P1093 Low Rail Pressure
#22
07-02-2014, 04:59 PM
ok... did some testing and heres what I got.
injector balance rates at 750 rpms
1)2.30
2)1.41
3)-0.69
4)-0.13
5)-0.78
6)-0.39
7)-0.81
8)-0.45
5310 psi in the rail
after winding it up a bit and then letting it idle I got
1.16
4.98 (sitting and watching the numbers at idle this one slowly decreased to like 3.70 after a bit)
-1.64
-1.22
-1.06
-0.17
-0.98
-1.03
rail psi - 4853 at 685 rpms.
also commanded 17k psi and got 4,800 psi.
injector balance rates at 750 rpms
1)2.30
2)1.41
3)-0.69
4)-0.13
5)-0.78
6)-0.39
7)-0.81
8)-0.45
5310 psi in the rail
after winding it up a bit and then letting it idle I got
1.16
4.98 (sitting and watching the numbers at idle this one slowly decreased to like 3.70 after a bit)
-1.64
-1.22
-1.06
-0.17
-0.98
-1.03
rail psi - 4853 at 685 rpms.
also commanded 17k psi and got 4,800 psi.
The following users liked this post:
geatgavi (11-08-2014)
#25
07-03-2014, 12:47 AM
in park the balance rates should be +-4 or less and in drive with foot on brake idling is +-6 or less
if your numbers are from park then they are fine. #2 with its lazy response is suspicious.
the low rail pressure could also be from high return rates on one or more injectors and thus exceeding the capability of the CP3. the only way to know with out replacing expensive injectors or fuel pump is the "bottle test" the LB7 is difficult to do as the injectors are under the valve covers and the return lines are too. the best way to do it without taking the covers off is test each bank of injectors. on each head there is a fuel line coming out of it. i believe the passenger side is towards the firewall and the driver side bank comes out towards the front. it is a banjo bolt holding on a fuel line for return fuel. you will need to test how much fuel is coming out of bank.
you pull the fuse for the FCIM so the engine wont start. place a hose connected to the return line on the bank of injectors in to a bottle and crank the engine over for 15 seconds. and measure the amount of fuel in the bottle. it is wise to repeat this test several times to make sure you get all of the air out for accurate readings. it is also a good idea to check if fuel is leaking past the fuel pressure relief valve.
anyways this is what GM says:
Fuel Injector Return Flow and Fuel Pressures
The fuel return from the fuel injectors to the tank will vary based on the API value of the fuel. Measure the Fuel API with the Diesel Fuel Quality Tester. For this reason the Fuel System Diagnosis-High Pressure Side values will vary for identifying a fuel injector or fuel pump concern. Use the following tables when referred to by the diagnostic. The first table is to be used during the initial diagnosis to identify the worst fuel injectors. After the fuel injectors that fail the first part of the test are replaced, the return flow from each fuel injector must be measured again. This is because the fuel system is returning less fuel to the tank, and thus the fuel pressure is higher during the retest. Failure to use the correct table may result in the replacement of good fuel injectors.
API Rating = Maximum Single Fuel Injector Return Flow = Maximum Fuel Injector Bank Return Flow
30-34 = 3 ml = 12 ml
35-39 = 4 ml = 16 ml
40-44 = 5 ml = 20 ml
Retesting Fuel Injector Return Flow Values
API Rating = Maximum Single Fuel Injector Return Flow = Maximum Fuel Injector Bank Return Flow = Fuel Pressure at cranking speed (FICM Disabled)
30-34 = 4 ml = 16 ml = 176-180 Mpa
35-39 = 5 ml = 20 ml = 134-178 Mpa
............................... 176-180 Mpa
40-44 = 5 ml = 20 ml = 114-135 Mpa
• Only perform this test when the fuel is more than 18°C (65°F).
• The fuel return volumes vary based on the API rating of the diesel fuel.
• A fuel injector may have high fuel return flow only at higher engine temperatures.
if the return rates are good then replace the CP3.
hope this may help
if your numbers are from park then they are fine. #2 with its lazy response is suspicious.
the low rail pressure could also be from high return rates on one or more injectors and thus exceeding the capability of the CP3. the only way to know with out replacing expensive injectors or fuel pump is the "bottle test" the LB7 is difficult to do as the injectors are under the valve covers and the return lines are too. the best way to do it without taking the covers off is test each bank of injectors. on each head there is a fuel line coming out of it. i believe the passenger side is towards the firewall and the driver side bank comes out towards the front. it is a banjo bolt holding on a fuel line for return fuel. you will need to test how much fuel is coming out of bank.
you pull the fuse for the FCIM so the engine wont start. place a hose connected to the return line on the bank of injectors in to a bottle and crank the engine over for 15 seconds. and measure the amount of fuel in the bottle. it is wise to repeat this test several times to make sure you get all of the air out for accurate readings. it is also a good idea to check if fuel is leaking past the fuel pressure relief valve.
