Fuel pump only comes on sometimes.

There is an oil pressure switch in parallel with fuel pump relay, if that switch is closed, the fuel pump will run all the time. Click on the link. At the fuel injectors, top wire is voltage, bottom wire is ground. If there is no ground, both wires will have voltage. Make sure battery has a full charge.

https://s17.postimg.org/vijjimikf/fuel_injectors.jpg

check or replace fuel pump relay, or you have a bad starter/ignition switch, not to be confused with "key switch".

did your car come equipped with prolock? prolock starter kill can become defective and prevent your car from starting. or it can be your ignition switch. I had a defective ignition switch and the van 's fuel pump would turn on but no start...

When you first turn on the key, no crank, the fuel pump should run for a few seconds then stop. There shows to be an oil pressure switch in parallel with the fuel pump relay. It's just a redundant power supply to the fuel pump. Once it cranks enough to bring up oil pressure and the switch closes, the pump will run even w/o the relay. Both the fuel pump relay and oil pressure switch get voltage from the ecm b fuse.
At the fuel pump connector, check voltage and ground, when problem is ongoing. Gray wire is voltage--black wire is ground.
You could pull the fuel pump relay, use jumper wire between terminals on load side of relay. The load side is that line with arrow head, when energized that line moves to the other terminal. As you can see, the orange wire is hot all the time, the gray wire carries voltage to fuel pump. If it runs with jumper, then check control side of relay, the control side has to have voltage and ground, dark green wire with white tracer is voltage comes from pcm. The black wire is ground.

I don't know if Yukon's had this problem or not, but once I changed a fuel pump on a Chevrolet Blazer only to find out that then the ignition switch went bad. I changed out the ignition switch and it started fine after that. Please rate my answer so I get my points for it.

YOUR PROBLEM IS MOST LIKELY,DUE TO BAD FUEL AND SITTING FOR A LONG TIME, YOUR FUEL PUMPS. I AM ASSUMING THAT THE TRUCK IS REGULAR GASOLINE FUEL AND NOT DIESEL. ENGINE. MOST FUEL PUMPS IN THE TANK WILL FAIL IF FUEL IS ALREADY GUMMED OUT, FUEL WILL SMELL LIKE "TURPENTINE" FLUID. CHECK FUEL PUMPS IF THEY COME ON FIRST BY OPENING THE FUEL CAPS AND HAVING ANOTHER PERSON TURN THE IGNITION KEY TO THE "ON" POSITION, AT THE SAME TIME PUT YOUR EAR CLOSE TO THE FUEL FILLER TUBE OF THE TANK THAT THE FUEL TANK SWITCH IS ON. YOU SHOULD HEAR A LOW GROWLING SOUND, IF NO SOUND COMING FROM THERE PUMP IS NOT WORKING. SOMETIMES (VERY RARE) YOU CAN JAR THE PUMP TO WORK BY BANGING ON THE BOTTOM OF FUEL TANK WITH A RUBBER MALLET, DO THIS AS SOON AS THE IGNITION KEY IS TURNED TO THE "ON" POSITION.

the engines oil pressure switch activates the fuel pump upon turning the ignition to the ON position..Replace that next

rewire it to teh ignition switch

It is most likely the Ignition switch. I have replaced several for this exact simptoms.
To check it, you test the power for the fuel pump relay.
When the ign switch starts acting up it will not power up the fuel pump relay portion of the fues box.
You can check it with a test light.
On a day when it starts and runs fine, pull the fuel pump relay, turn key on and probe the socket with test light and make note which ones have power to them.
Then one day, when it dont start, dont turn the key off yet, pull the relay, and retest, and compare.
If there is no power where there used to be power, replace the ignition switch.
Hope this helps
If it does please leave some testimony

either the pump is going bad, your connection to the pump is bad, your fuel filter may be bad, or your fuel pump relay has gone bad. In other words, there are many things to check. Get a test light and go to town. Also check the fuel pump fuse.