Truck Fires but won't crank

[Big Hoss]

Been having problems with it past few weeks. Mostly when it sits over night or for long periods of time the wife was having problems getting it cranked. I go out Sunday morning to crank it and it won't. So Tuesday I take it to the shop. Checked new fuel filter its fine,checked spark plugs needed replaced so replaced them,checked for fuel pressure was getting somewhere in the 50-60 range. Sprays some starter fluid in the fuel injection area cranks right up. Turns it off doesn't spray it won't crank but fires, so we get it pushed most of the way into the shop they spray some more starter spray get it cranked its acting like its running out of gas jumping and jerking trying to stall etc. Gas is good gas I get gas same place every time and its got 3/4ths of a tank in it so I know its not out of gas. Guy says it sounded like it jumped timing Friday when he tried to crank it gonna be Monday before he can check that but just curious what else should we check? Also can the Fuel Pump still be bad even though its getting fuel pressure to it? I replaced the fuel pump but even when the truck gets low on fuel it starts acting funny like jumping and jerking a little bit.Anyone know the correct pressure a 98 GMC Yukon is supposed to have for the fuel pump?

[Airjer_]

I think what you are describing is a crank no start. It will crank all day long but will not start. Just to clear that up.

Fuel pressure is the key. These injectors are a poppet style meaning they rely on a specific minimum amount of fuel pressure in order to operate correctly. This should be at least 60 psi. 1 psi can be the difference between starting and not starting. Not starting pressures are usually 57 and less PSI. Typically they will start with more that 58psi.

The symptom you described is a classic symptom of low fuel pressure. The reason is starts and continues to run with a shot of starting fluid is once the engine starts the poppets are under a vacuum and require less fuel pressure to "pop" open.

[Big Hoss]

Ahh. Yeah I looked at the pressure gauge when he was doing it. It was only going to about 54 or 55 so that is probably it. I will let them check the timing tomorrow and if that doesn't work I will order a fuel pump. Thanks for the quick answer.

[Big Hoss]

Does anyone know the correct fuel pressure for my vehicle I can't find it and really don't know where to look.

[maxwedge]

Look at post #2!

[Airjer_]

60-66 is the actual spec.

[Big Hoss]

Alright thanks. I got the truck Tuesday from the idiots I had working on it and the guy says yeah its been cranking up just fine ok wtf? He never told me this so I crank it right up no problems, I go to town take it to a diff shop so he can take a look says to come back if I have problems with it again well go to store and library home again and back into town for something else never had a problem with it all day Tuesday come yesterday morning was gonna go to wal mart get in it and refuses to crank. WTH? Frustrated beyond belief at my truck. I have read it could be the pump,the fuse,the relay etc etc etc. Course I took it to the shop last night and was gonna call the guy this morning tell him just to diagnose it and let me know well he ain't there for some reason done called 4 times with no answer so gonna take it to someone else. He only wants 50$ to diagnose it and hopefully I can get my cousin to install new pump for me once I know what it is. Can anyone tell me what fuel pump is on mine? Called a salvage yard he says there is 2 different kinds for my vehicle but I can't get to it unless I get someone to drop the fuel tank. Its a 1998 GMC Yukon 4 door 3.7 engine.

[Airjer_]

Call the dealer with you vin number handy. They will be able to tell you whaty pump you have. You can either go with an o.e. or have the aftermarket cross the part number the dealer gives you.

BTW, you are describing a crank no start. When you describe it as you did above you are saying the engine wont crank or the starter will not turn the engine over. I know its petty but you will avoid a lot of confusion by using correct terminology.

So if you hear the starter turning the motor over and it will not start you have a crank no start. the engine is cranking over but it will not start or "fire" up.