anyways this is what GM says:
Fuel Injector Return Flow and Fuel Pressures
The fuel return from the fuel injectors to the tank will vary based on the API value of the fuel. Measure the Fuel API with the Diesel Fuel Quality Tester. For this reason the Fuel System Diagnosis-High Pressure Side values will vary for identifying a fuel injector or fuel pump concern. Use the following tables when referred to by the diagnostic. The first table is to be used during the initial diagnosis to identify the worst fuel injectors. After the fuel injectors that fail the first part of the test are replaced, the return flow from each fuel injector must be measured again. This is because the fuel system is returning less fuel to the tank, and thus the fuel pressure is higher during the retest. Failure to use the correct table may result in the replacement of good fuel injectors.
API Rating = Maximum Single Fuel Injector Return Flow = Maximum Fuel Injector Bank Return Flow
30-34 = 3 ml = 12 ml
35-39 = 4 ml = 16 ml
40-44 = 5 ml = 20 ml
Retesting Fuel Injector Return Flow Values
API Rating = Maximum Single Fuel Injector Return Flow = Maximum Fuel Injector Bank Return Flow = Fuel Pressure at cranking speed (FICM Disabled)
30-34 = 4 ml = 16 ml = 176-180 Mpa
35-39 = 5 ml = 20 ml = 134-178 Mpa
............................... 176-180 Mpa
40-44 = 5 ml = 20 ml = 114-135 Mpa
• Only perform this test when the fuel is more than 18°C (65°F).
• The fuel return volumes vary based on the API rating of the diesel fuel.
• A fuel injector may have high fuel return flow only at higher engine temperatures.
if the return rates are good then replace the CP3.
hope this may help
The following users liked this post:
geatgavi (11-08-2014)
.....
#27
11-07-2014, 12:32 PM
So just a quick update. haven't had much time to mess with the truck but I noticed that the issue has gotten a bit worse. I data logged the truck again yesterday. under full throttle it commands 19k and is getting about 7700. injector balance rates are roughly the same aside from cyl 1... it appears to be flowing 10 mm3s. looks like it is injector time! still doesn't smoke which surprised me at first but it seems as only one injector is bad..
#28
11-07-2014, 04:48 PM
something doesn't sound right. i am surprised your truck even starts with pressures as low as that. i would look into the possibility of a bad fuel pressure sensor. with only 7700 PSI fuel at WOT your truck should be slow and likely smokey.
does it seem to run fine still until the CEL comes on from low fuel pressure?
have you checked the return fuel lines yet for excessive fuel?
how about the pressure relief valve? not normally an issue with the LB7s but still a possibility
does it seem to run fine still until the CEL comes on from low fuel pressure?
have you checked the return fuel lines yet for excessive fuel?
how about the pressure relief valve? not normally an issue with the LB7s but still a possibility
#29
11-07-2014, 05:01 PM
Truck starts and runs fine. The CEL has always been on. It did peak around 10k psi I believe but for the most part it stayed low under full throttle. Faint haze on the top end too but not at idle. Dont get me wrong it's reallyyyy slow right now. But I don't know what to make of that balance rate. I haven't checked my returns yet either
#30
11-07-2014, 05:18 PM
i think your truck is in limp home mode right now with the CEL on. what it does is limit the max amount of fuel allowed and limits the RPMs to 2K. this makes it real hard to drive, transmission shifts weird etc.
first thing i would do is check for excessive injector fuel return as that cost about nothing. then i would look into the return fuel on the pressure relief valve and CP3. if all checks out then look into a stuck injector by pulling the glow plugs and the FCIM fuse and cranking it over while watching for fuel coming out of the glow plug holes.
if you don't have any excessive fuel being returned, no bad fuel hoses and the injectors seem fine then the next step is to replace the CP3.
but... from your description of "it seems to run fine otherwise" makes me think the pressure can't be as low as being reported so that points to the fuel pressure sensor.
how about, with your EFI Live, turn off each injector while commanding max pressure to see if the pressure goes up?
first thing i would do is check for excessive injector fuel return as that cost about nothing. then i would look into the return fuel on the pressure relief valve and CP3. if all checks out then look into a stuck injector by pulling the glow plugs and the FCIM fuse and cranking it over while watching for fuel coming out of the glow plug holes.
if you don't have any excessive fuel being returned, no bad fuel hoses and the injectors seem fine then the next step is to replace the CP3.
but... from your description of "it seems to run fine otherwise" makes me think the pressure can't be as low as being reported so that points to the fuel pressure sensor.
how about, with your EFI Live, turn off each injector while commanding max pressure to see if the pressure goes up?
