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  #1  
Old 8th September 2006
li Arc's Avatar
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Default 'Judding' on throttle release

Hi guys,

I have a question about something that's been bothering me about the car. Normally, when I'm accelerating, then suddenly release the throttle (while still in the same gear), the car will start to swing back and forth (not velocity, but acceleration), like a springs without dampeners, as if there were some play or ambiguity in the drive shaft or something. This effect is less evident on the upper gears than on the lower gears. In other words, the deceleration is not dampened, or smooth, when the throttle is suddenly released. Toe-heel downshifting is not affected (mainly because the engine and transmission sync are mated purposefully).

I have several theories as to what this is, the first of which is directly mechanically driven, such as a worn drive shaft or something (I may not even know what I'm talking about!), causing some play (too much work for me to fix). The second is the deceleration dashpot some people talk about in the throttle body (haven't checked it yet, but sounds easy), and third would be boost backpressure (not sure).

Of course, this is just me ranting...it's not bothering me too much, but I know my passengers sure don't like it (they say the ride is "rough", I say hah! that is what performance feels like!...although not really ). However, if it's fixable, it's probably best if I do so, cuz my lunch buddies are starting to rethink letting me drive

li Arc
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  #2  
Old 8th September 2006
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SteveTheFolkie SteveTheFolkie is offline
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Default Swing back and forth?

It starts to "swing back and forth" - do you mean you're getting lateral oscillation in the vehicle (right to left rocking) ??? That would be curious.

If you mean that in, say, 2nd gear while coasting/crawling to a traffic stop the car seems to jerk front to back as though the engine were dying - but the jerking stops when you put in the clutch or put the car in neutral, well that'd be normal if you had the engine slowed to the point (low on the RPM register) that it was about to die -

past that I don't have a freakin clue!
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  #3  
Old 8th September 2006
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I'm a bit confused as well, but it sounds as if something is binding up or its a broken motor mount.

Broken A-frames can cause a bit of a judder feel as well, it will pull away and whoperjaw the axel around at the end of the tranny cuff either on acceleration or deceleration, depending on which side is broken. Thats getting a bit far out there though since we still don't know exactly what its doing.
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  #4  
Old 8th September 2006
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Okay, well, it is a little hard to describe...it's not left and right, it's kind of oscillating front to back. However, it's not like I'm suddenly "accelerating and decelerating", but kind of almost feels like engine lug, but it's not the engine (the engine is usually at high RPM, given I just accel'ed and suddenly released the throttle).

The engine sounds healthy too, so that's not it. However, you can hear the engine being driven by the wheels, then the engine is driving the wheels. It's kinda on, then off, then on, then off...quick and short, and not very forceful.

Picture it in terms of passenger movement...you can feel yourself getting thrown from your seat a little bit, then gettting scooped up by it, then back to thrown from your seat again. It's not as extreme as I describe it, since it is a very short period, about .25s; in other words, it oscillates at about 2-4Hz.

If you dampen it with the brakes, it will stop the rocking motion. However, I'm more of a toe-heel driver, so I use brakes as little as possible...

Am I making any sense??

li Arc
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  #5  
Old 8th September 2006
kemosaabey kemosaabey is offline
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sounds like worn engine mounts - or possibly play in the diff/driveshafts.
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Old 8th September 2006
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I'm not sure I follow your description ;-), but does your 900 have a dashpot?
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Old 8th September 2006
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Let's say you're kinda bouncing back and forth in your seat with an undampened deceleration on throttle release...key is undampened, since it's like springs without shocks (except it's forwards to backwards)...ghaa...so hard to explain!

I'm not sure which dashpot you are referring to, but I think I will go looking in the throttle body for the deceleration dashpot and see if I can find it (or the remains of it)...that might need to be fixed, as I have never witnessed a "gradual deceleration" that the dashpot is supposed to give you...

li Arc
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Old 8th September 2006
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Quote:
Originally Posted by li Arc
throttle body for the deceleration dashpot and see if I can find it (or the remains of it)...that might need to be fixed

li Arc
Yes, that's the one... and now I see it in your original post ;-)!
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Old 8th September 2006
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So, I checked the dashpot, and it was backed up all the way, so I adjusted it, but I couldn't adjust it according to the Bentley manual as I think you need 2 people for that. Anyhoo, I just put it so that it will just *click* when the throttle is closed. However, I notice that the dashpot didn't do much, like it was really weak; should that be replaced? It didn't do much in terms of stopping the throttle lever...

The other thing I noticed is what I believe to be the turbo bypass valve; there's no pic of it in the Bentley manual so I can't confirm. At any rate, it's connected to the hose just before the turbo on the air filter end. Is this supposed to have something to do with deceleration? It's completely unused!! But I can't find where it's supposed to connect to neither; from the Bentley manual, I can see it looks like the diaphram end is supposed to connect somewhere on the throttle body, while the input is supposed to come from somewhere after the intercooler but before the throttle body (so, that just leaves the alum intake hose that has no additional ports??!?) Perhaps this can cure my problem? It looks as if it has not been used in a while, maybe someone thought the car didn't need it at all (I am also getting low boost, maxed out at mid-yellow, something to do with this?). Someone clear my swirling cloud of confusion?

li Arc
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  #10  
Old 9th September 2006
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The dashpot shouldn't click. Sounds like you adjusted the throttle position switch instead.

Inside the dashpot is a diaphragm and a spring. Those two can become weak over time.

Only one person is required to adjust the dashpot.
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  #11  
Old 9th September 2006
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What I meant was that I adjusted the dashpot to the point where the TPS would click, thereby making it possible for the ECU to know when the throttle is fully closed. It is possible for the dashpot to be adjusted out far enough that the TPS will not click, which I don't think is a good idea.

I may need a new dashpot, as it seems not to do anything, but I am having difficulties finding it on eeuroparts. The manual mentions that the throttle lever should touch the dashpot at 2600RPM, which I can't really test myself (I suppose I could listen to the engine and determine if it's the right rev or not, roughly). But at any rate, it's more like 1000RPM or less when it touches right now (pretty much useless).

I'm going to make a trip to the junkyard today and see if I can find out where the darned bypass valve is supposed to be connected, and get a few other parts. It's buggin' the heck outta me!

li Arc
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  #12  
Old 9th September 2006
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The dashpot is supposed to be just an emissions device anyway. The car should work fine without it.
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  #13  
Old 9th September 2006
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I suppose so, but at the same time, it does abruptly stop the flow of air. I'm beginning to suspect the bypass valve, but I still don't know where it goes yet. I'll see if I can find out somehow.

li Arc
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