LMM: Idaho Rob? I am told you hold a solution....

[fastline]

Alrighty then..... New over here because everyone else has failed me. Have an 08 with LMM, 200k miles, and the plagued low rail pressure codes. I won't bore anyone with those specifics because it seems very common, BUT I have realized most people just seem to chase their tail on a solution, and just buy and install parts without really finding the core issue..... I am not built like that!

It has been well observed that at around 75% throttle, ECM requests 26ksi rail pressure, and the rail pressure holds that without issue, every day, all day, whatev, meaning the pump CAN hit that pressure. However, if we get towards 100% throttle, 26ksi is still requested, but actual started dropping to around 18ksi. You all know what happens if I hold it there long enough.

I have verified the relief valve NEVER leaks a drop. That is not the issue and don't want to hear a word about it. This issue is between performance of the pump, and injectors bypassing fuel. One of them is not getting the job done. I will buy into the lift pumps to a degree but smart people know the HPP is positive displacement so as long as it gets a complete mouth full, if it can't build pressure, something else it as fault. If I was seeing nasty vaccum on the fuel line, I might go there, but again, the truck won't even stumble, EVEN at WFO, it will limp but still run fine. I was getting ready to just pressurize the tank for a quick test but doubt that is the issue. Vacuum is on spec with what the CP3 can handle.


Appears prev owner swapped all the suspect fuel lines already, filter is clean and then some, and vacuum has been tested between tank and CP3 both at idle and WFO. It all works as intended.

Truck is 100% stock trim. believe injectors are original. However, this truck could NOT run any better up to 75% load. It will start at 0*F clean, has never stumbled, no smoke, no knock, injector balance looks good, nada. I just cannot point a finger towards pathetic injectors yet.

Given the price and time considered here, it is VERY hard to hear someone say "try some injectors". That seems ridiculous without proof. I was going to start with some leak by testing on injector lines but someone mentioned ATP might have something to try? Apparently it fixed a friend's truck. I am told (hard to believe) there is a designed orifice between the pump and injectors that won't allow the pump to move any more fuel and needs drilled out. Obviously if this is true and works, that means the injectors are leaking by, but it would buy me some time and since this truck gets 3k miles/yr, it would buy a LOT of time!!!

 

[Woodster]

Have you tried calling and asking ATP?

 

[jwrkansas]

The "orfice" that you're talking about is the fuel rail fittings. And trust me, that wont buy you a cup of coffee. When most guys do the knee jerk "try injectors" its because they're decently well known to return too much fuel back to the tank when they have higher miles. I guess since you don't wanna do that then you could just pony up and buy the bosch return line tool kit that measures fuel return off of each injector into a graduated cylinder. If you're not up for that then I suppose you could just pull them and send them to be tested (usually to the tune of $50-ish each)


ATP trucks would've no doubt been playing with rail pressure diagnostics to buy a weak system more time. But the truth is that ATP trucks no longer programs trucks anymore so you're probably not going to get any help from them.


When you're talking about 26k-75% throttle, that may be well and fine. But when you're talking about 26k-100% throttle its a completely different scenario. The difference is the amount of fuel being injected. It simply cannot maintain 26k at 120mm3. It may be able to sustain 26k at (just for illustration we'll say 75% mm3).

 

[fastline]

Its not really that I cannot be bothered by considering the injectors, and rather, they do seem to be highly suspect, I think the diagnostics on the situation seems really poor! When GM and their dealers have to go through many hours of testing just to prove the faulty, that seems an issue. Why the frig was this not considered in the beginning.

Is there not a flow sensor on the rails or pump to show actual flowrates? Obviously if the pump pressure and flowrates can be examined at different demands, it could easily prove where the culprit is.

I do have enough stuff in the shop to start testing the injector return rates but it seems obvious that either they or the pump are going, and possibly a combination and NEITHER are any fun........ I was shocked to see what it took to get to that pump! OMG..... Obviously I am just frustrated because I have a big dually that can only go get groceries....... I can't even haul the fat girls in it!

