LB7 No top end - have scan tools/codes

bluthndr

bluthndr

New member
Joined
Mar 16, 2008
Messages
139
Of course, a P1093 - no power above 2K or so, but will run higher if you give it room. I am mostly a Cummins guy but have a decent level of ability with drivability. I have been reading up on this ('net, and Mitchell, and Snap-On, and TSBs) but can't sort it out. I have plenty of tools and Snap-On and factory HD (not Tech 2) scan tools at my disposal...

Anyways, it does have the p1093 code for low rail pressure on power enrichment:
  • If I command the pressure up at idle the actual keeps up to spec (160 des = 140 act), same thing if I floor it in park (actually pressures match even better).
  • New fuel filter, no change.
  • Older oil change, no fuel in oil, oil level ok.
  • If I clear the code it comes back first time I drive it and lean on it, but I can get the pressure to spike to over 170 at least once or twice before it trips the code and goes into limp - then it seems like rail pressure flattens out (low) above 2000 RPM and will NOT come even close to desired.
  • Also does not want to upshift after code is set, but this may be common with this code?
  • No program at this time but used to have a bullydog program before all this happened.
My Cummins brain screams lift pump, but alas...

Not sure where the "vacuum test port" is but I could not see anything restricted on visual inspection of the lines.

At this point I guess it could be:
  • Injectors (common problem but the attainable high rail pressure makes me think not).
  • A CP3 pump???
  • An internally collapsed line (not firggin likely at all but not impossible)
  • A "Fuel/Rail Pressure Control Valve?" (but it seems on LB7 trucks this is rare?)

I plan on checking the vacuum thing tomorrow, but I am annoyed not being much closer than I was 4 hours ago...

Any other ideas? We are talking way too much money to guess at...

Again, not a DMax guy, but hopefully someone can help me pin the tail on the donkey (no insult intended).

Thanks in advance to all the guys out there smarter than me on these...:redx:
 
The bottom line is the p1093 is telling you the system cannot hold rail pressure under load. So you have a leak in the system or the CP3 is going out. Just holding pressure under a no load condition isn't good enough in many cases. Use the scanner and look at commanded rail pressure versus actual rail pressure when driving and you will see when it starts happening and maybe that will show you something.

I would start by adding a lift pump to the system, this fixes it many times. Once the p1093 is set the ECM will limit the engine RPM to around 2000 so that's normal. The no upshift again is normal as it cannot reach the speed required to shift.
 
The actual rail pressure is exactly as you said after you get on it one time.

Am I correct in thinking that it is not injector related because it can initially create the desired rail pressure and there is no fuel in the oil?

Is there any way to test the return flow without thousands in dealer tools?

We may go with the lift pump initially... Other recommendations/tests?

Thanks again!
 
Typically if you have an injector leak it will show up in the balance rates. Also take a look at the DC on the CP3. They typically run in the 35 range at idle. If that's where it is then most likely there is no leak.
 
First test is to check the injector return rates as an injector could be returning excess fuel and still have a good balance rate(this is the only failure that gM covers under the special policy anymore is excess return that will not allow full rail pressure to be built). Since you say you commanded 160 and it only hit 140, there is a leak, failing CP3, or a failed FPR not allowing it to reach desired rail pressure. First thing you can do is unplug the FPR and see if it will reach 160 MPA or higher for rail pressure. If it doesn't next you want to check the injector return amounts. If that is good then chances are you will need a new CP3. I know mine is getting weak and I can barely hold 160 anymore, but I'm gonna run her until she codes.

And there is no lift pump on the DURAMAX to replace, so unless your running it tuned you don't really need one(although it is a good upgrade to add).
 
The FPR on an LB7 is right behind and under the A/C compressor, but ahead of the pedestal in the valley right?

I assume this is the same/similar part as the FRPV on the newer Dmaxes that everyone replaces?
 
bluthndr said:
The FPR on an LB7 is right behind and under the A/C compressor, but ahead of the pedestal in the valley right?

I assume this is the same/similar part as the FRPV on the newer Dmaxes that everyone replaces?
Click to expand...


