Help: hard starting lb7

[james13]

just checking to see if anyone is having the same problems that I am having with my truck? When you shut the truck off after it sits for at least an hour or longerit has a hard time starting again, it turns over butit acts like it has to build fuel pressure before it will start again. Sometimes if you hold the gas pedal down it starts right up. Are there any kind of check valves on the fuel system to keep fuel pressure? If there is where are they located? Or if anyone has any ideas what it might be please let me know. I did put a new
cp3 in about 30000 miles ago and new injectors about 35000 miles ago. Other than that the truck is all stock. The truck currently sits at about 126000 miles now.

 

[S Phinney]

Several things could be the culprit. There could be a high pressure leak on an injector. There could be a restriction in your fuel supply. Check your fuel line under the truck where it changes from metal and turn to go p by the tranny. The braided line will sometimes weaken and allow it to collapse. Then you will loose fuel supply. Holding the throttle shouldn't affect the cranking unless there was something faulty about the TPS, I would't think.

 

[wdino73]

I just had this problem yesterday, I did all the troubleshooting blah,blah, come to find out the engine wasn't turning fast enough to build 1500psi, to start, rpm should be between 150 and 160, my driver side battery check out bad, replace the battery and vroom, it may help but that was what my problem was, I took it to advanced auto, and had them check the battery lucky for me it was covered under the battery warranty, hope this helps good luck. Dino :thumb:

 

[james13]

Yeah, I had bad batteries I replaced them not that long ago but this problem started after that. I think my problem is my lift pump or I should say my pump in my tank. Does anyone know what the rail pressure should be before you start the motor? Does it hold pressure or does it build pressure as you crank it? Thanks, James.

 

[wdino73]

Mine when the key on varies from 150 to 200 but when u start cranking it goes up, it takes 1500 to start then it goes on up to 5000 ish at idle. hope this helps, if u got efi u can monitor your pressure.

 

[james13]

with the key on i'm sitting at 2mpa =290.0754 psi and it peaks out at 159 mpa =23060.9943 I don't know what it idles at I will have to check that agian, but this is off my snap on scanner.
Thanks, James

 

[S Phinney]

Cranking pressures will be 200-300's. Upon starting it should spike in the 7k's then fall back to the set parameters usually around 5k depending on temp. That is if you are running the stock 35 kpa rail pressures.

 

[wdino73]

what's the cranking rpm's it should be 150 to 160, I'm not trying to start an argument but it takes 1500 psi to start if u watch your scanner as u crank it the pressure will raise, if the rpm's aren't high enough it will not build enough psi to start, check your driver side battery, low voltage will cause alot of strange things,

 

[knagel]

would be interested in hearing the out come of this as I am having the same problems with my LB7 with 211000 miles. I know the fuel filter is new and the batteries check fine and it cranks over at the right speed. It just seems to crank for about 3-4 seconds or so before it fires. Not like my LBZ where the first piston over it would fire. I'm guessing it is loosing pressure when it sits also. Would be interested to hear what you find with yours.

Kevin

 

[james13]

the wierd part is it doesn't do it all the time. some times it starts right away and other times it fills like it takes 20 revolutions befor it fires that what i can't figure out.

 

[JoshH]

Yeah, I had bad batteries I replaced them not that long ago but this problem started after that. I think my problem is my lift pump or I should say my pump in my tank. Does anyone know what the rail pressure should be before you start the motor? Does it hold pressure or does it build pressure as you crank it? Thanks, James.

What kind of lift pump did you put in the tank?

 

[james13]

it's the stock intank pump

 

[Drholliday]

it's the stock intank pump

No in tank pump on these trucks from the factory.

 

[james13]

sorry but gm said mine does! i don't have the framerail pump

 

[Drholliday]

sorry but gm said mine does! i don't have the framerail pump

There is no lift pump on these trucks period. They use the CP3 in the valley of the motor to suck fuel from the tank. Only the Dmax vans come with a lift pump IIRC, not sure about the kodiaks.

 

[02bigstrokin]

There is no lift pump on these trucks period. They use the CP3 in the valley of the motor to suck fuel from the tank. Only the Dmax vans come with a lift pump IIRC, not sure about the kodiaks.

thats why the damn cp3 pumps wear out. gotta love it.

 

[JoshH]

There is no lift pump on these trucks period. They use the CP3 in the valley of the motor to suck fuel from the tank. Only the Dmax vans come with a lift pump IIRC, not sure about the kodiaks.

No lift pump on them either. Cab and chassis trucks have a pump to transfer from the rear tank to the front tank, but that's it.

 

[Drholliday]

thats why the damn cp3 pumps wear out. gotta love it.

I think mine is wearing out and I've had a lift pump on it for a while

 

[wdino73]

I hope that this isn't a dumb ??? but is it possible for these trucks to lose prime???

 

[james13]

oh i asked the geys at gm and they told me that the sender had a pump too
so i guest they know more thin i do about the trucks, but i've never looked
for my self so i don't know