 

[fastline]

I just wanted to clarify that I am very familiar with variable flow positive displacement pumps and the relations in pressure and volume. I very much understand that the 'demand' on the pump at 75% load is such that the pump can hit the required flowrate and pressure. I cannot recall if I determined the actual duty cycle of the solenoid in the pump or not and cannot recall if 0% or 100% is WFO for the pump but regardless, once that threshold is met, she can't do no more bobby!


It is reasonable to consider if the rail pressure sensor is way down the line from the pump (cannot remember location), there could be considerable pressure loss from fluid friction as the fluid flowrate demand goes up. If I can see the pressure right at the pump, that would at least prove it is not restriction related.

 

[monster50iii]

Check injector return rates.

 

[Bdsankey]

Check the injector return rates first.



Just because the pump CAN hit 26ksi doesn't mean it is able to move enough fuel at higher injection rates. Also, as said previously ATP no longer tunes ECMs.

 

[Dozerboy]

Yep need to check return rates on injectors and cp3 it’s easy. If you search it’s been discussed.

It’s obvious you are frustrated with this, but if you want help you might try and not tell us what you want to hear. A simple I have checked that will be sufficient.


Sent from my iPad using Tapatalk

 

[JoshH]

I guess the first question I have is, has the truck been tuned, or is it still stock?

 

[gmduramax]

You have a shop and all the tools but you’re too lazy to check your cp3 and injectors and expect us to have some magic fix for you. But I mean I guess your 200k mile injectors are perfectly fine. Try making sure your bleeder screw on the filter head points north.

 

[Mike L.]

" Try making your bleeder screw on the fiter head point north"

Learn something new every day. :thumb:

 

[clrussell]

Fuel injector return rate test.. if truck is 100% stock you’ll find your issue. The rest is a bandaid

 

[jwrkansas]

Dude you're trying to geek out too hard on this. The location of the rail pressure sensor works fine for gm so why can't you buy into that? If you're really as smart as you're trying to sound then you shouldn't need this forum. Maybe you've already thrown a lot of good advice in the trash?

Talking about fluid friction loss on a foot long fuel rail seems a little crazy. When you see the size of these fittings/lines you'd think it was amazing that they do as well as they do!

 

[Bdsankey]

I just wanted to clarify that I am very familiar with variable flow positive displacement pumps and the relations in pressure and volume. I very much understand that the 'demand' on the pump at 75% load is such that the pump can hit the required flowrate and pressure. I cannot recall if I determined the actual duty cycle of the solenoid in the pump or not and cannot recall if 0% or 100% is WFO for the pump but regardless, once that threshold is met, she can't do no more bobby!


It is reasonable to consider if the rail pressure sensor is way down the line from the pump (cannot remember location), there could be considerable pressure loss from fluid friction as the fluid flowrate demand goes up. If I can see the pressure right at the pump, that would at least prove it is not restriction related.

0% is full pump flow.


As for losses, that is not a realistic concept. You also are not fighting a restriction. Guys make damn good power (you won't find the limit on stock injectors) on stock rail feed fittings. During the course of obtaining my engineering degree we studied the effects of friction on fluids flowing in pipes and determined the benefits to be negligible on a "decently" smooth surface. Sure, if it is super rough you will cause the fluid to not be laminar and take excess force to drive the fuel to the rails/injectors but in our case it is not enough to matter. The placement of the rail pressure sensor is strategic for a reason. The pump feeds the driver's side fuel rail and the sensor is in the passenger rail. What this ensures is that desired pressure is met on the passenger side injectors. While this COULD lead to a pressure imbalance it isn't enough to be a huge issue as well. Sure there are guys that feed the passenger side rail fuel as well (I am doing it in my dual fueler kit) but that is simply because I am OCD and don't want to rely on a single crossover line to take the fuel volume from both of my pumps.




