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Discussion Starter #1
Hi guys,

I’ve been dealing with an underpowered truck for a while now. Finally got an edge insight to see if I can figure out what the deal is. (Out towed by a 6.0 gas). It often seems like a safety mechanism when it happens. Pulling hard and then like I’ve rolled off the pedal. Like some computer nanny trying to keep me from speeding but it really hangs me at 40-50mph climbing hills.

Anyway, I am getting p1093. After some review on the site I see it could be a fuel pressure regulator, cp3, filter, blockage in the line, collapsing hoses, filter head, maybe more. Any advice on where to start?

I tested the balance rates with the edge. It shows 3.1 was the worst but that’s in spec if my research is right. I’ve recently changed the filter but this has been an issue for several years and I never realized the issue was as bad as it is until towing with dad who has a gas truck. I change that filter almost as regularly as the oil. I have recently changed the fuel lines between the filter head and the pump because I thought they were old and letting it lose prime. I can compare the desired fp and actual fp with the edge and it’s obvious where I lose power. Under 25% throttle the fp is pretty spot on with the desired. Any hard acceleration or towing though sees the desired at 23-24k and actual at 5.5k. Yesterday with my empty stock trailer I saw it get as low as 2.3 and I thought it was going to totally quit. I’ve seen the actual pressure reach 23k but that’s after letting out of the pedal and I’ve seen it as high as 18k when being a lot less aggressive with the pedal.

I don’t have a lift pump and this has been an issue for years. Occasionally I see an overboost code but that maybe unrelated. 270k miles on a what I think is a stock truck. I got it with a cai and magnaflow exhaust.

Where do I start or are we still missing data?
 

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When my truck was under powered like that throwing the 1093 code the CP3 was taking a crap . Almost the same issues. It could be a collapsing fuel line somewhere but with your mileage I bet its the CP3. You should probably learn how to do a return rate test too I think if the injectors are returning too much fuel it can cause the low rail pressure code to pop and be down on power. Also highly recommend a lift pump with filtration . The older LB7's need all the help they can get to keep those injectors happy .
 

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Discussion Starter #3
When my truck was under powered like that throwing the 1093 code the CP3 was taking a crap . Almost the same issues. It could be a collapsing fuel line somewhere but with your mileage I bet its the CP3. You should probably learn how to do a return rate test too I think if the injectors are returning too much fuel it can cause the low rail pressure code to pop and be down on power. Also highly recommend a lift pump with filtration . The older LB7's need all the help they can get to keep those injectors happy .
I saw on the air dog site they claim to resolve the p1093 so I was considering trying that first before the cp3. Especially if it needs one anyway. I’ll study up on the return rate test.
 

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Discussion Starter #4
In my continued search for whatever is causing this error I have a few more clues. Yesterday, towing a load of hay (13 4x5s on a 24’ flat 14k lbs minimum), I saw the egt temps hit 1500 degrees on some big grades. The desired rail pressure was 23k usually and the actual was around 4K. When cruising on fairly level ground the desired and actual were around 16k so it can make pressure. It just can’t keep up with the volume it seems on a big hill. Egt was under 1k when cruising on level ground.

Does the high egt temp tell us anything?
Could this thing be tuned or should I expect those high egt numbers with the low pressure?
 

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Go a little slower and drop a gear. See if the egt's come down. At 1500 the truck may be defueling to save itself, but I would think the desired would come down too.
 

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Discussion Starter #6
Go a little slower and drop a gear. See if the egt's come down. At 1500 the truck may be defueling to save itself, but I would think the desired would come down too.
It was in third gear and slowed down to around 40mph before it topped the hill. This is the load.
1083544
 

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You need to get the fuel pressure problem fixed before it leaves you with a dead truck.
Are you able to work on it yourself?
 

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Discussion Starter #8 (Edited)
You need to get the fuel pressure problem fixed before it leaves you with a dead truck.
Are you able to work on it yourself?
Some, I think if it needs a CP3 I may let a mechanic do that. I have been talking to a guy familiar with these things and trying a few troubleshooting steps to narrow down the problem. I put on some clear fuel lines between the FICM and Filter and then another between the filter and the pump. No air before the filter but a little after. Is this normal or could this amount of air cause the low pressure I am seeing? Sorry about the dog barking in the video. He hates vehicles. Also, that cheap clear hose was garbage and only on there long enough to get these videos. Yesterday's issues were with the rubber lines back on there without any kinks or junk like you can see in the video. I also switched back to the rubber one before the filter near the end of the video and still it shows air.

