LBZ won't hold rail pressure at idle

[Chevyhead3]

Hello, New to the forum, thanks for having me. Lots of good info on here. So I'm having issues with an LBZ, just got done doing head gaskets studs, EGR delete. The problem is since It's back together it will not hold rail pressure, once I unplug FPR, I get about 12,000 psi of actual and desired pressure, with FPR plugged in actual is between 900-3300psi and desired is between 4300-7000, both all over the place, truck lopes and surges. Once FPR is unplugged truck smooth's out? Drove truck with FPR unplugged, won't get over 13,000 psi under heavy load, runs well and doesn't stumble or surge. I've replaced FPR since, no change, cleaned grounds and ecm connections, new fuel filter. Tested fuel pressure sensor have 179 psi key on actual and 7249 psi desired. Unplugged FRPS get 29008 psi. Shouldn't i have more like 23k-26k psi with FPR unplugged? Any help would be appreciated. Thanks

 

[2004LB7]

Do you have the tools to command fuel pressure? IE: EFI live or another bidirectional scanner?

 

[Chevyhead3]

Yes, I have a scan tool and am unable to command the FPR, I can't watch fuel rail pressure at the same time, but when I bump the pressure all the way up with the scan tool, it still makes no difference?

 

[2004LB7]

Maybe the pressure relief is leaking or weak. Have you tried measuring the return rate to see if it's higher than spec?

 

[Chevyhead3]

I'm going to do that now, I just don't know if they are going to be accurate with the truck surging and barely idling?

 

[2004LB7]

Unplug the regulator while doing the test

 

[Chevyhead3]

So I was able to command FPR, and and watch the pressure, desired goes up, but actual stay the same all over the place, here's balance rates and fuel rail pressures

 

[2004LB7]

That doesn't tell us how much is returning. Test fuel returning on the pressure relief. Maybe even injectors and CP3 too. If anyone of them are returning too much the CP3 can have difficulty maintaining pressure. If all are good you may have a weak CP3

 

[Chevyhead3]

This is with FPR unplugged

 

[Chevyhead3]

Ok, Yes that makes sense, I'm going to do the bottle test, on the fuel rail next. So if truck run's good while driving and under load, with FPR unplugged, low pressure side of the fuel system should be fine correct? or is there something I might be missing? Also I have to pump up the fuel primer only when FPR is plugged in, with it unplugged it starts right up first crank

 

[2004LB7]

Never assume the low side is ok when the engine doesn't run right. Always check. You can use a clear hose to the CP3 to see if you have any air in the lines. Air will reduce the pressure the pump can make and give you incorrect diagnosis if you don't catch it first. Also make sure you aren't suffering from high suction from a clogged filter or collapsed fuel line

 

[TheBac]

Moved to LBZ section.

 

[Chevyhead3]

Ran clear fuel line between filter head and CP3, and also on suction side to filter head, bubbles at first obviously, until I got it all bled. After that no bubbles, no change in performance or rail pressures, rail bottle test tomorrow, and return on CP3, and see what happens? I'm really leaning towards the CP3? But haven't ruled out injectors or FPRV? Just pulling my hair out because truck ran when it came in. Should have looked at some PIDs before I tore it down for head gaskets? for reference, but oh well. Only did head gaskets because it was getting combustion pressure in cooling system @ heavy load. Thanks for all the input so far.

 

[Chevy1925]

So your a tech? You need to go back to what you have pulled apart and start there. Very rarely do things like this just go bad.

If you unplug the fpr and you don’t see 29k along with a knocking/rattling truck, the pump is not making full pressure.

Have you gone over the whole high pressure system looking for ANY leaks? Unplug the fuel pressure regulator and look at every single fuel rail and injector fitting looking for fuel leaks. It doesn’t take much to drop rail.

You unplugging the fpr and commanded going to 12k makes no sense. It should stay at 5k while actual goes to 29k.

Did you pinch any wires when putting all the wiring back in? Did you put the right connectors in each spot?

 

[juddski88]

I have a had several LBZs that the ECM had defaulted to commanding the last known good value and stayed like that until the wiring issue was resolved. The ecm will not always report rail pressure has maxed out when the FPR is unplugged. The OP needs to load test each part of the circuit from the FPR connector to the ecm connector.

So your a tech? You need to go back to what you have pulled apart and start there. Very rarely do things like this just go bad.

If you unplug the fpr and you don’t see 29k along with a knocking/rattling truck, the pump is not making full pressure.

Have you gone over the whole high pressure system looking for ANY leaks? Unplug the fuel pressure regulator and look at every single fuel rail and injector fitting looking for fuel leaks. It doesn’t take much to drop rail.

You unplugging the fpr and commanded going to 12k makes no sense. It should stay at 5k while actual goes to 29k.

Did you pinch any wires when putting all the wiring back in? Did you put the right connectors in each spot?

 

[Chevy1925]

I have a had several LBZs that the ECM had defaulted to commanding the last known good value and stayed like that until the wiring issue was resolved. The ecm will not always report rail pressure has maxed out when the FPR is unplugged. The OP needs to load test each part of the circuit from the FPR connector to the ecm connector.

I can’t say I read it that way? He’s saying he’s unplugging the fpr and then it’s saying commanded is 12k. Far as I’ve seen, that still shouldn’t happen

 

[Chevyhead3]

Yessir, I've went over everything double and triple checked, no fuel leaks whatsoever. Agreed that fuel pressure should jump up to max pressure along with fuel knock and rough Idle, when FPR is unplugged, but it does not. It runs smooth, sounds and runs good. When I unplug FPR scanner reads between 12-13k both desired and actual. Also agree that the CP3 shouldn't have just went out, but it isn't building more than 13k with FPR unplugged, no matter what load or RPM.

 

[2004LB7]

Since you had all of the top end taken apart. What are the chances of something getting into one of the fuel lines and restricting the flow causing low pressure?

 

[Chevyhead3]

I capped all the lines, when I pulled everything apart, I've ruled out the low pressure side, CP3 is getting plenty of clean fuel, also cleaned all high pressure fuel lines, before I re installed them.

 

[08lmm72mm]

That is strange it shows commanded 12k with the fpr unplugged. Is there any codes? I wonder if full flashing the ecm with your current tune would change anything possibly from a corruption issue or if one of the pins the the main bail connectors on the drivers side of the engine isn't fully seated.

Have you logged the truck driving it with the fpr plugged in watching dfpr and fpr? Sounds like cp3 is suspect to me. How many miles on it?