To recap, you're making the problem to difficult and trying to over complicate things. Your fuel system is perfectly fine (design wise) the way it is for 99.99999% of people making even the biggest tune on stock injectors/stock turbo. Take the advice on this forum. Check the injector return rates if you have already verified that the relief valve is not the culprit/is not leaking. If the injectors are not leaking look at commanded regulator percentage. If it is dropping to 0% (or check the ma sent to the reg) then you have found the issue.

 

[fastline]

0% is full pump flow.


As for losses, that is not a realistic concept. You also are not fighting a restriction. Guys make damn good power (you won't find the limit on stock injectors) on stock rail feed fittings. During the course of obtaining my engineering degree we studied the effects of friction on fluids flowing in pipes and determined the benefits to be negligible on a "decently" smooth surface. Sure, if it is super rough you will cause the fluid to not be laminar and take excess force to drive the fuel to the rails/injectors but in our case it is not enough to matter. The placement of the rail pressure sensor is strategic for a reason. The pump feeds the driver's side fuel rail and the sensor is in the passenger rail. What this ensures is that desired pressure is met on the passenger side injectors. While this COULD lead to a pressure imbalance it isn't enough to be a huge issue as well. Sure there are guys that feed the passenger side rail fuel as well (I am doing it in my dual fueler kit) but that is simply because I am OCD and don't want to rely on a single crossover line to take the fuel volume from both of my pumps.




To recap, you're making the problem to difficult and trying to over complicate things. Your fuel system is perfectly fine (design wise) the way it is for 99.99999% of people making even the biggest tune on stock injectors/stock turbo. Take the advice on this forum. Check the injector return rates if you have already verified that the relief valve is not the culprit/is not leaking. If the injectors are not leaking look at commanded regulator percentage. If it is dropping to 0% (or check the ma sent to the reg) then you have found the issue.

I did look a bit more at this elusive fuel fitting and appears to have a tiny orifice just before feeding the driver's fuel rail?

I wasn't generally saying the factory fuel rails would have significant pressure loss due to friction as the actual flows are pretty low, I am considering a restriction between the pump and fuel rails and this fitting seems to be at question. If there is high enough demand on the outlet side, pressure could drop.

However, it doesn't sound like guys here have run into that as an issue. ?


Is it not as likely to be an issue with the CP3? I realize return rates may be the only think the solve the riddle as that's about how a pos disp pump is proven worn typically.

 

[Bdsankey]

I did look a bit more at this elusive fuel fitting and appears to have a tiny orifice just before feeding the driver's fuel rail?

I wasn't generally saying the factory fuel rails would have significant pressure loss due to friction as the actual flows are pretty low, I am considering a restriction between the pump and fuel rails and this fitting seems to be at question. If there is high enough demand on the outlet side, pressure could drop.

However, it doesn't sound like guys here have run into that as an issue. ?


Is it not as likely to be an issue with the CP3? I realize return rates may be the only think the solve the riddle as that's about how a pos disp pump is proven worn typically.

The rail fitting is the only thing that can be seen as an orifice in the factory fuel system other than the injector. Even then, you won't see a major dip. Has a ported fitting helped some guys with rail dropping issues? You bet, but its a bandaid for an underlying cause (excessive return wasting pressurized fuel or a weak pump).




Again, check the fuel coming from the return size of the injector. Some is normal but too much will cause an issue. If that isn't the problem you need to data log the the regulator command and see what the percentage/amperage is doing. If it is going to 0%/540ma (IIRC that is the value, I can't recall but its something like like that) and you have normal injector returns/properly functioning relief valve then you've got a weak pump.

 

[fastline]

So you are saying the pump should not need to go to WFO even at WOT? I mean, that would make sense as there should be some headroom designed in.

 

[Awenta]

Get off the keyboard and into the garage. Come back with the return rates.


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[Burn Down]

Good lord:rofl:

 

[JoshH]

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