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Lope at idle/rpm drop

13K views 19 replies 6 participants last post by  lousieg55 
#1 ·
This is my first post here so if I did something wrong feel free to correct me. I have a 2001 duramax with around 255,000 miles on it and it has a lope at idle when it's cold outside and when it's warm outside it doesn't really lope anymore but it's kind of more like a slight drop or surge. The truck got new injectors at around 180,000 miles so I don't think it's that. I rebuilt the fuel filter housing and when that failed I went ahead and replaced the whole thing and the fuel lines coming from the housing which helped a lot cause it used to lope no matter what so I don't think it would be that either. When I went and had it diagnosed the first place said it threw the p0094 code and said "small fuel leak" so I was kind of thinking it's the FPR but I don't think it would make it lope like it's doing. My uncle did some work to the fuel tank where it connects so that also makes me think that there could be a leak in between the tank and the cp3 somewhere but I'm not sure if that's even possible. I'm really at a loss here and I was wondering if any of you guys have had the same problem. Thanks.


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#2 ·
Lopes of all sorts

Hey there, maybe i can shed some light on your problems. Im not a mechanic nor have i owned a dmax prior to 3 mths ago but ive had issues of sorts. So I had a lope at idle, just started out if the blue one day. Could hardly even notice it. Then it got progressivly worse to the point that it would push through the brakes. It was not consistant at first, it would be soft and slow than hard and rapid. Once it became at uts worst you could almost time it to the second. So anyway. I changed it with a bostech fpr. Started right up but loped again. This time hard on cold start and once warm you could barely notice it. Couldnt see it on the rpms or the oil pressure such as the prior lopes. Put a scanner on it and was jumping around lime crazy. But fast this time, like 1/2 to 1/4 sec surges. Changed out with a bosch oem and completely gone. Now im dealing with a power loss at 100kph at wot on the hwy. Trying to figure it out now. Anyway just my opinion based on recent loped ive experienced. Just an example of the many different lopes that a faulty fpr can cause. Hope that helps.
 
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#3 ·
Hey there, maybe i can shed some light on your problems. Im not a mechanic nor have i owned a dmax prior to 3 mths ago but ive had issues of sorts. So I had a lope at idle, just started out if the blue one day. Could hardly even notice it. Then it got progressivly worse to the point that it would push through the brakes. It was not consistant at first, it would be soft and slow than hard and rapid. Once it became at uts worst you could almost time it to the second. So anyway. I changed it with a bostech fpr. Started right up but loped again. This time hard on cold start and once warm you could barely notice it. Couldnt see it on the rpms or the oil pressure such as the prior lopes. Put a scanner on it and was jumping around lime crazy. But fast this time, like 1/2 to 1/4 sec surges. Changed out with a bosch oem and completely gone. Now im dealing with a power loss at 100kph at wot on the hwy. Trying to figure it out now. Anyway just my opinion based on recent loped ive experienced. Just an example of the many different lopes that a faulty fpr can cause. Hope that helps.


Thank you, that sounds like the exact problem I'm having. That's kinda what I was thinking it was.


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#6 ·
My FPR is also starting to act up. I heard about dumping a quart of 2cycle oil from Wally World in the fuel tank at fill up to see if it made any difference. It did and that was 100k miles ago. So as soon as I sense it acting up, I put in another quart at fill up which usually is about 10k interval. It's worth a try.
 
#7 ·
Im not exactly sure how 2 cycle oil would help with a fpr related issue but i have heard of people believing that the fpr is the issue in some cases and lube does clear some of the problems. I run approx 8 0z of atf as well as cetane boost in every tank but it has never solved my fpr issues. Like you say though its worth a try for the few bucks it costs.
Btw, why so many injector failures as your sig describes in your '04?
 
