LB7 Cold Start Problems. HELP PLEASE [Archive] - TheDieselGarage.com

: LB7 Cold Start Problems. HELP PLEASE


2500HD03
10-13-2009, 02:17 AM
So i'm new to the duramax's I have had cummins till this. I got my new pickup and just after buying it found that i had bad injectors took it to the dealership to get them done under warranty. Got pickup back and truck ran great and everything. I let it sit over the weekend though and started it up truck ran for about 2 sec. and then died. Crank and no start. Opened the hood and pumped the fuel primer a few times and it fires right up. Well now that it is getting colder it does this about every morning. I read a post that the duramax does not use a lift pump? is this correct, i'm a diesel mechanic and the only engines i've ever seen that do not use a lift pump is one with the tank above the engine, AND A SMALL ENGINE aplication at that. I would think that the cp3 pump would need something helping feed it. Is there one in the tank. Also if there is no lift pump how come there is aftermarket lift pump options for the duramax. Also hooked up a snap on modis and there are no stored codes. Sorry i did not have the opportunity to hook it up on a cold start though to monitor pressures. I would think if they caused an air leak in the system it would have some running issues when i first got it running. But i just pump the primer a few times and it fires right up, no smoke or nothing.

New 0mega
10-13-2009, 03:24 AM
It is true that the Duramax does not have a lift pump on the truck the CP3 pump draws the fuel by vacuum. Make sure your fuel filter is on good, if the fuel line sucks air then it will not flow properly to the engine. There are aftermarket lift pump options because there is no lift pump. You might have to run an anti-gel till you are able to get a batch of winter diesel.

2500HD03
10-13-2009, 04:05 AM
Thanks, for the info. By cold start i don't mean cold enough to need anti jell. I just mean sitting for 12 hours. temp is only been like 40 f. I don't think it is sucking air at the filter though because I've seen this on other diesels and when they suck air they run bad till air clears. This shuts off like you turned the key off. And then once you pump fuel back in a few times it starts normal and runs normal. (i don't mean it cranks and cranks then starts) Sorry I'm not trying to say your wrong just trying to add more info on the problem and discuss reasons.

New 0mega
10-13-2009, 05:06 PM
I was just going to the most common won't start problem, air in the fuel line.


If you are thinking of getting a lift pump, I'd go for a Fass or Airdog 2, cause they add an extra filter and a water separator which is always a bonus.

BombDocDiesel
10-13-2009, 05:16 PM
I'm still betting on the filter. This is kind of how that works. I know a lot of us have thought we the fuel filter on tight enough to have these kind of issues.

Also, warm truck and hot engine bay with even the slightest of air leaks equals cold moist air drawn in the leak.

I will second the AirDog, but you probably won't need it until you do some tuning.

2500HD03
10-14-2009, 05:10 AM
Pulled it in the shop today to take a look myself. First time i have had time to look at it since the dealer did the injectors. The fuel filter is on tight, i thought maybe they forgot to put a clamp back on a rubber line, but they were all on and good. I could not find anything. I was able to push the quick coupler line ( in front of the drive about at the valve cover.) in a little tighter, but it was connected correctly so i don't think this was it. Is there a check valve in the cp3 pump? That is about all that makes sense to me. It only happens when it sits for extended periods of time. I just cant see a filter issue because a leak at a filter gets worse when running because it would actually be under a vaccum. Say like 10 lbs vaccum pressure trying to suck air through the leak. But engine off it would just go to 0. If a filter was at cause this should be present while running and definately under a load shouldn't it? But if it were a check valve in the cp3 (i dont even know if they have one) once it were running the check valve would not matter because it would be open anyhow. But once off if it were stuck slightly open the fuel could drain back out of the pump and into the tank causing it to start but die quickly and not start again untill fuel is reprimed back up to the pump. Then it would run fine.
Were do you guys put lift pumps on these? Just somewhere between the tank and fuel filter?
Thanks again for feedback

New 0mega
10-14-2009, 04:48 PM
There is a problem with leaving these trucks sit for a long period like you said. You can install a lift pump/Fass/Airdog 2 on the frame in front of the rear tires.

BombDocDiesel
10-14-2009, 05:16 PM
It loses prime. Still sounds like a leak or hole. Take it back to the shop that did the injectors. I will ask this to some Duramax guys that may have seen this before.

BombDocDiesel
10-14-2009, 06:16 PM
A fuel leak from injector lines could do this. Did you check the oil level? If your oil level is rising, it could be leaking under the cover and into the pan.

BombDocDiesel
10-14-2009, 11:03 PM
If the oil is good try this.

Pump up the primer several times and look for fuel leaks at the filter housing area and at the fuel inlet and return line area at the drivers side of engine.

Your leak is most likely outside engine. Check bleed screw o-ring and then never touch it again. Check filter housing rubber hose connections and fuel connections at the FICM (right behind the alternator) these are most of the places that they could have touched when doing the injectors.

2500HD03
10-15-2009, 03:39 AM
Yeah I have done that. Pumped up primer rock hard. The engine compartment is spotless. An injector line makes sense though I haven't checked my oil lately. So you guys aren't sure if there is a check valve in the cp3 though?, I got on AllData and looked on there to try and find a diagram to see if there was and found that there was no good diagrams of the fuel system. On another note though the truck did not die after starting this morning. I will have to see what it does tommorow morning.
Thanks again

TheBac
10-18-2009, 01:03 PM
You also could have a small pinhole leak in the metal feed line from the tank. The only way to test for it is to pressurize your fuel system, tank and all.

The CP3 does have a pump within it. Its a small gear pump on its rear that pulls fuel from the tank. It is not a vacuum system, per se. Adding a lift pump is a good idea, even in a near-stock app, as it lessens the stress on the CP3.

dmaxalliTech
10-19-2009, 02:38 AM
Some scanner data will do wonders. What is the rail pressure doing while cranking?

Check the fuel supply hose on top of the tank, there is a spot in that hose that can rub on the bottom of the cab under the truck and put a small air leak in it. Does it only do it cold? How does it start hot? Problems exist before injectors? What does the shop that did the work have to say?

2500HD03
10-21-2009, 01:08 AM
I'm aware it is not a vaccum system but at any given spot before the cp3 if there is no lift pump fuel is being pulled and not pushed. Fuel being pushed is +psi fuel being pulled is -psi therefore the cp3 is technically creating a vaccum on the inlet side.
Sorry I have not ever had a scanner hooked to it when it has the problem as it only happens when truck has sat for an extended period of time.