Hi everyone this is a mystery for sure. I have picked up a 99 yukon slt 5.7 L I have had this 20 yr old baby to a mech and lo they both stood there with blank looks on their faces. not a good sign lol. No codes came up at all. They said i don't know. so this is what is going on. I have had full crank and no fire unless i spray quick start into the intake. I have given it a complete tune up. wires. sparkplugs, distributor, rotor, coil, module, crank position sensor, all the O2 sensors on a banks, changed the Mas sensor . Coolant temp sensor also. but not the gauge sensor on the drives side below the plugs. figure it just went to the cluster on dash. still cranks like it wants to jump out to the truck but no start till i use spray in intake. i hate using this due to what it does to the engine and mas if i get some near it. When it starts and drives it usually has not issue starting again that day unless it is cold then hell no. have to spray again. warm days always starts. I have been working on trucks since 77 and well i am lost on this one myself. I am just a backyard mech and Know when to say this is for the boys and girls who have been trained. Could this be the CRANKSHAFT SENSOR ABOVE THE STARTER BY THE FLY WHEEL. I have a mech who wants to have a look at it but wow i can imagine if they don't know and cost . lol i trust people that own our trucks and have had similar experiences with their own .