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Discussion Starter · #1 ·
Ok so there’s been hundreds and hundreds of threads about LLYs lopeing in park and neutral at operating temp and I have found zero that have helped. Mine is a 2005 LLY with 295,000 miles on its second set of injectors. To start off with things I have a noticeable lope at idle when warm only in park and neutral. It started doing the day I got it back from the shop in March of 2022. I noticed it right away and took it back to them the next day. They called me the day after and said there was a faulty injector and they got it fixed. So the next day I picked it up and it started fine with no issues until about 15 mins later when I stopped to get some fuel and noticed the lope. It only does it once the oil pressure drops below the 30psi dash on the gauge and the coolant temp is around 180. So I drove back to the shop and told them I was still having the issue. Long story short they told me I needed a lift pump and they quoted me 900$ for one if there fass systems. I held off on it until about August of 22 when I ordered an Airdog 100gph for about 650$ and did it myself. I did notice better throttle response and a few other things but it did not solve my lope in park. Since then I’ve done a new FPR, fuel filter head, fuel filter, new fuel lines to cp3, new ficm, ice picked all the injector pigtails, and a new pigtail for the fpr, still having the lope. I’m completely stumped. I’ve checked for the wire harness rub at alternator bracket and nothing there. The only thing I could think is faulty injector, or some weird electrical issue, and possibly an ecm reflash which seems to be highly unlikely. The engine is pretty much stock besides egr delete and exhaust system. Stock tune as well.
 

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For most of these, it is a well known, but not well understood problem. It is in the programming in the idle fuel tables. It is such a subtle issue that you have to log the relevant parameters as it is happening to see where in the tuning the problem occurs and why. Any load usually gets rid of it. Like putting it in drive, turning the wheel, turning on the AC, even using a thicker oil. It is especially noticeable on very "loose" engines that need less fuel to idle. It can be fixed by a programmer who knows what to adjust. A data log of rpm, mm3, pulse, and actual vs desired FRP will show the relationship of the rpm lope to the actual programming parameters.
 

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Discussion Starter · #3 ·
For most of these, it is a well known, but not well understood problem. It is in the programming in the idle fuel tables. It is such a subtle issue that you have to log the relevant parameters as it is happening to see where in the tuning the problem occurs and why. Any load usually gets rid of it. Like putting it in drive, turning the wheel, turning on the AC, even using a thicker oil. It is especially noticeable on very "loose" engines that need less fuel to idle. It can be fixed by a programmer who knows what to adjust. A data log of rpm, mm3, pulse, and actual vs desired FRP will show the relationship of the rpm lope to the actual programming parameters.
It’s just so weird that after I got new injectors it started. You are correct about putting a load on the engine such as turning on the ac compressor. What do you mean by “loose” engine btw? Does that mean low compression? Blow by? Mechanically I think the engine is sound. There is a small amount of steam/vapor from the oil filler but from what I’ve heard it’s normal for high mileage Duramax’s.
 

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Some LLYs use less fuel at idle than others. For whatever reason, I refer to them as loose. It is when those low fuel idle tables are being used, there is fuel fluctuation and a latency for rpm feedback. <it is idle rpm, afterall that the ecm is trying to stabilize. The lope is a bit of a feedback conundrum. A bit too much fuel commanded results in rpm overshoot, and vice versa. You can probably see it on the rpm gauge. When you add even a little load, the fuel demand increases, and the engine operates in a different fuel demand range where the tuning is more stable, and less fluctuating. Sometimes the tuner can also get rid of it just by upping the idle set point. 720 rpm instead of 680, for example.
 

