts.
If it isn't then do the Power and Grounds test in the tutorial.
Yes as already suggested only use an oem sensor don't use some cheap Chinese made one that could be the whole problem. But if not that means it's running on the computer when it gases but doesn't want to get the data from the mass answer which would either mean you have a massive air leak somewhere and the sensor gives skewed data because of the air leak or you have a failure somewhere in the wiring to the math sensor or in the driver circus of the computer for the sensor
