LB7: LB7 fuel pressure relief valve.

briencolgan

briencolgan

brien colgan
Jan 26, 2017
85
4
8
Ithaca, NY
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Well this is the elusive LB seven fuel pressure relief valve. It's so special that the dealer wants about $700 for it. It's only sold as part of the fuel rail junction block. None of the shim kits or fuel race plugs available will work for the LB7 I don't believe. (Correction. PPE has a plug. Not sure if there is a shim kit for the LB7)

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As you can see I have disassembled the valve and I am going to shim it with a few nuts, to improve spring tension.

Total cost zero dollars! I had some on the truck.

I have heard lots of talk on the forums about the LB7 fuel pressure relief valves and how they weren't prone to fail. But this one definitely did fail. I confirmed it two separate ways.


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Last edited:
briencolgan

briencolgan

brien colgan
Jan 26, 2017
85
4
8
Ithaca, NY
I couldn't find one anywhere. Do you know who has one?

I have the old frpv shimmed and installed and it does not leak.

My first choice was to replace but not for $700.


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briencolgan

briencolgan

brien colgan
Jan 26, 2017
85
4
8
Ithaca, NY
I'm a little wary to reengineer a great engine as well. Although I guess that's exactly what I'm doing.

I've got it shimmed but have no idea what pressure the valve will release at.


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briencolgan

briencolgan

brien colgan
Jan 26, 2017
85
4
8
Ithaca, NY
Well that did it! Half second start time. Up from 10 to 15 seconds of cranking. White smoke is disappeared. I'm glad I took the time to perform the test in the service manual instead of throwing injectors at it.

Some of the injectors are at high threes, but they still pass. So I'll hit then with some injector cleaner and see what happens. A mechanic friend of mine who does fleet maintenance on diesels says they're always putting injector cleaner in the trucks. Like every other Fill.


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Last edited:
briencolgan

briencolgan

brien colgan
Jan 26, 2017
85
4
8
Ithaca, NY
Yes. I guess I should have been more specific and left off the race plug option as I didn't want to gamble not being able to dump pressure. I figure it must be there for a reason.

I kind of assume guys that are putting plugs in are racing their trucks and working on them all the time. I'm a builder so my focus is just trying to keep an old truck rolling with as little "under truck" time as possible.


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Chevy1925

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
21,145
4,901
113
Phoenix Az
Lb7 fprv's don't normally pop but they can. When they do, they will continue to pop easily time after time. Many guys just weld them shut as they usually don't pop even when you have rail pressure issues/spikes so they don't do much good. Mine never did with 26k rail pressure and dual cp3
 
briencolgan

briencolgan

brien colgan
Jan 26, 2017
85
4
8
Ithaca, NY
I considered that. And I'll probably do that if it leaks again. I am guessing the engineers put that valve in to protect the injectors in the case of a runaway pressure spike. Like if the regulator failed.


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Robby Avery

Robby Avery

GM TECH @ FENDER GMC
Jul 31, 2008
820
2
18
Norman Park, Georgia
briencolgan said:
Well that did it! Half second start time. Up from 10 to 15 seconds of cranking. White smoke is disappeared. I'm glad I took the time to perform the test in the service manual instead of throwing injectors at it.

Some of the injectors are at high threes, but they still pass. So I'll hit then with some injector cleaner and see what happens. A mechanic friend of mine who does fleet maintenance on diesels says they're always putting injector cleaner in the trucks. Like every other Fill.


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Click to expand...

Was 10-15 seconds when its hot after driving and getting to operating temp then shut down and start up?
 
briencolgan

briencolgan

brien colgan
Jan 26, 2017
85
4
8
Ithaca, NY
Yes exactly. The fuel gets hot and gets thinner. Once the fuel is nice and thin, it can slip through that needle valve easier.

So normally would fire right up in the morning, but lots of cranking even when cold. Drive to the coffee shop, come out, cranking cranking cranking till the battery is dead. And also tons of white smoke at idle, which cleared up with the shim.


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briencolgan

briencolgan

brien colgan
Jan 26, 2017
85
4
8
Ithaca, NY
I was sort of shutting out the idea of a bad fprv. Because the standard wisdom in the forums is that the LB seven fuel pressure relief valve is not prone to failure. So I was calling around to mechanics, investigating fuel injector replacement, even considering buying another truck. It was about a week of handwringing, and being lazy sort of about not wanting to run the physical tests that involved a little bit of engine breakdown. But the actual break down and test of the FPRV was only in an hour or two of my time. I think buying that five volume service manual was the smartest thing I ever did. That big ass book has saved me a lot of time and money. I just ran the troubleshooting test by the book, and it solvedthe problem. I should've done that before I got into worrying about injectors and new trucks.

If this helps anybody, the dealer specific hoses for the actual fuel injector return tests are about five or $600. If anybody gets that far on the return test it may be cheaper to have a mechanic run it that has those hoses. Or maybe there's a workaround where you don't need that kit.

For $600 you could take a brass billet to a machinist and have them build you some fittings for a lot less than that probably.


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briencolgan

briencolgan

brien colgan
Jan 26, 2017
85
4
8
Ithaca, NY
Sure. Hope that does it. Just take the extra step of having a friend turn over the motor with the crank relay out.

Do this after u have removed the banjo bolt and pulled the line off the back of the fprv. You're gonna see fuel leaking out the fprv while cranking if it's bad. If it's not leaking. It's something else.


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briencolgan

briencolgan

brien colgan
Jan 26, 2017
85
4
8
Ithaca, NY
Also noticed that the turbine whine had slowly disappeared and that nice turbine sound is back with the fprv repair. Not really sure if the turbo is tied to good fuel rail pressure or not. So wondering if a lower volume turbine sound would also point to a weak fprv valve.


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