Low fuel rail pressure, need help diagnosing - Chevy and GMC Duramax Diesel Forum
04.5-05 LLY Duramax Powertrain Discussion of components that are directly involved in the power production and all that is needed to get and keep the truck moving . Engine , Transmission, Etc.

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post #1 of 14 (permalink) Unread 01-01-2020, 01:24 PM Thread Starter
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Low fuel rail pressure, need help diagnosing

I started off with a p1093 code thought it was the fuel filter on my fass. I replaced the fuel filter with the same one a racor R90s. That did not solve anything. I then replaced the whole filter housing and then put on a car fuel filter with the adapter, still didn’t fix the pressure. When I first start the truck all the injectors are in spec at #1 -1.3 #2 1.7 #3 -.08 #4 1.8 #5 -0.4
#6 0 #7 -1.8 #8 0.9 this is at 96 ECT at that temperature I did the return flow test and unplugged the return line on the cooler unplugged my fass and cranked it over for 15 seconds I got about 90ml. I then drove the truck around until it got to 138 ECT and these are the balance rates #1 -1.0 #2 4.6 #3 2.3 #4 1.5 #5 -0.5 #6 1.9 #7 -5.1 #8 -3.0 all these number are in park. I then unplugged the fass and return line and measured the fuel. It was at 50ml then I tried it with the fass plugged in and it was at 170ml. Stock tune, no smoking at idle or driving. I don’t know what to do.
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post #2 of 14 (permalink) Unread 01-01-2020, 01:59 PM Thread Starter
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Also I do have a fuel race plug installed, and replaced the fuel hoses going from the filter house.
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post #3 of 14 (permalink) Unread 01-01-2020, 02:04 PM Thread Starter
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When desired from is above 15k actual won’t go above 15k it will stay the same below 15k though. Truck has 207k on it by the way and it’s a 2004.5 lly
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post #4 of 14 (permalink) Unread 01-02-2020, 12:35 PM
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Many reports about the FASS fuel pressure being too high. You need to be in a 5-7 lbs/in2 range.

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post #5 of 14 (permalink) Unread 01-02-2020, 02:29 PM
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Originally Posted by Ron Nielson View Post
Many reports about the FASS fuel pressure being too high. You need to be in a 5-7 lbs/in2 range.
Would that cause low rail pressure though? it sounds like the trucks holding commanded vs actual up to 15K then falling on its face which sounds like a bad regulator or pump.

2015 Chevy Silverado 2500 HD 6.6L LML
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post #6 of 14 (permalink) Unread 01-02-2020, 04:28 PM
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Would that cause low rail pressure though? it sounds like the trucks holding commanded vs actual up to 15K then falling on its face which sounds like a bad regulator or pump.
The return rates with the FASS plugged in are quite high. If the injectors are returning too much fuel, that MAY be at least a part of the problem. The truck SHOULD run just fine with no pump at all. That's why I suggested changing the pump pressure. Very easy, very low cost. If there is no improvement with lower pressure, then on to other things. As you say, could be regulator, pump, injectors, who knows what.



The fuel pump I run in my truck ,I believe, runs 12-15 lbs, but it has never caused any problems that I have discovered. Truck runs just fine. I was surprised to see that people with FASS pumps have had problems with the pressure of their pumps.

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post #7 of 14 (permalink) Unread 01-03-2020, 08:51 AM Thread Starter
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When I first got the truck it came with the fass and it’s has adjustable pressure, I never had any problem with frp. I don’t know if I should just buy a fake tech 2 and test the cp3?
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post #8 of 14 (permalink) Unread 01-03-2020, 09:14 AM Thread Starter
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I’m also confused if your suppost to leave your fass plugged in for the return test and leave it unplugged. When I leave it unplugged frp will only go to about 7k if anything higher is commanded. I have a older fass that is adjustable preset to 10psi at 95gph it’s a fass FA C08 095 with a addon filter kit. I’m not sure if this has a pass through or too much restriction for two fuel filters when it’s not on.
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post #9 of 14 (permalink) Unread 01-03-2020, 09:36 AM
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Lift pumps can be a band aid to the real culprit....find the leaks 1st, without LP confusing things.
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post #10 of 14 (permalink) Unread 01-03-2020, 09:52 AM
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One thing to keep in mind is the balance rates don't mean anything unless the truck is up to full operating temperature. Until then, they are not accurate.

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