Answer to engine rock/bounce on throttle lift

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I used to have a problem (and I know others have posted about this) with the engine rocking or bouncing when I let off the throttle gently. Last night I adjusted my throttle cable and the bounce went away!

The main problem for me was I had about a thumbswidth of play on my gas pedal, caused by a similar amount of slack in the cruise control box. As near as I can figure out, with the cruise not moving with the engine (it's attached to the battery mount), when the engine shifts upon revving it rotates towards the rear of the car giving a disparity between actual throttle and the intended throttle. Letting off the gas causes the engine to move forward, but with the extra slack there's a slight delay... depending on the amount of engine movement and throttle lift, a situation can be set up where the engine and cruise's throttle spring move in syncopation. In other words, you get
1 throttle lift
2 engine shifts forward
3 cable from cruise stays in same position for half second, causing throttle to open again
4 engine shifts back at same time cruise cable finally reacts, closing the throttle more than intended
5 engine shifts forward
6 cycle continues

So the long and the short of it is if your car experiences this behavior, check your throttle slack in the cruise control box. It's like a 5 minute job to fix.

[arcasmachineracing]

Ahh... so how exactly do you adjust this throttle cable... heh, mabey I should grab a FSM and actually look at it huh? LOL... :p

Good post tho!

[wontonjohnny]

this may help get rid of that but it will also render you cruise control disabled

check if your CC still works

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Hmm, guess I should test that actually. Unless it needs all that slack though, I'm guessing it should still work alright. In my particular CC unit, there are two throttle actuators... there's a top disc that's connected to the pedal, and a bottom disc that's connected to the throttle. Pulling on the top (pedal) disk makes the bottom one move, however the bottom one can also pull independantly on the throttle with no input from the pedal.


Arcas: it's kind of hard to explain even though it's straightforward to actually do. Look at your throttle up by your intake manifold. It's C shaped. Your throttle cable connects to that and runs through a sort of metal flange then down to the pedal (or cruise). On either side of this flange are a couple nuts, you basically tighten/loosen on either side to move the cable forward or backward.

Kind of like this ascii art:
[code]
C-------:][:-----
1 2 34 3

1. Throttle
2. Cable
3. Locking nuts
4. flange

[/code]
Be careful in your adjustments that you don't cause the throttle to stay open when the cable is at rest, though.
The cruise has a similar set up, but you have to take the cover off the box to get at it. I'd take pics, but I don't have access to a digi camera right now.

[wontonjohnny]

yeah i did this awhile back and as soon as i did my CC stoped working

im thinking that CC needs the slack because any tension on the throttle cable cancels the CC right??

if you wanna get rid of the rock/bounce i think it would be better to raise the idle up just a bit

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Huh, I dunno cause I just took my car out for a drive and found my cruise still works fine. Accelerate and decelerate also work, so I don't know what I did different to prevent breaking it. By "not work" do you mean it wouldn't set the speed/engage, or was flakey?

Based on how the mechanism seems to work, I don't see how slack between the pedal and the cruise would be necessary. But I dunno.

I will say that my first adjustment was way too much, to the point where I'd let off the gas but the motor would stay revved. I could see how that would cause problems with the CC. I went back and deliberately left a teeny tiny bit of play at the cc, but not enough to feel with my foot. Maybe that's what turned the trick.