2012 GMC LML won’t start, have CP3 upgrade questions

rrh13441

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Mar 24, 2022
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I have a 2012 GMC 250hd with 150k is deleted and in the shop for head gasket replacement because of a coolant leak on the passengers side. All was well until I was told the exhaust manifolds were warped, which I kind of expected from talking to other owners. I expected a call that my truck was done after three weeks in the shop. Yesterday I called the shop and asked for an update and they put the owner on the phone. He told me that “when they went to fire it up it would not start, it’s not getting fuel pressure to the injectors. They spent two days troubleshooting the fuel system, came to the conclusion that the fuel pump was not building enough pressure, bad pump.” I said “I brought it in it was running great just the small coolant leak, WTF?”

He’s said they need to replace the fuel pump and they really don’t know why it failed. He quoted a new CP4.2 pump replacement. I said, oh hell no, if it’s replaced we’re doing a CP3 conversation. The owner said I’ll make it right with you and eat labor costs, which we’re still negotiating.
My question is, what in the world could have been done by the shop to make that pump fail, or could I be that lucky that it just gave out?

The truck has 150k was running like new when I took it in. Is there a chance the mechanic could have caused this pump to fail when doing the head gasket swap? If so they are going to buy me a new CP3 conversion, this is BS.

Do you think I should have to pay the full price for the conversion and they pay labor, or should I pay the difference between cost of a CP4 vs the CP3 conversion? Also any thoughts on what could cause this would be appreciated.
 

Chevy1925

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Pumps can just fail like this. The cp4 can go from lack of fuel after the engine is re-assembled. We usually use a fuel pump to draw fuel up through the pressure port and pre-fill as much of the pump as we can prior to install to give the pump the best chance at surviving that long crank.

Thing is, the pumps shouldn’t need all this to stay alive.

Did they pull any of the regulators to see if there is metal in them? Did they try back pressurizing the return system and cranking to see if it’s the return regulator that died? These trucks need 43.5psi min on the return fuel side to shut injectors and get to fire.

Shop sounds pretty decent if they are already willing to help cut out labor. I get it hurts you got a big cost ran up in this truck already with HG so that’s not helping you right now. If they are doing the proper checking to verify it is the pump and there is no metal on the regulator, I’d go cp3 conversion too. If there is metal, the fuel rails and lines need to come off to be cleaned or replaced. Then you can either hope and pray injectors are ok or they realistically should be replaced to get you the truck back asap and keep you from additional diag time to figure out why it may not be running right.

Did they check your pistons real well for cracks? Not uncommon to find cracks starting, specially if tuned. We have a lml that needed hg and 5 of the 8 pistons are starting to crack. Engine is down at the machine shop now getting a very mild build.
 

rrh13441

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Mar 24, 2022
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Do you need to run a lift pump when you do a CP3 conversion on an LML?

Which CP3 conversion kit should I buy if I want to run stock power on my deleted 2012 GMC LML?


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rrh13441

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The shop said there is no contamination in the system and changed both regulators. They said during troubleshooting they hooked up an external source to isolate the pump and it still would not pump. They did not say anything about cracked pistons.

I will replace with a CP3 fir shire but diesel that mean I still should run a lift pump (FASS system) with the CP3 pump?


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darkness

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Always a lift pump. Still want to keep the cp3 happy and not doing all the work.
 
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Chevy1925

don't know sh!t about IFS
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Well at this point, kinda late for a cp3 but it’s not going to hurt to have one.

If they know their shit and I’d venture to guess you trust them for such a large job, then it should be a sound diag.

We are being fed second hand info, nothing against you but it’s just how it goes.

Do you believe and trust their diag at this point?
 

Chevy1925

don't know sh!t about IFS
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No but it’s not a bad idea. More filtration and less likely to cavitate. Something the cp4 was highly susceptible to before, the cp3 is far more resilient to.

I like the wcfab or s&s kit. Some massaging is required for fuel lines but they are the better fitting kits. S&s has 3 options as well for these kits
 
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Cougar281

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The shop said there is no contamination in the system and changed both regulators. They said during troubleshooting they hooked up an external source to isolate the pump and it still would not pump. They did not say anything about cracked pistons.

I will replace with a CP3 fir shire but diesel that mean I still should run a lift pump (FASS system) with the CP3 pump?


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'Technically', no, you don't need a lift pump if you believe GM... But the 2010 Jetta my wife had, had a Bosch ECM and fuel system and had a lift pump.... Sooo... In reality, it's NEVER a bad idea to add a lift pump, be it FASS, Air dog, Kennedy, whatever. All of them will reduce the stress on the CP3.
 

2004LB7

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Dec 15, 2010
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As james said

The CP3 for the Duramax has a gear pump attached to the back to suck fuel. But this requires it to suck fuel all the way from the tank. So any air leaks and restrictions can hurt fuel supply a lot. A lift pump helps keep the lines full and air free. If you get one with filters then added bonus
 

rrh13441

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Mar 24, 2022
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I have a 2012 GMC 2500hd deleted with an H&S mini Max tuner. My CP4 pump just quit working so I’m going to do the CP3 upgrade. Has anyone with the H&S tuner had issues with this upgrade, and will I need a special tune to get the CP3 conversion to work with my H&S stock tune?


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Chevy1925

don't know sh!t about IFS
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I have a 2012 GMC 2500hd deleted with an H&S mini Max tuner. My CP4 pump just quit working so I’m going to do the CP3 upgrade. Has anyone with the H&S tuner had issues with this upgrade, and will I need a special tune to get the CP3 conversion to work with my H&S stock tune?


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Not if you use a cp3 kit that does not require tuning. They cost more but saves the headache of a tune
 
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TheBac

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Merged all three of your current threads into the original issue thread in order to keep everything straight.