Don't believe so. The FPR is part of the CP3 on all of the DURAMAX's and controls how much fuel goes to the CP3 to control pressure output. It is located in the valley of the engine in the CP3. On the LB7 it uses a straight plug, but LLY+ use a 90 degree plug on it. The valve that most of the guys with newer trucks mess with is the pop-off valve that prevents the fuel pressure from exceeding a safe limit. LB7's have them as well, but they don't give as much trouble as later models do.
 
THEFERMANATOR said:
Don't believe so. The FPR is part of the CP3 on all of the DURAMAX's and controls how much fuel goes to the CP3 to control pressure output. It is located in the valley of the engine in the CP3. On the LB7 it uses a straight plug, but LLY+ use a 90 degree plug on it. The valve that most of the guys with newer trucks mess with is the pop-off valve that prevents the fuel pressure from exceeding a safe limit. LB7's have them as well, but they don't give as much trouble as later models do.
Click to expand...

So the popoff valve is what I'm seeing? Do you know if a bad popoff valve will cause the same result on an injector return rate flow test?

For those fllowing/interested in this thread, injector balance rates are all fine. Worst is 3.2 (spec is 4.0) hot - checked in and out of gear. All the others are 1.2 or less.

Have and airdog 100 and kent-moore tool on the way to find the truth of the matter. Still unsure about how to test popoff valve... Just pull the connector?
 
bluthndr said:
Of course, a P1093 - no power above 2K or so, but will run higher if you give it room. I am mostly a Cummins guy but have a decent level of ability with drivability. I have been reading up on this ('net, and Mitchell, and Snap-On, and TSBs) but can't sort it out. I have plenty of tools and Snap-On and factory HD (not Tech 2) scan tools at my disposal...

Anyways, it does have the p1093 code for low rail pressure on power enrichment:
  • If I command the pressure up at idle the actual keeps up to spec (160 des = 140 act), same thing if I floor it in park (actually pressures match even better).
  • New fuel filter, no change.
  • Older oil change, no fuel in oil, oil level ok.
  • If I clear the code it comes back first time I drive it and lean on it, but I can get the pressure to spike to over 170 at least once or twice before it trips the code and goes into limp - then it seems like rail pressure flattens out (low) above 2000 RPM and will NOT come even close to desired.
  • Also does not want to upshift after code is set, but this may be common with this code?
  • No program at this time but used to have a bullydog program before all this happened.
My Cummins brain screams lift pump, but alas...

Not sure where the "vacuum test port" is but I could not see anything restricted on visual inspection of the lines.

At this point I guess it could be:
  • Injectors (common problem but the attainable high rail pressure makes me think not).
  • A CP3 pump???
  • An internally collapsed line (not firggin likely at all but not impossible)
  • A "Fuel/Rail Pressure Control Valve?" (but it seems on LB7 trucks this is rare?)

I plan on checking the vacuum thing tomorrow, but I am annoyed not being much closer than I was 4 hours ago...

Any other ideas? We are talking way too much money to guess at...

Again, not a DMax guy, but hopefully someone can help me pin the tail on the donkey (no insult intended).

Thanks in advance to all the guys out there smarter than me on these...:redx:
Click to expand...



Bluthndr,
I'm sure you've probably checked this out but anyway here it goes: The GM filter housing primer pumps are famous for sucking air. I've been through a few of them and EVERY one had problems eventually...even the one I have now is leaking! The systoms are if your filter is less than full when you pull it to change filter and after filter change you try and hand prime the filter and get no where after 1000 pumps....most likely the filter housing is bad. Another symtom is long cranking to start, the motor should fire off real quickly under normal ambient temps. The guys at PPE showed me if you can EASILY depress the primer bulb with engine off your filter housing seals are shot (DON'T force this test as you can damage the seals).
I have more than a few Dmax buds who all had the same problem. A FASS or AirDog type lift pump system will solve the problem for sure but I like the idea of making sure the stock filter housing is leak free too.