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Discussion Starter · #5 ·
Some LLYs use less fuel at idle than others. For whatever reason, I refer to them as loose. It is when those low fuel idle tables are being used, there is fuel fluctuation and a latency for rpm feedback. <it is idle rpm, afterall that the ecm is trying to stabilize. The lope is a bit of a feedback conundrum. A bit too much fuel commanded results in rpm overshoot, and vice versa. You can probably see it on the rpm gauge. When you add even a little load, the fuel demand increases, and the engine operates in a different fuel demand range where the tuning is more stable, and less fluctuating. Sometimes the tuner can also get rid of it just by upping the idle set point. 720 rpm instead of 680, for example.
Ok I gotcha 👍. The truck is on its stock tune btw. The last time I checked the demand vs actual pressure was about a month ago and if I remember correctly it was within 50-100psi at operating temp in drive. It was commanding 4350 psi and the actual was pretty darn close to it. It’s the smoothest idle I’ve ever felt in an LLY until you put that sucker in park and it goes haywire. What tuner do prefer btw? I’ve never been interested in high output numbers or tuning in general but maybe a custom tow tune or something would help. The truck is deleted btw and I don’t know if that plays a factor or not.
 

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Discussion Starter · #6 ·
Some LLYs use less fuel at idle than others. For whatever reason, I refer to them as loose. It is when those low fuel idle tables are being used, there is fuel fluctuation and a latency for rpm feedback. <it is idle rpm, afterall that the ecm is trying to stabilize. The lope is a bit of a feedback conundrum. A bit too much fuel commanded results in rpm overshoot, and vice versa. You can probably see it on the rpm gauge. When you add even a little load, the fuel demand increases, and the engine operates in a different fuel demand range where the tuning is more stable, and less fluctuating. Sometimes the tuner can also get rid of it just by upping the idle set point. 720 rpm instead of 680, for example.
Another thing is on my lift pump I’ve got the pressure set all the way down. If I do crank up the pressure a bit it does help the idle to an extent but with different pressures ranges at different operating temps it doesn’t help the idle. Also with my lift pump operating at the lowest pressure while I’m driving I do notice a miss/lope/knock when I let off the throttle around 40-50 mph while coasting. It only does it for about 3 seconds and then it smooths out when I hit the brakes and slow down. But with the lift pump fuse pulled it doesn’t do what I just mentioned.
 

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If by deleted you mean you have a high flow exhaust, that is one of the ways a lower load can be experienced. Try this...try stuffing your tailpipe with something porous (load) and see if that lessens or eliminates the lope. FWIW, 99% of the tuners out there won't have a clue how to adjust your idle fuel tables for this problem. And while on this subject, it isn't usually pressure that fluctuates, at least not dramatically. It is pulse and mm3. Log that against rpm. EFILive is the best for this kind of thing. If you have a log file, I can look at it to say for certain.
 

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Discussion Starter · #8 ·
If by deleted you mean you have a high flow exhaust, that is one of the ways a lower load can be experienced. Try this...try stuffing your tailpipe with something porous (load) and see if that lessens or eliminates the lope. FWIW, 99% of the tuners out there won't have a clue how to adjust your idle fuel tables for this problem. And while on this subject, it isn't usually pressure that fluctuates, at least not dramatically. It is pulse and mm3. Log that against rpm. EFILive is the best for this kind of thing. If you have a log file, I can look at it to say for certain.
I’ve got a scan tool the measures balance rates. I’ll check them tomorrow after about an hour drive so it should be good to test and check them. And yes I’ve got an egr delete and a 4 inch straight pipe. I get what your saying about stuffing something up the pipe to see if it eliminates the lope by adding back pressure I’m assuming. Would a performance muffler possible help?
 

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This is not a balance rate issue. If you just want to observe the overshoot issue, monitor pulse when it is loping. If it cycles to zero, or near zero, then you are closing in on this being a tuning issue. IIRC you should be seeing pulse cycling between 500 us to 800 us to keep idle steady within 20 rpm.
 

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Discussion Starter · #10 ·
This is not a balance rate issue. If you just want to observe the overshoot issue, monitor pulse when it is loping. If it cycles to zero, or near zero, then you are closing in on this being a tuning issue. IIRC you should be seeing pulse cycling between 500 us to 800 us to keep idle steady within 20 rpm.
How do you measure the pulse?
 
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