#8 ·
Hey, so I dealt with the same problem that everyone else is having. It is now virtually gone, to the point that I don't even think to worry about if my truck is going to drop into limp mode. I went and got an OEM FPR cause that's what I had read would help fix the problem. I still have yet to actually replace it. I however went and replaced the "fuel pressure valve" with a valve delete, those valves pop open and create a pressure problem. I replaced it and it fixed about 60% of the problem, so I went a step further and installed a airdog 150 lift pump. The LB7 is very bad at functioning with air in the fuel, coming from the tank sloshing around. Now that the deleted valve and lift pump are on, I have no problems. It has a smooth, quick start up and I'm getting the most out of my fuel. Hope this helps a little, I spent a lot of time wrenching and scratching my head, trying to figure out what was up.
 
#12 ·
Yea, the valve is about $60, it can be a bit of a pain to replace. I wouldn't bother trying to disconnect the wiring harness, no need, you can loosen and disconnect what you need, but I wouldn't unhook everything. Go ahead and get new thermostats while you're at it, you'll have to remove the casing that covers them. All in all, it's not too bad of a job, just some tight spots. You'll have to know hand yoga to get in and around some areas lol. Well good luck, hope this helps and don't listen to some of these guys about putting 2 cycle oil in your tank. Just a myth, if anything, stick to your regular diesel fuel additives.
 
#14 · (Edited)
Not a myth, it worked for me and some of us on the other forum have done this for the FPR issue. Mind you it's not something you add to every tank which I do not advocate. Try it if you want. Nothing to loose.
 
#15 ·
I hear you, wasn't trying to bash your comment. I tried it with mine cause I read the same thing and hoped it would help. Never changed anything with my LB7, the fuel additives have helped a lot more though. Like you said, sometimes it's not for every truck or tank.
 
#16 ·
Yeah, for me it was try the oil or replace the FPR. My buddy in FL had luck with it on his 03 so I gave it a go. Guess I got lucky. Not so much with injectors though. It's now my son's truck and it runs like new at 228,000 miles and still uses no oil. Just spent some money to fix both differential seal leaks and axle seals on the front and encoder motor sensor on the 4wd. Good to go again.
 
#18 · (Edited)
I too have the idle surge. I posted on it awhile back, got some good advice about adding 2 cycle oil, putting in some good fuel additives (the low sulfur diesel fuel we have does not help), and after 4 tanks of gas, it did not help. So either I can install a good OEM FPR, or as another poster mentioned, a "fuel pressure valve" with a valve delete and a lift pump. The lift pump is expensive (for me at least), and I'm figuring that this engine did not come from the factory originally with an idle surge, so something is worn out.

My guess is the fuel pressure regulator (FPR).

Is the "fuel pressure valve" the ball and spring in the fuel filter housing? In the upper housing/"hat" that has the primer?
 
#19 ·
I would definitely get the "fuel pressure relief valve delete plug", I got one on ebay for about $60. I got a replacement FPR and haven't put it in yet, the delete plug helped a lot. It isn't located by the fuel filter, its kind of a pain to get to, but it can be replaced in a couple hrs. Its located by the turbo horn, just under the wire harness. You should get new thermostats when you go to install the plug cause you will have to remove the casing that covers the thermostats.

I would save up for getting a lift pump system, the LB7 was made with a manually purged fuel system. That's why there is a pump for you to purge the system after a fuel filter change. The lift pump system will force fuel to the block without the process of having the line pre pressurized via the manual pump. It also gets rid of the air and particles that come from the tank, so you have pure fuel going to the block, which for the LB7 helps significantly.
 
#20 ·
I did one on my old truck... If you have a Ca version not federal truck its a pain in the ass to replace the FPR as all the EGR stuff is in the way. I could not take off the wiring harness or anything like the federal trucks could.

I would highly recommend getting a Bosch. I had to install (2) as the first one even bosch was bad out of the box.

Took my about 4 hrs from start to finish.

If it was a federal truck probably 2 hrs.

I also would recommend while you are in there if you have not done the air horn mod for the turbo a good time do it. Should be able to do a quick search to pop this up. Spooled a little quicker and could here to turbo a little more.
 
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