Duramax Diesels Forum Truck of the Week
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Old 09-16-2018, 01:33 PM
Johnrt Johnrt is offline
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Exclamation 08 lmm limp mode issues

hey guys I am new to this but have been reading a lot of the conversations, I got a 08 lmm 120k miles and have issues with rail pressure with or with out efi tune from Duramax tuner. went back to stock fuel filter, got a bottle test done yesterday, no fuel. have read post about soft lines. When I did the bottle test I noticed the 3/8 line that is next to what looks like a 1/2 line is bent over pretty good, could see as to where that could cause issues. Is this the line that gm did a cut and replace with better fuel line? Is there a way to view balance rates through an autoal?
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Old 09-16-2018, 04:38 PM
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ZeroGravity58 ZeroGravity58 is offline
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Have you shimmed to relief valve or replaced it with a race plug? Might be causing your low rail problems
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Old 09-16-2018, 06:13 PM
Johnrt Johnrt is offline
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I did the bottle test no fuel from the the relief valve
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Old 09-16-2018, 06:27 PM
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Do you have a lift pump? What were you using previously as a fuel filter ?
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Old 09-16-2018, 06:34 PM
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Quote:
Originally Posted by Johnrt View Post
I did the bottle test no fuel from the the relief valve
The 3/8 line is the return line, the 1/2 would be the supply line. They are on the driverside back where they come up if that's what you are referring to. Did you try to command fuel rail pressure to see if the relief valve is popping open? Give us more info on the truck. What all do you have done
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Old 09-16-2018, 07:02 PM
Johnrt Johnrt is offline
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No lift pump, started having issues towing so went back to stock tune and continued to have issues after awhile after. Was running wix filters as I was going through acdelco ones every 10k or less.
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Old 09-16-2018, 07:53 PM
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Quote:
Originally Posted by ZeroGravity58 View Post
The 3/8 line is the return line, the 1/2 would be the supply line. They are on the driverside back where they come up if that's what you are referring to. Did you try to command fuel rail pressure to see if the relief valve is popping open? Give us more info on the truck. What all do you have done
Truck is stock other than dsp5 tune. While the tune is loaded it will show low power mode, dump the fuel filter but does not go into limp mode. Stock tune will dump a brand new filter and go into limp mode. The rail pressure will drop with wot. But catches back up after shifts. Only 120k on truck, pulling less than 10k most of the towing life.
Sad part is it is doing this not even towing lately. On 3 rd filter in less than 10k miles. I have a data log of 3 12%grade wot pulls. Just not great with
Efi live to break it down effectively
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Old 09-16-2018, 09:13 PM
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Is the truck over heating?
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Old 09-16-2018, 11:27 PM
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Kink or collapsing fuel line would produce the symptoms you discribed
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Old 09-17-2018, 04:58 PM
Johnrt Johnrt is offline
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Truck runs a little warm when towing, sounds to be the same as every other lmm of people I know That have one. I will have to crawl back under it and inspect lines.
What is the thoughts on brands of quality lift pumps?
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Old 09-17-2018, 06:15 PM
Blackmax123 Blackmax123 is offline
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As stated above, check the fuel lines on top of the drivers valve cover. A lift pump won't help it the supply line is collapsing shut. But I'd hook the hose from the lift pump right up to the metal hard line at the valve cover and remove the factory 1/2" supply.
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