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Old 07-28-2012, 03:36 PM   #1
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Car bucks and jerks while coasting

So I've had this issue from day one. While driving at 50 mph and let off the gas and let the car coast, when the car gets to about 45 mph down to about 40 the drivetrain will randomly buck a few times. It's only noticeable at operating temp. Its really hard to describe the bucking. The tach will bounce down a few hundred rpm in sync with the bucking and it feels like someone is applying and releasing the rear brakes really fast. But I can clearly feel the driveshaft clunking sometimes if it's bad enough. Any other speeds from 39 down it will coast smooth, and from highway speed it will be smooth coast until that 45-40 range.

Any help as to what this could be would be great. It's quite annoying.

85 305 TPI 700R4 3.42. All stock. No codes.
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Old 07-28-2012, 04:44 PM   #2
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Re: Car bucks and jerks while coasting

This happens when you are backing off, or cruising?
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Old 07-28-2012, 04:59 PM   #3
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Re: Car bucks and jerks while coasting

Happens at no throttle. Any throttle at all and the problem is gone.
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Old 07-29-2012, 01:25 PM   #4
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Re: Car bucks and jerks while coasting

How does it shift?
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Old 07-29-2012, 01:42 PM   #5
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Re: Car bucks and jerks while coasting

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Originally Posted by Motown View Post
Happens at no throttle. Any throttle at all and the problem is gone.
I was think it could be the lock up in the torque converter, but it wouldn't happen when backing off.
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Old 07-29-2012, 03:17 PM   #6
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Re: Car bucks and jerks while coasting

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I was think it could be the lock up in the torque converter, but it wouldn't happen when backing off.
This is a good idea, it's possible that it is not properly disengaging.


When driving, GENTLY press the brake pedal, just enough to depress the brake switch. Your rpms should jump a couple hundred. If it starts bucking, than you know you have an issue with your TCC.
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Old 07-29-2012, 04:09 PM   #7
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Re: Car bucks and jerks while coasting

The time I had lock up problems it would go in and out of lock up, and it was the switch activated by the break peddle. I propped my shoe under the peddle and it stayed in lock up.
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Old 07-29-2012, 04:35 PM   #8
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Re: Car bucks and jerks while coasting

Wow never even thought about a trans issue. This could be possible since it only happens at operating temp. And this would also make since since it happens right at the speed where lockup happens. Ill try the brake pedal idea and let you know.

Thanks for the tips so far!
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Old 07-29-2012, 04:38 PM   #9
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Re: Car bucks and jerks while coasting

Check the trans fluid level and condition while you're at it. Easy and free.
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Old 07-29-2012, 04:38 PM
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