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Low fuel rail pressure

J

JSC

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I have a 04 lb7. The injectors have been rebuilt with new steel lines the Cp3 pump rebuilt all new rubber lines . Primer head rebuilt. And plugged fuel pressure valve. 3 new FPR and 3 new FPS. I completed the return flow test 3 times all under 100 ml . What is making me crazy is my edge programmer will truck is cranking low fuel pressure. But when I check for codes there is on glow plug code. The battery’s are brand new. I have check the whole wiring harnesses all the grounds. I even removed all the snow plow wiring all the strobe lights. If it did not come with the truck I removed. If any one can help I’m game.
 
I was thinking the same thing. The last couple trucks that beat me up on diagnostics all came down to corrupt tune files. Just had a LML I went through HELL for 2 weeks.

A lb7 will be easy to go back to stock and test drive since its shouldn't go into limp mode like a deleted truck will.
 
100ml for a return rate test is beyond excessive. It's 30 ml for all 8 injectors(4ml per injector) and 20 ml for the cp3, but iirc GM specs anything more than 40ml cumulative can cause low rail pressure codes(it's been a long time since I found the specs buried in a service manual online). And do you have an actual tuner, or is it a plug in like an edge juice?
 
THEFERMANATOR said:
100ml for a return rate test is beyond excessive. It's 30 ml for all 8 injectors(4ml per injector) and 20 ml for the cp3, but iirc GM specs anything more than 40ml cumulative can cause low rail pressure codes(it's been a long time since I found the specs buried in a service manual online). And do you have an actual tuner, or is it a plug in like an edge juice?
Click to expand...
Yes I have a CTS tuner. What I’m baffled about is there is no codes coming up. Sounds like I should pull the injectors and send them back.
 
1.5 MPa is only 218psi. A good fuel system should produce over 2500psi while cranking. 90% of duramax no starts I get in the shop is air in the system. Usually it's a rusted sending unit from our Ohio winters. When you open the bleeder and pump the primer do you get a steady flow of fuel with ZERO bubbles? Alot of times I bypass the tank and force feed the pump directly with a lift pump pulling fuel from a 5 gallon bucket. Just to confirm the truck runs before I sell fuel lines and a sender.

If your 100% it's not air in the system then you might have an injector issue. Your return specs seem too high. You can by a block off tool to test them. You remove the injection line from the rail and install the block off cap. Then crank the engine and see if rail pressure changes. Do that to every injector one by one and see if rail pressure changes. I bought 4 myself and usually do an entire bank to save time.

Where did you get your parts from? I only offer genuine bosch parts to my customers to avoid issues. I also recommend adding a lift pump to every truck that comes in without one.
 
I would suspect air with that of rail pressure. Even with bad injectors I would expect to see at least at least 5 mpa of rail pressure. 1-2 is almost always air.

Unplug the ficm, take the return line off from over the valve cover(the main one going to the tank), put a rubber hose on the metal portion, run that hose into a quart or larger jar, crank the engine over until fuel begins to flow into the jar, clamp the hose off, empty the jar, put the hose back in, unclamp the hose, and crank it over for 15 seconds while observing rail pressure.

IIRC, GM says you should be able to obtain 30+ mpa during the 15 second cranking period(mine had no trouble reaching 45 with a shot cp3), and measure how much fuel was returned during the cranking test. Anything more than 40ml is a sign of a problem. If you get less fuel with low rail pressure it's sucking air. If you get a ton of fuel you need to further isolate your returns to see if it's the line coming off your cp3, injectors, or popoff valve(very unlikely to be bad on an lb7, but does seem to be happening more as they get older.

Now if your returned fuel is in spec with the ficm unplugged and it builds rail, then you have an injector(s) returning to much fuel when actuated, and will need the plugs to isolate the leakers.

You need at least 1450 psi of rail pressure to get the injectors to actually pop and open to inject fuel. The solenoids will open at any pressure, but you need the pressure differential to actually open them.
 
THEFERMANATOR said:
I would suspect air with that of rail pressure. Even with bad injectors I would expect to see at least at least 5 mpa of rail pressure. 1-2 is almost always air.

Unplug the ficm, take the return line off from over the valve cover(the main one going to the tank), put a rubber hose on the metal portion, run that hose into a quart or larger jar, crank the engine over until fuel begins to flow into the jar, clamp the hose off, empty the jar, put the hose back in, unclamp the hose, and crank it over for 15 seconds while observing rail pressure.

IIRC, GM says you should be able to obtain 30+ mpa during the 15 second cranking period(mine had no trouble reaching 45 with a shot cp3), and measure how much fuel was returned during the cranking test. Anything more than 40ml is a sign of a problem. If you get less fuel with low rail pressure it's sucking air. If you get a ton of fuel you need to further isolate your returns to see if it's the line coming off your cp3, injectors, or popoff valve(very unlikely to be bad on an lb7, but does seem to be happening more as they get older.

Now if your returned fuel is in spec with the ficm unplugged and it builds rail, then you have an injector(s) returning to much fuel when actuated, and will need the plugs to isolate the leakers.

You need at least 1450 psi of rail pressure to get the injectors to actually pop and open to inject fuel. The solenoids will open at any pressure, but you need the pressure differential to actually open them.
Click to expand...
I have installed a new popoff valve. Solid one. I did the return test. Every time it's just under 100 ml. The injectors have been rebuilt. But there must be something wrong
 
You need to measure the return rate for each bank and the pump SEPARATELY to isolate the issue. Each bank is 12ml per 15sec cranking time. If your over 12ml on either bank, cap of the injector supply line one at a time with the block off tool I mentioned in my last post. And see if the return rate return to spec. If both banks are under 12ml them you more than likely have a injection pump issue.
 
JSC said:
I have installed a new popoff valve. Solid one. I did the return test. Every time it's just under 100 ml. The injectors have been rebuilt. But there must be something wrong
Click to expand...
100 is way to high. You need to next take the line off of the cp3 and isolate it, and repeat your test. And WHO did you get the injectors from? As to the solid plug, I hate those. LB7's don't need them.
 
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