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Discussion Starter · #1 ·
After searching through many threads, yes I do think I have the bump from behind problem. But I have some other problems that I haven't found on here yet and don't know if they're related or not.

First, yesterday while driving home from work, doing between 60-65 it happened probably 6-7 times. It feels like the truck lunges forward or something is slipping. There's also like a clunking sound like something is dropping out of place. This has happened before but not nearly as frequent as yesterday.

Second, if I drive at 50 and slow down to 40ish, I will still be in 6th gear and the rpm's are at about 1000. This I can replicate almost anytime I want if there's a slight hill. While driving up a slight hill (and I mean very slight) at 40-42mph in 6th gear, the truck will change gears to 5th and it feels and sounds like the rear diff/drive shaft/tranny is going to drop out. It's a little more than the bump from behind that many seem to have.

So my questions are, should I have the tranny fluid and rear diff looked at be someone other than the dealer? Or could it be the rear diff pin is out of place and I should look into an Auburn diff? I dont really know what's going on here. Any insight would be very helpful.
 

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What year truck? how many miles? Warranty? Any lift?
 

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Discussion Starter · #3 ·
I've got a 2007 5.7 DC with 75k miles, warranty ended in december last year and its got the Rancho Quiklifts with LT275/65-18. Other than that, the outside is completely stock.
 

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Check your tranny and motor mounts, not likely an issue but rule them out first. I'd guess it's your torque converter, a fluid change could make a difference, couldn't hurt.
 

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Discussion Starter · #7 ·
The strange thing is that it doesn't happen all the time. It's an every now and then type thing except for yesterday's episode. What would I be checking for with all the mounts? And going with a new LSD would be the very last resort although I think it would be something I'd like to do at some point.
 

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After searching through many threads, yes I do think I have the bump from behind problem. But I have some other problems that I haven't found on here yet and don't know if they're related or not.

First, yesterday while driving home from work, doing between 60-65 it happened probably 6-7 times. It feels like the truck lunges forward or something is slipping. There's also like a clunking sound like something is dropping out of place. This has happened before but not nearly as frequent as yesterday.

Second, if I drive at 50 and slow down to 40ish, I will still be in 6th gear and the rpm's are at about 1000. This I can replicate almost anytime I want if there's a slight hill. While driving up a slight hill (and I mean very slight) at 40-42mph in 6th gear, the truck will change gears to 5th and it feels and sounds like the rear diff/drive shaft/tranny is going to drop out. It's a little more than the bump from behind that many seem to have.

So my questions are, should I have the tranny fluid and rear diff looked at be someone other than the dealer? Or could it be the rear diff pin is out of place and I should look into an Auburn diff? I dont really know what's going on here. Any insight would be very helpful.
This one I have, though it doesn't seem as severe as you describe. I can usually prevent it by modulating the throttle. Just giving it as little as possible without completely letting up.

Mine is an 08 with a little over 30,000.
 

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I've got an '07, 5.7, DC and have had that harsh back shift since new, but mine is from 5th to 4th gear with light throttle; yea it sounds and feels like you've just been rear ended by a semi.If I think of it while slowing, I'll come out of the throttle completely until I feel it back shift to 4th, then apply what throttle is needed.Had mine back to the dealer several times when it was new and they "couldn't duplicate"??
 

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Discussion Starter · #12 ·
I either have to let off the throttle completely or give it a little gas to jump the rpm's up some if I know I'm about to have that horrid gear change. I think I'm going to have someone other than the dealer look at it this weekend and hopefully get a better answer other than the "we couldn't duplicate it' typical response.

Another thing, is it normal that the truck won't/can't get into 6th gear when the truck is cold?
 

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I either have to let off the throttle completely or give it a little gas to jump the rpm's up some if I know I'm about to have that horrid gear change. I think I'm going to have someone other than the dealer look at it this weekend and hopefully get a better answer other than the "we couldn't duplicate it' typical response.

Another thing, is it normal that the truck won't/can't get into 6th gear when the truck is cold?
Yes, that's typical;It's a component temperature function. Seems to be engine temperature controlled, but there is interaction between engine and transmission temperature controlled by the transmission fluid heater(engine coolant). It won't shift into sixth gear until a predetermined temperature is reached
 

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First, yesterday while driving home from work, doing between 60-65 it happened probably 6-7 times. It feels like the truck lunges forward or something is slipping. There's also like a clunking sound like something is dropping out of place. This has happened before but not nearly as frequent as yesterday.
This happened on my dad's '95 Forerunner. He wanted to take it to the dealer and have them fix the tranny. I was with him and told him to stop, let them figure out the problem and report back.

It turns out after 120k miles and 11 years he needed new spark plugs. haha

The engine was missing/stumbling and this was causing the driveline to feel like it was lunging and slipping and the slop in the mechanicals was causing banging noises.
 

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Discussion Starter · #15 ·
I hope it'll be a simple thing like spark plugs to fix it. Since that last episode Monday afternoon, nothing has happened since but I'll see what happens today after work if anything happens since I'll be driving probably 3 times the mileage.
 

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How about bad gas? Try another gas station....
 

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Discussion Starter · #17 ·
I'll be doing that today actually, down below a 1/4 tank now and probably closer to an 1/8th by the time I get to a station.
 
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