Stumbling from stop
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Junior User
Joined: Aug 2013
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From: Napa California
I checked the readings for the petal and acc low shows .33v, my cousin came over with his 07 and I checked my values against mine and his shows 0v. Maf reads 1.88v at idle and climbs to 3v + without load. In gear under load it’s only going to 1.88v. Don’t know what the Acc low should read.
Fleet Owner
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Light acceleration MAF should be around 2V. Cruise should be around 2.2V. Heavy acceleration it should be 4.2V. Coast 1.7V.
Sounds like the turbo isn't performing. I know you've cleaned it, but it may not have "done the trick".
Sounds like the turbo isn't performing. I know you've cleaned it, but it may not have "done the trick".
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Junior User
Joined: Aug 2013
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From: Napa California
I really don’t think it’s the Turbo but I’ll move to it and see if I can determine if it is. Help me understand how it works exactly.
when I’m at idle my vgt reads 85%
with slight acceleration it stumbles and moves to say 68% and then climbs to 85%.
My truck when driving around the amount is 60% but coasting it moves to 85%.
under full throttle it will drop all of the way down to say mid 40’s %. Is the percentage that I’m reading just what the pcm is commanding and the Turbo doesn’t actually have to be at that point or does it follow the actual movement of it as it moves. Hopefully this makes sense. The thing is my Turbo does work and make boost so I’m wondering what isn’t working right. I’m sorry for so many questions just trying to understand how it works. Thanks for your help.
when I’m at idle my vgt reads 85%
with slight acceleration it stumbles and moves to say 68% and then climbs to 85%.
My truck when driving around the amount is 60% but coasting it moves to 85%.
under full throttle it will drop all of the way down to say mid 40’s %. Is the percentage that I’m reading just what the pcm is commanding and the Turbo doesn’t actually have to be at that point or does it follow the actual movement of it as it moves. Hopefully this makes sense. The thing is my Turbo does work and make boost so I’m wondering what isn’t working right. I’m sorry for so many questions just trying to understand how it works. Thanks for your help.
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Junior User
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From: Napa California
I just took it out for a test drive and still the same. The numbers on the Maf were right on what you said they should be. I can hear the Turbo spooling as I’m moving along and see the boost building. The only thing is when I’m at full throttle it says that the pedal is only at 78% shouldn’t it go to 100% and the lowest throttle voltage stays on.33v. Do these numbers look right to you I’ve not been able to find anything that shows what they should be.
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Hopefully this helps
APP (Accelerator Pedal Position - %):
Key on: 0%
low idle (690 rpm): 0%
High idle (1200 rpm): 13$%
Light Accel: 18%
Cruise: 19.5%
Hard Accel: 74%
Coast: 0 %
APP1 (Accelerator Pedal Track 1 - in Volts):
Key on: 4
low idle (690 rpm): 4
High idle (1200 rpm): 3.4
APP2 (Accelerator Pedal Track 2 - in Volts):
Key on: 1.4
low idle (690 rpm): 1.4
High idle (1200 rpm): 1.9
APP3 (Accelerator Pedal Track 3 - in Volts):
Key on: 0.9
low idle (690 rpm): 0.9
High idle (1200 rpm): 1.4
APP (Accelerator Pedal Position - %):
Key on: 0%
low idle (690 rpm): 0%
High idle (1200 rpm): 13$%
Light Accel: 18%
Cruise: 19.5%
Hard Accel: 74%
Coast: 0 %
APP1 (Accelerator Pedal Track 1 - in Volts):
Key on: 4
low idle (690 rpm): 4
High idle (1200 rpm): 3.4
APP2 (Accelerator Pedal Track 2 - in Volts):
Key on: 1.4
low idle (690 rpm): 1.4
High idle (1200 rpm): 1.9
APP3 (Accelerator Pedal Track 3 - in Volts):
Key on: 0.9
low idle (690 rpm): 0.9
High idle (1200 rpm): 1.4
Fleet Owner
Joined: Nov 2003
Posts: 27,881
Likes: 3,595
I checked the readings for the petal and acc low shows .33v, my cousin came over with his 07 and I checked my values against mine and his shows 0v. Maf reads 1.88v at idle and climbs to 3v + without load. In gear under load it’s only going to 1.88v. Don’t know what the Acc low should read.
I don't see anything wrong w/ APP percentages.
As stated before, I donlt think VGT should be going to 85%.
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Junior User
Joined: Aug 2013
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From: Napa California
Thanks for hanging with me on this Bismic. I did clean it following Diesel Tech Ron’s video. I sure miss that guy. Checked clocking out and everything. I’ll pull the Turbo later today when I get home from work and see what I see. I’ll try and post some pictures of the disassembled turbo.
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When you replaced the VGT solenoid did you happen to move the brown plastic piece that covers the screen. There are small oil passages that can be blocked and will result in a stumble especially when cold. Ask me how I know.
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Junior User
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From: Napa California
No I was very careful with it, straight out of the container and inside the Turbo. It was doing it before I replaced it. The thing is if I bring idle up to 100 rpms the vgt’s go to 60%. If I’m driving slow like 20 mph and give it an easy throttle it’ll drop off back to 85% and then start working down to low 40’s%. The Vgt seems to sweep back at fourth smooth with not anything jumping around. Still driving around and watching for some signs before pulling out the Turbo later.
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Junior User
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From: Napa California
Before I pull the turbo out and look at it what would cause it to be at 85% and then drop down in percentage like it should normally and then go back to 85% at idle or coasting. I’m trying to understand. With all of my readings where they are, I just don’t understand. Is it something that a sensor is telling the computer or something mechanical with the turbo. It seems that if the turbo Vgt’s are moving that it couldn’t be stuck or what? Could the unison ring be worn and then it just slips back down into the 85% area. I hope this doesn’t sound stupid. I’m stuck on what I’m going to look for.
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As stated above, the 85% number (VGT % duty cycle) is simply a command. No way of knowing what position the turbo inlet guide vanes are actually in.
Simply stated, the computer (PCM) is looking at things (like boost, EBP, IAT, and Mass Air Flow) and issuing a command to the VGT actuator to move the vanes (open more or close more) to produce the desired air flow.
Also, the unison ring can have spots of high wear (or high corrosion and/or carbon deposits) where it tends to stick more than other spots.
Simply stated, the computer (PCM) is looking at things (like boost, EBP, IAT, and Mass Air Flow) and issuing a command to the VGT actuator to move the vanes (open more or close more) to produce the desired air flow.
Also, the unison ring can have spots of high wear (or high corrosion and/or carbon deposits) where it tends to stick more than other spots.
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Junior User
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From: Napa California
I understand it now, even though I cleaned it up good it could somehow be stuck in the middle of its range and it would still make boost but have a lag down low causing the bogging I’m having. I’m pulling it out and see if I can figure out something. I did some dealing with a buddy that has one off a 07 that he’s dropping off for me. It’s easy enough to try it out and see if it solves anything.
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Junior User
Joined: Aug 2013
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From: Napa California
So After taking the turbo off and looking at the internals I came up with it’s not working right. The actuator that moves the unison ring was gummed up where the rod passed through the housing. So I’ll need to see if I can get it back to where it should be. I was lucky enough to have a buddy with a good one that gave it up. I dropped it in and everything cleared up and it’s driving great. Thanks everyone for your input it really helped me to track down my issues.
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