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Discussion Starter #1
I just wanted to share my experience and my fix that has worked for me.

About 2 years ago all of a sudden my fuel mileage around town went from about 16-17 to 11 mpg and the truck was really sluggish. The mileage of 11 would not change between just driving around town or towing my 12000 lb fifth wheel. I changed fuel filter every 3000 miles, used all different injector cleaners, brought it to a shop who said, without even looking at it, said I needed new injectors. I had no bad injector issues, no smoke. I had a different shop check the balance rates and they said they were ok.

Found some threads talking about the fuel pressure relief valve, figured for $30 give it a try. Best $30 and a busted knuckle I ever spent. I have only driven for about 100 miles on the highway but I am getting 16-17 mpg. Replacing it was pretty easy once I got the correct 18" flex head 3/8" rachet, 4" extension and 18 mm 6 sided socket. I removed the glow plugs controller bracket, not sure if you can do it without. I would recommend anyone who has a LLY who hasn't done this should.
 

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Did u get a stock valve or the Poe race valve? Also any other symptoms? I have a loapy idle and suspect #2 injector. I have a fuel control actuator/valve I’m gonna throw on first just curious if u had any other symptoms and if u got a stock valve or a race valve
 

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Discussion Starter #3
I got the PPE race valve. I think you can get stock ones for cheaper. I have no mods on the truck just figured it might last longer than the stock one.
The symptoms I really noticed were the poor mileage, low power off the line, regular driving and towing and the idle was a little rough.

My idle does seem smoother.
 

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Thanks I’ll see if it helps me out.
 

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I’m over 190k on mine and from the previous owner sounds like the truck was never messed with. #2 injector harness was repaired a cat 2micron adapter and a edge set on mileage is all the changes done
 

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My truck ran great 100% stock, high mileage, when I bought it, but my fuel mileage daily driving no load was only about 7 mpg.

I'm not even sure I could sit here and think up all the things I've done to it, exhaust, air intake, lift pump, higher output CP3, tuner, "race plug", larger boost pipes... not one single thing seemed to make any noticeable difference in fuel mileage or performance, it's been apparently many little progressions adding up but my mileage is now up to a consistent 18 mpg. Still runs and drives exactly the same, which is kind of disappointing for all the money I've put into it, not that I was ever disappointed in how it ran and drove before, but to read all the hoopla on all these HP increasing parts I've put into it over time you'd expect me to now be doing at least 8-9 second 1/4 miles while towing a 20' flatbed and two Bobcats on it.
 

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Discussion Starter #7
So, replacing the plug on my 163k lly may not have corrected all of my issues. Over Labor Day towing my 5th wheel I got a P2563 code. I cleared the code and it has not come up again. After the code my fuel mileage decreased radically from 16-17 to about 12 not towing. I only got the code one other time and replaced the sensor 2 years ago, but looks like I might need to clean or replace the turbo.
 

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I had that happen once when pulling my 42' 5th Wheel, I pulled over when it happened, cleared the code, changed my Banks Tuner from program 5 to program 2 (though I've yet to ever find any documentation on what the difference is on any of their different programs) and it never turned up again, almost 2 years ago now.

It probably just needs cleaned, its amazing the things work at all between how the factory design dumps nasty exhaust soot back into the system with the EGR compounded by also dumping tar from the crank case breather hose directly into the turbo inlet. Routing the crankcase breather hose to a different location is an easy fix. I bought a larger opening throat on Amazon that doesn't have the connection point for that smaller hose, they also sell plugs to plug it off at the turbo inlet, then just ran longer hose down past the fuel filter and zip tied it to my frame underneath.

At first I was worried it was going to always be spilling oil out the hose on my driveway, but it really hasn't, there's just a slight hint of oil residue around the open end of the hose, but oil doesn't pour out of it. I did fail a Nevada emissions test for having it like that, they physically inspect for emissions "tampering" on diesels only (racist) and failed me for the "crank case breather hose venting to atmosphere". I popped my hood in their parking lot, cut the zip tie off the frame and stuffed the end of the hose up into the opening on the bottom of my air filter box, they passed it, I zip tied it back to the frame and went on down the road.
 

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I have to say rebuilding the turbo really woke my truck up. Over 24mpg on the highway. I get 14-16 driving around town
 

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Will this work on an LB7,

I was getting 14-16 around town 18- 20 on the highway. I just put in SAC00 and a FAS165. Truck is stock Now no matter towing one of my boats or not towing a boat. I get 11 MPG. I just replaced all of the fuel filters maybe thinking they pick up residue from being installed. Nope all the same 11MPG. Will my FPRV do this for me too? Maybe. Im at 217,XXX miles
 

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Will this work on an LB7,

I was getting 14-16 around town 18- 20 on the highway. I just put in SAC00 and a FAS165. Truck is stock Now no matter towing one of my boats or not towing a boat. I get 11 MPG. I just replaced all of the fuel filters maybe thinking they pick up residue from being installed. Nope all the same 11MPG. Will my FPRV do this for me too? Maybe. Im at 217,XXX miles
I honestly can't imagine how changing the relief valve with a plug is going to make any difference what so ever on your fuel economy. I changed mine out just for the "one less thing to go wrong" theory. Don't waste $30, $40, $60 on one just because it has a fancy engraved name on it, once installed the only way you'd ever see it is if you cut your firewall out and installed a window, and even then only the driver would ever see the fancy engraving. They sell stainless steel ones with the fancy o-ring and no engraving for $6-$8, there is absolutely no justification for charging someone more for one.
 

