LB7: Another LB7 That won't start when warm...

[poweredmotives]

So its an 03 CC/SB with 130k on it. Edge has been disconnected at least for T/S purposes. Other than that it has a K&N drop in and 4" straight pipe. It starts and runs fine when its cold but won't restart for an hour or so when its warm. Tried about 3 different brands of fuel filters (Wix, Baldwin, Car Quest). I've changed the filter head because of a leak at the primer pump, pressurized the system via 10psi at the tank and cant find a leak. Hooked up to the OTC scan tool it cranks at 170rpm cold and 210-215 warm.Replaced the batteries because I was only getting 165rpm warm before and, they were old. Balance rates are within 1.2 of each other. Commanded and actual fuel pressure are dead on while running but i only get about 6.5- 7mPa while cranking during one of these no start situations. Like i said if it sits for an hour or so it will always start right back up. Also getting an occasional P0335 Crankshaft Position Sensor A Circuit Malfunction. Due to cranking so long? Injectors were replaced at 58k for the campaign. I know I'm forgetting something. Anyway... HELP ME PLEASE! lol.


Thanks,
Matt

 

[Mike]

What does your mass air flow sensor read for air volume and air temperature?

 

[poweredmotives]

Didn't pay attention to that. IAT was pretty normal looking but I don't remember exactly. I forgot to save the freeze frame I just looked. I'll check it in the am.

 

[Mike]

Not sure that would be your problem, but if mine on either my 04 lb7 or 07.5 lmm are unplugged, it's hell to start if it does at all. Out of curiosity, what does your injector Pw show while cranking it. Probably zero, but at least that's a starting point.

Curious too, what is the fuel rail pressure a few minutes after you kill the engine and after turning on the key and not cranking?

 

[madmatt]

Do you have a gauge you can out on the fuel system test port to read fuel system vacuum?

 

[othrgrl]

7MPa is just over 1,000 PSI which isn't quite enough to make it start. With the other troubleshooting steps you've taken I don't think it's a fuel supply issue at all. I also doubt it's a CP3 or regulator issue but wouldn't completely rule it out either. I have seen injectors that were returning so much fuel to the tank the CP3 couldn't build enough pressure to get it to fire up. As stated check the commanded PW - if there is a command for PW when it doesn't fire then it could be an FICM issue; if not than it is just the lack of pressure.

 

[Mike]

7MPa is just over 1,000 PSI which isn't quite enough to make it start. With the other troubleshooting steps you've taken I don't think it's a fuel supply issue at all. I also doubt it's a CP3 or regulator issue but wouldn't completely rule it out either. I have seen injectors that were returning so much fuel to the tank the CP3 couldn't build enough pressure to get it to fire up. As stated check the commanded PW - if there is a command for PW when it doesn't fire then it could be an FICM issue; if not than it is just the lack of pressure.

Good catch, I read it as 6.5 to 7k ( psi ) duh.

Ya, he needs 3kpsi plus to get it to start. This makes it a fuel supply problem for sure. Needs x amount of fuel pressure to even command an injection event.

If he can reach the fuel regulator on the CP3 and disconnect it, Rail pressure should sky rocket right away as he spins the engine over.

 

[othrgrl]

Good catch, I read it as 6.5 to 7k ( psi ) duh.

Ya, he needs 3kpsi plus to get it to start. This makes it a fuel supply problem for sure. Needs x amount of fuel pressure to even command an injection event.

If he can reach the fuel regulator on the CP3 and disconnect it, Rail pressure should sky rocket right away as he spins the engine over.

By fuel supply I was referring to the fuel system on the way to the CP3 which I think he has properly troubleshot and can rule out. You do not need 3,000 PSI to fire off - more like 1400 PSI for the command for injection to come on. The commanded rail pressure during cranking is typically ~6,000 PSI and unplugging the FPR will typically do nothing - the truck is already trying to build pressure.

 

[Mike]

I see.

Here is an lb7 spinning over to start. Not 3,000 for sure but, 3,000+ to register an accurate pw. At least that is with my hardware. Second picture. Notice the pw while setting still and not spinning over, not acurrate for sure.

Disconnecting the FPR would do as much as suspecting a high fuel return rate injector.

 

[poweredmotives]

Not sure that would be your problem, but if mine on either my 04 lb7 or 07.5 lmm are unplugged, it's hell to start if it does at all. Out of curiosity, what does your injector Pw show while cranking it. Probably zero, but at least that's a starting point.

Curious too, what is the fuel rail pressure a few minutes after you kill the engine and after turning on the key and not cranking?

It shows 0 while cranking and a couple minutes after i kill it it is showing 1.5mPa.

 

[poweredmotives]

Do you have a gauge you can out on the fuel system test port to read fuel system vacuum?

No I don't.

 

[Mike]

If you have someone there can help, have them pump the filter housing and watch the pressure. Just a thought, seems to me you aren't getting enough fuel. Although, could be the other suggestion as well.


Just out of curiosity, how many times can you pump the primer until it is difficult to press down?

 

[poweredmotives]

Im driving it to my dads house right now and we are gonna mess with it for the day. Ill post more when I get back. Thanks for the ideas though.

 

[Mike]

May have to pm or email info. Sorry, was there a while ago.

 

[Mike]

You can remove the return line going to the tank to see just what kind of leak you may have from the injectors. I remember seeing a thread here on it, just can't remember where. Although, at each head and before fuel is returned from the cp3 would be a good check if you need.

Also, if you think you need to check the ficm for failure, post up.

 

[Colt]

Seen a guy with an issue of iced/gel in the neck on tank top from pickup tube he pulled it in garage and hit the top of tank where line comes out with heat from hair dryer for 20 minutes and it worked with charged batteries. Must have had enough condensation to do that blockage. He said it has to start by the tank and go thru the lines so heat it up the crook in the pickup tube on tank was it though. Farm thing so just sayin...

 

[poweredmotives]

The rubber line from the filter to the cp3 was folded over a bit from a clamp and had a very small rub mark in it. Best I could tell it let air in while warm. put some shop air to it to see any kind of leak. Changed the line and everything is working great. Thanks to everyone for the advice! Good things to know because I am the mechanic in a family of 4 LB7s lol.

 

[Mike]

Cool, you are up and running. :thumb: