LB7: LB7 rail pressure problem

[C.bowles2]

Good friend of mine made a trade for an lb7. Dont gave much history on it besides it was a plow truck and the people who had it before said it was acting up most the time they had it. Anyhow im trying to diagnose the issue and am having a hard time.

Only code it has is P1094 - low rail pressure at idle and is a hard code that comes back right away after clearing.

Using bi-directional controls and monitoring desired vs actual fuel rail pressure it wont build rail pressure what so ever when i command an increase. I can take it all the way up to 160mpa and actual sits right at 24mpa. No change in engine sound either.

At idle actual rail pressure is 24 mpa and desired is 50 mpa. I can get actual rail pressure to increase when i give it throttle but it never keeps up with desired.

So my question is if i can get actual rail pressure to increase by giving it throttle but not from a command with scan tool this makes me think it is something electrical or the regulator and not the actual cp3 pump being bad?

Also when i unplug connector to cp3 regulator it doesnt change the actual pressure at all, so makes me think it is the regulator. Any insight guys?


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[DAVe3283]

That really sounds like a bad CP3 or injectors returning too much fuel. You can try to replace the FPR, but I would do a return test first. If the return rates are in spec, then you could try a new regulator. But I suspect the CP3 is toast.

 

[PureHybrid]

Is it hard starting?

 

[Chevy1925]

What is the main injection fuel rate and balance rates? might be a bit skewed with that low of rail pressure at idle but it can help (main injection fuel rate will help more here).

but like dave said, most likely injectors or cp3 are done. could be a sticky FPR but you can pull it, leave the electrical connector plugged in, turn the truck on and use the DVT tools to modulate rail pressure and you should see the fpr slide in and out from no mpa to 180mpa. if its working, id really look at cp3/injectors so return rates would need to be done next.

 

[C.bowles2]

Is it hard starting?

Nope, starts right up. No white smoke or raw diesel smell from exhaust either like bad injectors usually do.


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[C.bowles2]

What is the main injection fuel rate and balance rates? might be a bit skewed with that low of rail pressure at idle but it can help (main injection fuel rate will help more here).

but like dave said, most likely injectors or cp3 are done. could be a sticky FPR but you can pull it, leave the electrical connector plugged in, turn the truck on and use the DVT tools to modulate rail pressure and you should see the fpr slide in and out from no mpa to 180mpa. if its working, id really look at cp3/injectors so return rates would need to be done next.

I will check and see what the balance rates are and main injection rate is and report back.


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[C.bowles2]

That really sounds like a bad CP3 or injectors returning too much fuel. You can try to replace the FPR, but I would do a return test first. If the return rates are in spec, then you could try a new regulator. But I suspect the CP3 is toast.

Is the return test as simple as disconnecting the quick disconnect return line and seeing how much comes out from there into a bottle?


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[Chevy1925]

LB7's are a pain for return testing. technically you need to remove teh valve covers and put a line on each return port of the injector BUT i would plug into the return port for each bank off the valve cover/cyl head that is external and then test. i believe healthy injectors cranking for 15 sec should return no more than 5ml so you should see no more than 20ml at the port.

CP3, ive never found an exact return amount to determine if bad or not but if injectors are in check, balance rates/main injection rate is good and FPR moves as it should, that kinda only leaves the CP3 as the culprit.

 

[JoshH]

I would lean more towards a stuck regulator or more likely a bad CP3. If it isn't hard starting, the chances of it being injectors returning too much fuel are pretty slim.

 

[C.bowles2]

I would lean more towards a stuck regulator or more likely a bad CP3. If it isn't hard starting, the chances of it being injectors returning too much fuel are pretty slim.

I wonder if i can get an oscilloscope on that regulator wiring and watch it move through wave lengths, i may try that and see if i can watch it move. If it seems to move and sweep the range just gonna tell him it needs a cp3 then.


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[TheBac]

Moved over to LB7.

 

[WolfLMM]

Sure sounds like a bad cp3

 

[Yaryan92]

I've seen a couple videos of people un hooking the return line at the fuel cooler cranking for 15 seconds and with less than 100ml of fuel in a bottle the "Injectors passed" and most likely a cp3 for low rail pressure at wot? How accurate is that?

 

[LB7 Lover 1994]

What is a healthy main injection rate? I see this asked from time to time but never now? Don’t know a whole lot about Duramax I don’t work for a GM shop. But Navistars newest medium truck has the new L5P or different version at least in it. It’s literally a GM truck but with a straight front axle and International badges

 

[DPC]

Had on 04 LB7 do the same thing, ended up being a bad CP3. How many miles are on the truck?