LB7: Rail Pressure Issues

[durallymax]

Been looking around on the interwebby and not finding much on the issue i'm having with an 02 LB7. The truck surges/hunts at idle like an FPR going bad. When it was cold at idle in park and i first started it the Actual Rail Pressure was at 18,000 when it is being commanded to less than 6000 IIRC. When i would shift to drive it would drop down to the commanded pressure, about 2000psi per second. Then when shifted to park the rail pressure would go back up again. I let the truck idle for another 30 minutes and when i came back out pressure were back to within spec.

It was definately running very high rail pressure sitting there, you could hear and feel the clatter.

The original complaint was no power when he went to pass somebody. So I went out and drove it around and it holds steady until commanded pressure goes over 18,000psi then actual pressure drops to 2,000-2,500psi. Sitting in park i used EFI Live to ramp up pressure where again it kept up until it got to 18,000psi, then the actual pressure just sat at 18,000psi up until commanded was at 23,000psi. However at 2000rpm the actual pressure would be much closer.


I havent dealt with a bad Fuel pressure regulator before. The Book points straight to replacing it but after doing research I want a second opinion first thanks.

 

[blk smoke lb7]

Do you run 2 stroke oil in your fill ups? 1 quart per tank fill keeps the sticky fpr running much better. When i put my engine back in the fpr was sticking and after 2 tanks with 2 stroke it freed up and havent had another problem with it yet.

 

[durallymax]

Not my truck but he runs lucas oil additive i remember he said.

Im going to order an FPR today and see what that does.

 

[Burn Down]

Sounds like the regulator to me.

 

[durallymax]

Regulator fixed the surge which was obvious. However did not fix the pressure issue while driving. When it drops fro 18,000 to 2,000psi. It does it soo smooth and with no real hesitation, or sputtering, or noises. Just gracefully but quickly drops. Doesnt shudder or anything and no error codes whatsoever. As far as the 18,000psi at idle when cold I will have to check it this morning as it was sitting in a 60* shop last night when I ran it.

Would the rail pressure sensor cause this? Kinda seems far fetched but IDK. I could swap one from a different truck I have here.

Could it be the CP3 itself? Hate to throw a $900 part at it. Again I could swap one in just to see but thats a lot of wasted labor if it doesnt work.

I wouldnt think a leaking fuel filter head would cause this because I would think that would cause a lot of sputtering and more issues across the board. Could be wrong though.

I am going to pull the tune off of the engine to make sure it is stock. he said it did have a programmer in it before he bought it but it was supposedly returned to stock.

Ive heard of the rare instances of where reflashing the ECM cures odd problems like this, and then issue is so smooth that it almost makes you wonder.

 

[770JOHNNYW]

sounds like you got a leaky return line just had this problem....

 

[juddski88]

how would a leaky return line affect the hp side of the injector? I have seen high return PSI create high RP at idle, but not any RP issues created by loss of return pressure.

 

[05smoker]

The last one I had like that turned out to be injectors. Would throw a rail pressure code as soon as it dropped the pressure.

Also, when the filter head is leaking, it will start very hard since you lose prime. You can always spray soapy water on it and see if it bubbles.

 

[770JOHNNYW]

how would a leaky return line affect the hp side of the injector? I have seen high return PSI create high RP at idle, but not any RP issues created by loss of return pressure.

maybe it wont...every time this happened to me insted of building pressure on the hp side it was returning the fuel. injectors fixed it. balance rates were fine it just dropped pressure when it was hot and threw low rail psi code. he can throw a Pump at it but it wont fix it.

 

[durallymax]

talked to merchant they said pump or injectors. Injectors were done within the past year by GM. So then they said pump.

Id hate to throw a pump on it because then i cant return it. Just like the FPR I put on it, if it is the pump theres a wasted FPR that I wont be able to recoop all of the money out of. I could try a different pump but really dont wanna have to replace them pump, find out it fixes it, then replace it again with a new one. But i could do that if theres nothing else i can really check.

 

[juddski88]

maybe it wont...every time this happened to me insted of building pressure on the hp side it was returning the fuel. injectors fixed it. balance rates were fine it just dropped pressure when it was hot and threw low rail psi code. he can throw a Pump at it but it wont fix it.

I'm confused I guess, was the issue actually a leaky return line or the injectors?