Hope this helps!
 
lcyachts said:
Bluthndr,
I'm sure you've probably checked this out but anyway here it goes: The GM filter housing primer pumps are famous for sucking air. I've been through a few of them and EVERY one had problems eventually...even the one I have now is leaking! The systoms are if your filter is less than full when you pull it to change filter and after filter change you try and hand prime the filter and get no where after 1000 pumps....most likely the filter housing is bad. Another symtom is long cranking to start, the motor should fire off real quickly under normal ambient temps. The guys at PPE showed me if you can EASILY depress the primer bulb with engine off your filter housing seals are shot (DON'T force this test as you can damage the seals).
I have more than a few Dmax buds who all had the same problem. A FASS or AirDog type lift pump system will solve the problem for sure but I like the idea of making sure the stock filter housing is leak free too.

Hope this helps!
Click to expand...

The only drawback to a lift pump is then you get diesel leaking out when the seal go bad instead of air getting in. I know this happened to me with my single KENNEDY lift pump. A $.74 O-ring from NAPA fixed it though.
 
bluthndr said:
So the popoff valve is what I'm seeing? Do you know if a bad popoff valve will cause the same result on an injector return rate flow test?

For those fllowing/interested in this thread, injector balance rates are all fine. Worst is 3.2 (spec is 4.0) hot - checked in and out of gear. All the others are 1.2 or less.

Have and airdog 100 and kent-moore tool on the way to find the truth of the matter. Still unsure about how to test popoff valve... Just pull the connector?
Click to expand...

The pop-off valve is a mechanical valve designed only to open if pressures exceed the max limit for the system. They tend to not give alot of trouble on the LB7's like they did on 04.5+ trucks. And the +/- 4 spec is to high in my opinion. I had one at 2.7 and had tons of smoke because of it.
 
UPDATE:

Used the j-tool (GM special tool) for testing all the return volumes and ran through the first part of testing. Fuel return spec on cranking (whole system?) is 35ml. If it's over 35ml, testing continues to individual banks, then individual injectors. Ours tested out at 10ml 3 different times, for which GMs repair is "replace fuel injection pump."

Since we had the air dog we put it on, thinking maybe cp3 was not getting necessary supply volume. Same issue. Definitely runs better now, but at WOT act FRP drops to 25-29mpa after it first spikes to 179mpa! Oddly enough, at part throttle it will seem to maintain 120mpa actual FRP and runs pretty good. Also seems to run worse with more temp... Anyone think it could still be a pump? Also have run the command FRP and watch actual, and have seem it dead on (160/160) or only achieving 120mpa vs 160 commanded or somewhere in between...

The fuel press reg % goes to zero when the rail pressure dives at WOT, so the system appears to be trying to make the pressure, it just can't/won't at WOT. Looking for anything else to test/try.

Guess I will try out the j tool testing again tomorrow...
 
THEFERMANATOR said:
The only drawback to a lift pump is then you get diesel leaking out when the seal go bad instead of air getting in. I know this happened to me with my single KENNEDY lift pump. A $.74 O-ring from NAPA fixed it though.
Click to expand...

It is easier on injection components if you aren't sucking air. I would rather leak out diesel than suck air into my cp3.
 
160mPa Commanded VS. 140mPa Actual is not "keeping up". You're almost 3000psi short, under a 'no load' situation (as far as the injectors are concerned). Once you get into WOT, and your injector pulse width is up, you're sucking down more fuel, that 3000psi variation is going to grow and grow, till you drop to the 25mPa (3625psi) you speak of.

Hate to say it, but that sounds like a CP3 to me. I just had to swap mine last weekend. If your's ends up being the pump, I hope you have better luck that I did. Had to do it 1500miles from my shop, with 2 fist-fulls worth of handtools, in a parkinglot in 106* heat. Took about 10hours. Flatrate is 6.5hrs, so I didn't think that was too bad given the circumstances.

Best advice... have a plethora of extensions, and a helper. It's not a difficult job, theres just alot of stuff that has to be moved to get down into the heart of the beast.
 
You must log in or register to reply here.
Back
Top