When cruising in 4th 5th and 6th my car wants to buck back and forth. The lunge will start and get worse until I give it some gas to get the rev's up. It's not the cam or the tune its coming from the rear end. What does this mean?
my car did the same thing. i know mine is an auto but it would happen at low rpm in 3rd with the converter locked up. i fixed this by deleting lockup in third gear. maybe try to keep your rpm's a little higher while cruising.
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Location: On the Tour Bus....My bed goes 80mph, what does yours do?
Posts: 3,157
its a tuning issue, not a gears/axles issue...
typically it can be corrected by adding a little bit to the Throttle Cracker in the area of concern
or by fixing the fueling issues at that particular RPM/MAP/MAF value
multiple things can cause it...but its all in your tune
__________________ -Scott
408ci LSx / TCI 6x
Super Victor / 4150
Holley EFI Dominator
Dual Stage Direct Port Nitrous
Sounds like you have to tune your idle. Add some timing up to 1200 rpms; decrease the VE table idle region (I multiplied by 95%, 90%, and 85% with my small cam); add a little air to the Base Running Idle Airflow table (I added +1); maybe bump up your idle rpms.
Location: On the Tour Bus....My bed goes 80mph, what does yours do?
Posts: 3,157
Quote:
Originally Posted by Predator
Sounds like you have to tune your idle. Add some timing up to 1200 rpms; decrease the VE table idle region (I multiplied by 95%, 90%, and 85% with my small cam); add a little air to the Base Running Idle Airflow table (I added +1); maybe bump up your idle rpms.
I would not blindly tell somebody what to change without seeing the tune and some data logs to support it...
its more than likely Throttle cracker related...almost always is...(occasionally Throttle Follower related)
but sometimes if fueling is off it will cause the issue to be more exaggerated.
If its fueling related, it will be because you are super lean or way super rich(and I mean like 9:1 or worse rich) and it will have other issues that will be more obvious before you will notice bucking
if its bucking, its not "Idle", its while driving...so it has to be in "Driving" related tables...not "Idle" related tables.
and usually,timing doesnt cause bucking...I say usually because it can, but you have to be pretty far off for it to do that...
__________________ -Scott
408ci LSx / TCI 6x
Super Victor / 4150
Holley EFI Dominator
Dual Stage Direct Port Nitrous
I would not blindly tell somebody what to change without seeing the tune and some data logs to support it...
its more than likely Throttle cracker related...almost always is...(occasionally Throttle Follower related)
but sometimes if fueling is off it will cause the issue to be more exaggerated.
If its fueling related, it will be because you are super lean or way super rich(and I mean like 9:1 or worse rich) and it will have other issues that will be more obvious before you will notice bucking
if its bucking, its not "Idle", its while driving...so it has to be in "Driving" related tables...not "Idle" related tables.
and usually,timing doesnt cause bucking...I say usually because it can, but you have to be pretty far off for it to do that...
You're right, plus I didn't read his post carefully enough (the 4th, 5th, and 6th gear part). I was fixated on idle only.
I'm going to be getting in contact with another tuner to take a look at my tune. I'll keep you guys updated. I've had it tuned twice first on the dyno and a second time for driveablity.
Location: On the Tour Bus....My bed goes 80mph, what does yours do?
Posts: 3,157
as a test you can sometimes drive it, and have the tuner find the rpm its bucking at, and just use the real time IAC controls in the scanner to add some air.......then you can work on just how much it is to make the bucking go away
but it only works if your IAC position is below 127 when the bucking is happening...
otherwise, you have to find it, add it in the cracker, write the file, and go driving some more to test it out
the fact that giving it more throttle makes it go away, tells me that it is airflow related and its looking for more air at that particular spot.
__________________ -Scott
408ci LSx / TCI 6x
Super Victor / 4150
Holley EFI Dominator
Dual Stage Direct Port Nitrous
Location: Chattanooga, TN... Now: Little Rock AFB, AR
Posts: 2,851
I know the exact bucking your talking about. It may not be in the tune. My old 10 bolt would buck like no.other before it finally broke. Slop in the gears due to wear can cause it. Get under the car and rotate the driveshaft back and forth to see how much play you have between gear teeth.
as a test you can sometimes drive it, and have the tuner find the rpm its bucking at, and just use the real time IAC controls in the scanner to add some air.......then you can work on just how much it is to make the bucking go away
but it only works if your IAC position is below 127 when the bucking is happening...
otherwise, you have to find it, add it in the cracker, write the file, and go driving some more to test it out
the fact that giving it more throttle makes it go away, tells me that it is airflow related and its looking for more air at that particular spot.
Really thought giving it gas makes the bucking more intense until it goes up into the rpm's some more. I will drive it and make note of the RPM positions where it starts and stops.
Quote:
Originally Posted by HoLLo
I know the exact bucking your talking about. It may not be in the tune. My old 10 bolt would buck like no.other before it finally broke. Slop in the gears due to wear can cause it. Get under the car and rotate the driveshaft back and forth to see how much play you have between gear teeth.
I will also do this. Like I said I have 4.10's in a M6 10-bolt. I'm thinking its just on its way out. I posted a thread asking if to much back lash will cause this because I also just a rough jolt when going from off-throttle to cruise but never got a reply after 4 days and deleted the thread.
Location: Chattanooga, TN... Now: Little Rock AFB, AR
Posts: 2,851
Yeah off throttle to cruise I would feel a slop also. Or say going down a hill without throttle and lightly pressing the throttle you would feel the gear slop. Too much backlash is most definitely the cause. Backlash is exactly that: distance a gear tooth moves before it make contact with the next tooth.
I installed a Moser 9" back in September with a center section i assembled myself. I apparently didn't do it right and started getting a ton of play in the gears. It is now getting rebuilt properly by a shop as I'm typing this.
I'm weighing my options now, QP housing/axle package and digging up a trac-loc center section in a junk yard or a S60 with a spool. I could also get a stock ford center section with a spool for about 600 bucks from QP. Don't know yet, leaning towards the S60.
Location: Chattanooga, TN... Now: Little Rock AFB, AR
Posts: 2,851
I got a good deal on a Trac Loc center section on ebay and used it. It was all stock Ford parts but since I'm rebuilding the gears I decided to upgrade it all to a 1350 forged yoke, billet daytona pinion support and Moser lightweight Nodular case. Definitely cheaper to put together your own center section and toss it in a housing/axle package like I did.
Car bucking at steady state. 