Vince, an injector or some injectors may also be faulty even though they were just replaced. Dylan just did injectors on a truck with newer ones done by a dealer and it had an intermittent start issue. Sometimes it wasn't building pressure to start. FPR didn't fix it, new injectors did. It sucks to consider newer injectors unreliable, but it happens I guess.

 

[770JOHNNYW]

I'm confused I guess, was the issue actually a leaky return line or the injectors?

Vince, an injector or some injectors may also be faulty even though they were just replaced. Dylan just did injectors on a truck with newer ones done by a dealer and it had an intermittent start issue. Sometimes it wasn't building pressure to start. FPR didn't fix it, new injectors did. It sucks to consider newer injectors unreliable, but it happens I guess.

no need to be a smart ass man:thumb:

maybe i worded my first post wrong ****in sue me!

 

[Chevy1925]

talked to merchant they said pump or injectors. Injectors were done within the past year by GM. So then they said pump.

Id hate to throw a pump on it because then i cant return it. Just like the FPR I put on it, if it is the pump theres a wasted FPR that I wont be able to recoop all of the money out of. I could try a different pump but really dont wanna have to replace them pump, find out it fixes it, then replace it again with a new one. But i could do that if theres nothing else i can really check.

check return rates on both the injectors and CP3. its not simple to do but it will tell you what way to go.

is he running a stock tune? the ECM wasnt recently reflashed was it? almost sounds like a 1st stage engine limp or something

 

[durallymax]

that was my plan was to check the return rates.

I know new injectors can be bad or go bad quickly. Had it happen before but not with Bosch ones, but still have heard of that as well.

ECm hasnt been flashed to his knowledge although I was going to see what I could find with it. Ive got another 02 here that i have a license for the ECM and was thinking about throwing its ECM into the truck with the issue loaded with a stock tune for that OS and seeing if anything changed. just for laughs and since its a quick easy idea.

It just seems so smooth when it does it, and its extremely consistent.

Ive felt engine protection before and its not acting like that but i am going to look around and make sure that the logs dont show signs of it being in limp mode.

 

[JoshH]

Have you tried checking the fuel pressure relief valve? What happens if you command full fuel pressure with EFILive or some other scan tool?

 

[durallymax]

Have you tried checking the fuel pressure relief valve? What happens if you command full fuel pressure with EFILive or some other scan tool?

never even thought of that. Makes sense that it could be an issue.

If I command full psi at idle it will go to 18,000 and stay there, it will not drop.

I need to retest at 2000rpm to get accurate numbers for there as I do not remember them and did not log it by accident.

Is there really any way to check the valve or do I just try one from a different truck I have here?

 

[JoshH]

You could modify a bolt, but it would probably be easier to just swap it.

 

[PAT]

Is there really any way to check the valve or do I just try one from a different truck I have here?

There is a simple way to test the fuel pressure regulating valve. It's called a bottle test. It will let you know if your valve is returning to much fuel. If it returns to much fuel, it drains the fuel rails. Creating a low pressure situation.

I don't have the link to the DIY on the test. But a quick search will yield you the step by step process.

 

[blk smoke lb7]

I pulled the banjo bolt and plugged just the end of it because we thought my pump was returning to much it didnt make a difference but i guess i did eliminate 1 thing.


Oh yeah i have the same problem as you just not as bad we comand 22,000 and it delivers 17 or 18 mine will gradulay get up to where comanded and supplied are the same but out of the whole or even geting on it lightly its way off but i dont set any codes for low rail yet.I have a brand new lbz pump im going to put in the valley as mine is somewhere around 160,000 and is probably getting tired after that its new injector time.I was wanting 60 overs anyways

 

[PAT]

I pulled the banjo bolt and plugged just the end of it because we thought my pump was returning to much it didnt make a difference but i guess i did eliminate 1 thing.


Oh yeah i have the same problem as you just not as bad we comand 22,000 and it delivers 17 or 18 mine will gradulay get up to where comanded and supplied are the same but out of the whole or even geting on it lightly its way off but i dont set any codes for low rail yet.I have a brand new lbz pump im going to put in the valley as mine is somewhere around 160,000 and is probably getting tired after that its new injector time.I was wanting 60 overs anyways

When you pull it, let me know if you want some pointers. You can give me a call. I learned a thing or two pulling, installing, pulling and installing again.
Pretty sure I can get the job done in an hour- 1 1/2.