Mystery Crank

[Ironrod]

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 .

 

[Doubeleive]

so let me be sure I have this clear?
you basically have to use starter fluid to get it to start? then it runs fine?
if so, then that would lead me to it being a fuel pressure related issue
have you checked the fuel pressure?
am I missing something here?

 

[Ironrod]

Yes you are correct and no i have not done a pressure test on fuel line. all i am aware is their is pressure as it did spray out when i just pressed it but that does not tell me the pressure as i am aware. fuel pressure regulator
is something i never thought of either.

 

[Doubeleive]

not sure what the starting pressure should be on the obs but on the newer trucks it's typically 60-65 (for starting) then 45-50 running.
you may have a issue where it has just enough pressure to keep it running once it has started.

 

[iamdub]

Unless there's more to the story, I'd be questioning the aptitudes of the mechanics since performing a proper fuel system test is a basic, obligatory step in such a drivability case.

 

[liquify33]

You can get a cheap kit from an auto store, attach it to the port near the firewall and check with key in the On position.
As said, around 55-65 psi should be the reading.
I can send you pics if you want of the port location and reading.

 

[bigfootchiro]

Two mechanics, and no one checked fuel pressure? Once it was running, no one checked for vacuum leak? No one checked live data stream?

You need to run far away from these “mechanics,” especially if they are relying on CEL codes to fix their problems.

 

[OR VietVet]

What's been said already but also need to watch for pressure bleed off when sitting. You say it runs fine when started. Get the very very basic first step going with a fuel pressure test, cannot believe none of these tech people have not brought this up, and watch to see how it holds the pressure after shut down. Monitor that over a period of time. You could have a fuel pump problem or even a leaking injector(s) at the spider. I am also assuming you are not smelling fuel and do not have a leak in a line between pump/tank and engine.