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I have a 1999 expedition with an electrical problem. it will not start sometimes it seams to be an electrical issue because it comes and goes, some days it will start while other day it will only turn over.  It's parked in the garage every night.

I don't think it is a fuel problem because it will not try to start.  

do you mean it will crank anytime you try it..   but sometimes it won't run...

first.. does your truck have a built in antitheft system???    transponder in the keys????    do you have any Mobil speed pass fobs on your key ring..?? any new mini credit cards???

they might interfere with the transponder...    any keys to other cars... that might have a transponder built into the key on that ring???

checked for trouble codes?????   used an OBD2 scan tool with live data display to examine the live sensor voltages..

with the proper scan tool... you can see what the engine computer sees...   so if there is a problem why it won't start.. you will be able to see it..

the last thing...

is the fuel pump noisy..???    

there is a trick... when the fuel pumps fail to spin when cranking the motor...  

takes 2 people...    one to crank the motor... the other to pound on the bottom of the fuel tank... this will almost always get the stuck fuel pump started.. at least for as long as the key is on..  they are brush type motors...

when the fuel filters clog..  the extra resistance causes the pumps to work much harder..   more power through the brushes..  when they wear.. the pump may not start everytime..

really top end mechanics have a hand held scope.. with a low amp current probe.. they can record the amps the fuel pump draws while its running..   they can see each winding as the pump spins...    so if one of the 6 is shorted...  you will get a pattern like this.. ~~~~~^~~~~~^~~~~~^~~~~~^  

some times you will get a pattern like this~~~~~~`~~~~~`~~~~~`    since one of the armature windings is not contacting at all..   you will have a pump that is really hard to get started when it stops on that winding..