Post CP3 swap - won't stay running p0087

[DylOrr]

Working on my dad's truck. He lost the cp4 in it, 2011 single tune with 5" exhaust at 230k miles. The cp4 locked up, cracked the gear, and split the pump in half. Put a used lb7 cp3 in it i had laying around here and deleted the egr while it was all off. Got a new tune. There wasn't any metal on the fprv. Had the lines and rails cleaned anyways, but didn't replace injectors.

Got the truck back together and it took forever to finally get it started. It only idles at around 2.3k psi rail when desired is 5.5k and that eventually falls off to where the truck dies. Once it dies you won't get it started unless you let it sit for a few hours. I got it running once and took off and the truck wouldn't hold more than half of desired. 5k actual with 10k desired, 15k actual with 29k desired. If I let off the throttle at all the truck would die, I had to powerbrake it to keep it running.

The truck won't hold prime. I pump the primer on the FF housing and it'll gets hard and zero leaks. Let it sit 5 seconds and every bit of pressure has bled off. I pulled vacuum from the feed on the driver side of the engine to cp3 and it held 15in/hg no problem so I don't believe there to be any leaks.

My question is, could it be returning the fuel either through the injectors or through the cp3 with pressure. I've never seen one bleed off pressure like it's doing but hold a vacuum. It's almost like something is trying to return fuel. I have a set of injectors ready to go in but I'm just looking to be sure that I'm not wasting time doing injectors if somebody else has had this problem. Or if anybody has an idea on any other tests I can do to check for what exactly is wrong. Thanks.

 

[DAVe3283]

I'd do a return test on the CP3. It might be trash.

If you unplug the regulator, what does the actual rail pressure do?

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[DylOrr]

I'd do a return test on the CP3. It might be trash.

If you unplug the regulator, what does the actual rail pressure do?

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I unplugged it with same results. I tried to be as descriptive as I could. I also replaced the fprv with a known good one in case it was stuck open. The pump came off a good running lb7 that cracked a piston about a month ago and the guy upgraded to a sportsman, although I wouldn't rule that out.

 

[$chultz38]

I thought that you had to run a lbz/lmm pump or is your tuning adjusted for the lb7 regulator?

 

[DylOrr]

It does have an lbz regulator

 

[Chevy1925]

do a return rate test on the CP3 and injectors but my money is on the CP3 being your issue.

 

[$chultz38]

What about the injector return regulator could that be bad?

 

[DylOrr]

I forgot to mention the truck does white smoke a fair amount, which is why I believe it's injectors.

 

[2004LB7]

Double check the fuel lines are not restricted or put on the wrong fitting on the cp3 (return and supply backwards?)

 

[NC-smokinlmm]

LML injectors don't return much at all, my bet is cp3 also...

 

[Yngdmax92]

I find it very very odd that the truck is half of what ever the desired is for rail pressure.

At idle, at various throttle and wot. It just seems to perfect to me if the cp3 is a problem. I have seen some weird stuff.

 

[DPC]

Who's CP3 conversion kit did you use? Did you leave the injector return regulator in? Those injectors will return too much fuel without it.

 

[DylOrr]

Who's CP3 conversion kit did you use? Did you leave the injector return regulator in? Those injectors will return too much fuel without it.

It is a wcfab kit. Yes it's still there.

 

[$chultz38]

The return regulator has to be there. It makes back psi to shut the injectors. Without it the truck would never start/run. It might still be bad and letting to much fuel return though.

 

[DylOrr]

So I took off some lines and pumped the primer. All the pressure from the primer is bleeding out the return on the cp3. I'm going to start by replacing the cp3. I cranked and the truck only wants to build 400 rail and the cp3 is returning a lot of fuel while cranking. I'll update if that fixes it

 

[DylOrr]

Put in a tested good lbz cp3 and all seemed well at first. Same symptoms again except it doesn't smoke when its idling now. I'm going to double check the fuel to be sure it didn't get bad fuel, water, or gas in it. It's my dad's truck so who knows. But I'm going to throw a set of injectors in if the fuel all seems fine.

The truck will start right up after sitting overnight or even for a couple hours. It eventually starts dropping rail and dies.

 

[torqued2dmax]

I assume you drained fuel filter water separator into a white container to check for stuff first ! Then did you change the injector return regulator , it may be held partially open by something or an internal malfunction ! Good luck !

 

[TxDmax88]

More then likely injectors, I had a replaced my cp4 with a new cp3 in my LmL having similar issues. Thought it was the pump but I had a few injectors returning too much fuel. Try testing the return on your injectors, no more then 3 ml in 15 seconds or cranking.

This might help you: https://oregonfuelinjection.com/services-repair/diesel-diagnostics-repair/gmc-diesel-diagnostics/

 

[DylOrr]

Forgot to update. Did injectors and a return line setup and that fixed the issue. I went ahead and dropped the tank and there was a lot of metal in the tank and fuel basket. I would definately reccomend doing this if the cp4 goes.