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KTPhil 
Samba Member
Joined: April 06, 2006
Posts: 33991
Location: Conejo Valley, CA
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 Posted: Fri Jan 21, 2022 1:58 pm Post subject: Bucking (FI issue?) |
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My newly-rebuilt motor is sounding and running well, with a out 100 miles of easy driving on it, but I have the recurrence of an old problem. It's a kind of bucking that occurs only under specific circumstances. I searched for "bucking" in this forum but the hundred of mistitled threads was too much to go through, so I'll start anew.
When coasting, especially a little downhill, but not entirely off the throttle, I feel the engine cutting out. A little more or less throttle clears it up. When heading down a long, shallow drop on the freeway, it gets very annoying. I haven't tried serious throttle valve switch (TVS) adjustments, though that was identified in older threads as a cause of bucking. No air leaks anywhere.
The odd thing is that it seems to be amplified by a tranny/CV joint issue. I have more "play" than I would think is normal when jacked up and moving rear tires forward and back. I haven't done this carefully enough to separate tranny play with CV play. I repacked my CVs a long time ago, and there is about 250K on the car. No bluing or pitting back then, so I repacked them with fresh grease and new boots. No noises, either.
Tranny was rebuilt by a very good shop about 8 years ago and I have only about maybe $15,ooo miles on it.
But any extra play there makes the bucking seem worse.
Nothing that wills top the car or damage the engine, but it is less pleasant to drive when I have to adjust the throttle position to avoid the jerking.
Any thoughts? |
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Tram
Samba Socialist
Joined: May 02, 2003
Posts: 22711
Location: Still Feelin' the Bern- Once you've felt it you can't un- feel it.
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| Posted: Fri Jan 21, 2022 8:49 pm Post subject: Re: Bucking (FI issue?) |
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TP sensor needs adjustment or replacement. The C/D 4 prong TPS was notorious for this.
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raygreenwood
Samba Member
Joined: November 24, 2008
Posts: 21510
Location: Oklahoma City
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| Posted: Sat Jan 22, 2022 7:59 am Post subject: Re: Bucking (FI issue?) |
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| Tram wrote: |
| TP sensor needs adjustment or replacement. The C/D 4 prong TPS was notorious for this. |
Yes....especially some of the later build dates of them....maybe early to mid 1980s. It seemed to a pin spacing issue of the "on/off" poles either in the main rotating armature or in the floating forked switch.
I suspect it was a handful of processing issues during the injection molding of a couple of parts inside.
It made some of them very sensitive to adjustment......needing a level of detail a good deal above ......"just rotate until the VOM beeps and then give it 2° more".
A very few I have found enough distance/lash between the on/off poles in the armature....that they can never be adjusted properly. At a certain temperature when the expansion reaches maximum....they just cut out.
Read this:
https://www.thesamba.com/vw/forum/viewtopic.php?t=683230&highlight=tvs
Ray |
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KTPhil
Samba Member
Joined: April 06, 2006
Posts: 33991
Location: Conejo Valley, CA
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| Posted: Sat Jan 22, 2022 12:08 pm Post subject: Re: Bucking (FI issue?) |
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Thanks, guys, that's the link and info I was looking for!
It really feels like that spring/drag-related "first movement" play or sticking discussed there. ("floating forked switch")
I have a couple of spares I'll try tweaking per the link. The car is not very pleasant to drive with this condition, so hopefully this restores it to the joyful throttle response and smoothness I remember when it was new! |
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raygreenwood
Samba Member
Joined: November 24, 2008
Posts: 21510
Location: Oklahoma City
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| Posted: Sun Jan 23, 2022 11:53 am Post subject: Re: Bucking (FI issue?) |
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| KTPhil wrote: |
Thanks, guys, that's the link and info I was looking for!
It really feels like that spring/drag-related "first movement" play or sticking discussed there. ("floating forked switch")
I have a couple of spares I'll try tweaking per the link. The car is not very pleasant to drive with this condition, so hopefully this restores it to the joyful throttle response and smoothness I remember when it was new! |
A couple of other items that can cause the bucking....lets say 10-20% of the time...each...are the injector ground wires being flaky.....usually at the case ground star under the plenum...which is both beat expansion related and vibration related......and the three pin connnector for the trigger points.
Lastly.....and its a low percentile problem......is the ground strap on the transmission tail cone.
Any of these that can cause either complete ground loss or partial ground loss.....when you are at the part throttle steady cruise throttle plate setting....can cauze the problem as the engine shifts in its rubber mounts.
I had a serious bucking problem with the TVS long ago.....and could not adjust it out....because it was not 100% the only cause. It was the cause maybe 50% of the time.
It was maddening to track down. So go through all the grounds first....anx THEN adjust the TVS.
Ray |
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