 

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Replace the fuel filter housing. It might be leaking if you have bubbles in the line.
It's easy and might fix your problem.
 
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Discussion Starter #10
Replace the fuel filter housing. It might be leaking if you have bubbles in the line.
It's easy and might fix your problem.
I’ve ordered a filter head and decided to test the possibility of a restriction in the tank or lines up to the engine. This may look funny but my temporary fuel tank proves my issues are on the engine somewhere. Running it off the can here proves to not fix the issue. Going to try a return rate test while I’ve got the return line off and wait for the filter head next and see if that fixes it.
1083680
 

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Discussion Starter #11
Return rate on the return line after 15 seconds of cranking was 25ml. I believe 100 is what we need to stay under so now I’m down to thinking this either must be air from the filter head or the cp3 just can’t keep up at wide open throttle.
 

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Discussion Starter #12
I have replaced the filter head and that resolved my air in the fuel situation seen in the earlier video.


The bad news though is I still have the low pressure issue under wide open throttle. I am up 1k psi though so the bubbles were hurting. I just have more trouble than the bubbles. Now I see 6k under wide open when the desired rail pressure is 20k. It's starting to look like the injection pump is the last piece to my puzzle.
 

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Discussion Starter #13
Is there anything else I need to check at this point or is it pretty clear I need to replace the cp3?

This truck is a daily driver/tow truck. No sled pulling or racing for it. Should I go back with a stock replacement cp3 if that is the diagnosis or is there a simple upgrade I should take advantage of?

Is there anything that should also replace or upgrade while replacing this? I understand it’s pretty buried so there may be plenty taken off and easily upgrade while I am in there.
 

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Discussion Starter #14
I believe the last test that needs to be ran is the ability for the pump to reach max pressure at idle. I don't have a scan tool to command it but I read somewhere that you can unplug the regulator and it will go to max pressure. If that is the case, the CP3 is my problem. It can only make 15.5kpsi at idle and only sees 21kpsi at 1500 rpm with the regulator disconnected.
 

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I believe that 21,000 on the LB7 is the max, and idling should be around 5,000. That's with everything hooked up, I don't know about maxing out while idling with the regulator unhooked but with it making that much pressure at 1500 rpms I don't know if i would think the CP3 pump is the problem. Do you have a way of comparing desired fuel pressure against actual fuel pressure.
 

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Discussion Starter #16
I believe that 21,000 on the LB7 is the max, and idling should be around 5,000. That's with everything hooked up, I don't know about maxing out while idling with the regulator unhooked but with it making that much pressure at 1500 rpms I don't know if i would think the CP3 pump is the problem. Do you have a way of comparing desired fuel pressure against actual fuel pressure.
Yes, my CS2 will show the actual and the desired on the same screen. When towing I see a desired pressure of 23k at WOT and an actual around 5-6k. Idling it keeps it pretty close and even on level ground I see consistency between the two but it separates when I get over half throttle. That is where I see actual falling and desired pressure climbing. Today's test was to see what it shows at idle without the regulator plugged in and I see desired pressure of 5k and the actual was 15k or so. Just for my own curiosity I ran the rpm up to 1500 and the actual pressure went to 21k while the desired was around 8-9k. Assuming the regulator when disconnected, does nothing to limit the pressure, it should have been maxed at idle. I think this points to the pump being unable to make the pressure it needs.
 

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I did see where you checked the desired against the actual rail pressure after rereading your thread, you may be onto something with checking it under load. If you haven't already I would add a lift pump and see if it helps as these truck should have one anyways, and if it turns out to be the CP3 pump you would already have a lift pump.
 

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Discussion Starter #18
I added a vacuum gauge on my filter head to double check there was nothing fishy happening when it was under a heavy load. I see -1psi at idle and -2.6 or so under full throttle. When it staggers and junk I notice no change in the vacuum pressure. I ordered an Airdog 2 4g today and will report back if there are any changes once the cp3 is under positive pressure.
 
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Discussion Starter #19
Installed an Airdog 2 4g yesterday. I’m seeing 13.5 psi positive pressure at idle and 11 psi at wide open. The rail pressure gain was similar to the new filter head gain. I’m up another 1000 psi but still way off. 22k desired and 7k actual or so. Either I failed to get an accurate return rate test or my cp3 is the problem.
 

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Discussion Starter #20
Well today my p1093 code became p0087, p1093, and p1094. It didn’t have the power to climb on the trailer in 2wd. 4lo was low enough to get it loaded. Hopefully FedEx is right and the new injection pump is here tomorrow.

1085676
 
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