Gotta Problem--Need Help
10-04-2010, 02:10 PM
#1
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Gotta Problem--Need Help
gotta problem with my 07 4.8 NNBS RCSB. Been having the issue for a little while.
If I punch it from a roll, say 20-40 mph, when it downshifts to first, then runs up through the power band, at the top of second gear, just before the shift, its like the truck just falls flat on its face for a split second. At first we thought it was hitting the rev limiter just before the shift, but have since taken it to Wheatley and we have 400 RPM between the shift point and rev limiter. It only does this at WOT and only on a 1st gear downshift, when it gets the most out of the power. If I roll into a WOT run from first it won't replicate this issue. If really feels like the power is just cutout for a split second. I can feather the throttle just before the shift and not have it happen also. I have also noticed belt slip just before the shift at that same point, but I wouldn't think that it would be related.
Wheatley thinks it might be the tranny starting to slip. I've never experienced tranny slip so I have no idea what this might feel like, but I have a hard time believing that. Wheatley did not experience the issue I'm having when he re-tuned it so he's just guessing based off of my explanation to him. I usually drive like a grandpa and the truck is lightly modded, I'm not running any Tq Management, but rarely ever go WOT.
Does the tranny slipping feel like slamming on the brakes/losing all power instantly?
Any ideas or suggestions are appreciated.
If I punch it from a roll, say 20-40 mph, when it downshifts to first, then runs up through the power band, at the top of second gear, just before the shift, its like the truck just falls flat on its face for a split second. At first we thought it was hitting the rev limiter just before the shift, but have since taken it to Wheatley and we have 400 RPM between the shift point and rev limiter. It only does this at WOT and only on a 1st gear downshift, when it gets the most out of the power. If I roll into a WOT run from first it won't replicate this issue. If really feels like the power is just cutout for a split second. I can feather the throttle just before the shift and not have it happen also. I have also noticed belt slip just before the shift at that same point, but I wouldn't think that it would be related.
Wheatley thinks it might be the tranny starting to slip. I've never experienced tranny slip so I have no idea what this might feel like, but I have a hard time believing that. Wheatley did not experience the issue I'm having when he re-tuned it so he's just guessing based off of my explanation to him. I usually drive like a grandpa and the truck is lightly modded, I'm not running any Tq Management, but rarely ever go WOT.
Does the tranny slipping feel like slamming on the brakes/losing all power instantly?
Any ideas or suggestions are appreciated.
10-04-2010, 02:38 PM
#2
Kinda a guess, but it might be the fuel pump... When you roll into the throttle it does not use the transient (pump shot) like it does when you slam on the gas, meaning you use more gas when you stomp on the gas rather than rolling into it. I would rule out the fuel pump by putting a fuel pressure gauge on it...
10-04-2010, 03:52 PM
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Oh yeah, just thought of the best comparison to how it feels when this happens. It feels like hitting the top speed limiter when you have a stock truck.
10-18-2010, 08:00 AM
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Are your WOT shift qualifiers for just rpm or are they rpm and mph?? You may not be hitting one of those preventing the shift...I have experienced the same in my tahoe, best I can describe it as is a diesel defueling at shift. I have readjusted my WOT shift mph and rpm...problem solved.
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10-18-2010, 08:34 AM
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Are your WOT shift qualifiers for just rpm or are they rpm and mph?? You may not be hitting one of those preventing the shift...I have experienced the same in my tahoe, best I can describe it as is a diesel defueling at shift. I have readjusted my WOT shift mph and rpm...problem solved.