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Just changing it out because of the fuel mileage thing. I couldnt careless about engraving. Besides, mine ia a PITA to access. I saw a thread where a guy took his a part and put two minute washers inside to make the spring a little stiffer. Do i need to R&R it. I dont have codes. Im just not sure why im losing MPG.

I think my truck runs fine. I pulled 1500 miles this summer through the 120+ desert heat. Im in an office telling about it. I just want some MPGs back, not willing to tune it.
 

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It's not impossible to replace it, but there's one bolt under the glow plug controller that is a real pain, especially when it comes to putting that bolt back in. They also torqued it on at the factory to what seems like 2 billion foot pounds or used some type of they'll never have to remove it again locktite on it, one of those things where you can only get to it with a wrench, have to put everything you have into breaking it loose, then when it does break loose it goes from 2 billion foot pounds to -300 immediately. If I was going to go through the trouble of replacing it I'd just put the $6 plug in it with some really good chemical resistant thread sealant like Leak Lock.

Someone could have came up with an easier way to eliminate it for sure. There's a small hose you'll see just above it, that's were the fuel passes out when that valve opens. Seems it would be a whole lot easier to stop the flow from there instead, since that's all the plugs, sorry, "race valves" do is plug off the flow out that small hose.

As far as your fuel mileage improving from it, I'd really like the science behind how that could have any effect on it explained.
 

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It's not impossible to replace it, but there's one bolt under the glow plug controller that is a real pain, especially when it comes to putting that bolt back in. They also torqued it on at the factory to what seems like 2 billion foot pounds or used some type of they'll never have to remove it again locktite on it, one of those things where you can only get to it with a wrench, have to put everything you have into breaking it loose, then when it does break loose it goes from 2 billion foot pounds to -300 immediately. If I was going to go through the trouble of replacing it I'd just put the $6 plug in it with some really good chemical resistant thread sealant like Leak Lock.

Someone could have came up with an easier way to eliminate it for sure. There's a small hose you'll see just above it, that's were the fuel passes out when that valve opens. Seems it would be a whole lot easier to stop the flow from there instead, since that's all the plugs, sorry, "race valves" do is plug off the flow out that small hose.

As far as your fuel mileage improving from it, I'd really like the science behind how that could have any effect on it explained.

Thanks Col. I wont be doing that until absolutely needing to. I just did injectors. Didnt think to do it then. It wont be until i have to. Thanks for the info. Much appreciated.
 

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That’s the part I hate most about the deep into it repairs, wishing you would have thought to replace something else while you’re into it that far.

I try like hell to do that. Where do you stop. I did the entire front end every movable part i could think of. It worked. I got them all. Water pump did a pretty good job, pump, t stats,silicone hoses, housing, by pass pipe....When you tear the top half of the motor down....and forget or dont even think of replacing $6.00 hard to get ot parts , thats what hurts. I had no idea i exposed it and it was so hard to get to. Oh well live and learn....I always said " working on these trucks is like 10 pounds of shit in a 5 pound bag. Because most everything that breaks is so hard to get to.
 

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I replaced all the rubber fuel lines too. Again, I was surprised what heat and the suction from the cp3 does to them. All were dryrotted and brittle.
 

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Discussion Starter #18
I honestly can't imagine how changing the relief valve with a plug is going to make any difference what so ever on your fuel economy. I changed mine out just for the "one less thing to go wrong" theory. Don't waste $30, $40, $60 on one just because it has a fancy engraved name on it, once installed the only way you'd ever see it is if you cut your firewall out and installed a window, and even then only the driver would ever see the fancy engraving. They sell stainless steel ones with the fancy o-ring and no engraving for $6-$8, there is absolutely no justification for charging someone more for one.
I didn't think it would work either and not sure that it did as I stated before, my mileage is back down to 11-12 towing or not. I'm just really confused that after I changed the plug for about 300-400 miles I was getting 16-17 MPG, I tow my trailer, throw a P2563 code, and now I can't get the MPG above 12 not towing. This all happened in the past month. The first 2 years I had the truck, 2016-2017, 125k, I was getting 16-17 MPG. No other codes.
 

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See attached on P2563 and note which conditions are associated with this DTC.
 

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